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***STICKY*** - NA1 5-speed Transmission Rebuild Thread

30 October 2016
Austin, TX
There's little information about rebuilding these transmissions out there on the internet so I figured I'd take a bunch of pictures and post them for anyone who cares. We're getting to the point that some of these cars are getting up there in miles and might need it sooner or later, and some people just want better gearing so I hope this information helps someone else. Honda manual transmissions from the 90s are all fairly similar, this one is just a bit bigger and heavier and more annoying.

This thread is intended to be used as a reference in addition to the diagrams and instructions found in the factory service manual. If you don't have one, go download it and read the chapter on tearing down the transmission.

In an ideal world you'd have access to a big press, various pullers, and other special tools. But I don't have that stuff and most DIYers don't either. Part of being a good DIY garage hack with limited tools is knowing when you've reached the limits of your capability and to call a friend/pro/machine shop/etc. If something is moving like you think it should, stop and reevaluate, whipping out the sledge should never cross your mind.

Also, if you're having issues with your transmission, change the fluid before you rip it open. Old fluid can cause all sorts of strange issues and you might get lucky.

Parts List

Below is a list of parts one might use in a "typical" transmission rebuild. This is a good baseline for what to buy, but do your own research and make sure you order everything you need, there is a very good chance you will find other things that need to be replaced and find things that you probably didn't need to replace. Examples include clearance adjustment shims, needle bearings, worn hub/sleeve (especially on 5th), etc. I've found that parts are much cheaper if you order from Japanese suppliers like Amayama, but for some things such as clearance shims that you can't predict ahead of time I find it easiest to order them from local Acura or Honda dealerships since they can usually get them in much faster than you could order them and don't cost all that much more.

To minimize the cost of a rebuild, if you have a grinding issue, stop using that gear immediately or figure out how to make it not grind when you select it. The more the gear grinds, the more likely you'll have to replace parts that aren't normal wear items.

If you find anything strange with the part numbers or list items, please let me know, I'm only human and there is a non-zero chance that I might have screwed up when copying stuff over from my spreadsheet.


23641-PR8-010 qty 1 1st gear synchro
23646-PR8-020 qty 1 2nd gear synchro
23641-PR8-911 qty 2 3rd/4th gear synchro
23642-PG1-912 qty 1 5th gear synchro


91002-PR8-008 qty 1 Mainshaft bearing (bottom)
91004-PR8-008 qty 1 Mainshaft bearing (top)
91103-PR8-018 qty 1 Countershaft bearing (bottom, the one with no inner race)
91102-PR8-018 qty 1 Countershaft bearing (middle)
91003-PR8-008 qty 1 Countershaft bearing (top, the one with the snap ring groove)
91105-PR8-008 qty 1 1st gear needle bearing
91108-PR8-008 qty 1 2nd gear needle bearing
91104-PR8-008 qty 2 3rd/4th gear needle bearings
91107-PR8-008 qty 1 5th gear needle bearing
91121-PR8-008 qty 1 Differential taper bearing (bottom)
91122-PR8-008 qty 1 Differential taper bearing (top)

Oil Seals

91216-PR8-005 qty 1 Input shaft seal
91207-PR8-005 qty 1 Driver side driveshaft seal
91206-PR8-005 qty 1 Passenger side intermediate shaft seal.
91215-PR8-005 qty 2 Shift lever oil seals


90030-PG2-000 qty 2 Countershaft bearing retainer bolts (these are staked so must be replaced if removed)
90201-PR8-000 qty 1 Countershaft lock nut
23926-PR8-000 qty 1 Countershaft lock nut spring washer
90602-PR8-000 qty 1 Snap ring (I wouldn't replace this unless your transmission is in snap ring range, it's difficult to remove without damaging the transmission case)
22841-PR8-010 qty 1 Clutch fork boot
21180-PR8-016 qty 1 Oil strainer
21181-PR8-000 qty 1 Oil strainer spring
91307-PR8-005 qty 1 Oil strainer O-ring
91309-PX4-003 qty 1 O-ring for the cover plate above differential in clutch housing

OPTIONAL JDM Gearing Upgrade

23210-PR8-020 qty 1 JDM mainshaft
23441-PR8-020 qty 1 JDM mainshaft 3rd
23451-PR8-010 qty 1 JDM mainshaft 4th
23431-PR8-010 qty 1 JDM countershaft 2nd
23471-PR8-000 qty 1 JDM countershaft 3rd
23481-PR8-000 qty 1 JDM countershaft 4th

OPTIONAL NSX-R Differential Upgrade

41696-PR8-000 qty 2 Preload spring washer
41581-PR8-J00 qty 1 NSX-R Preload spring retainer

OPTIONAL NSX-R 4.23 Final Drive

23220-PR8-J00 qty 1 NSX-R countershaft
41233-PR8-J00 qty 1 NSX-R differential ring gear
21173-PR8-J00 qty 1 NSX-R oil pump drive gear

1) The shafts

Start off by removing the various sensors, wires, and engine mount.


Then use a 1/2" drive breaker bar to expose the snap ring and spread it to release the countershaft from the transmission housing. It should drop down a bit when it releases and the snap ring should stay in the open position. I had to balance on top of the transmission while pushing down on the breaker bar to get enough leverage, since the snout of my impact gun was rounded and started to round out the cover.


Once the countershaft has been released, undo all of the 12mm housing bolts in a criss cross pattern. There are 4 bolts that are longer, I'd keep them in separate baggies for ease in finding them later, the longer bolts go in the holes with the dowel pins. Also pay attention to the bracket that shares a housing bolt as you'll need to put it back in the same place.


Once you've lifted the housing off, there are a couple measurements to take with a feeler gauge to verify clearances.


The manual then states to remove the reverse gear lever, lift up the mainshaft a bit, remove the reverse gear fork, then remove the reverse gear shaft. I was able to get the reverse fork out, but I couldn't get the mainshaft to lift up enough so I strapped all 3 shafts and the 3 shift forks together with some big zip ties and removed them as one unit.

There is another method I've found that can easily be done with 2 people (or 1 if you're handy) and requires no zip ties. Have one person grab both shafts and lift them up 1/2 inch or so, then have another lift up the reverse shaft and pivot it around the reverse fork until it is free, then both can be removed from the clutch housing at once. Once the reverse shaft and fork are removed, grab the two shafts while holding the forks with your thumbs and lift them up and out of the case.


Once the shafts are out you can begin taking them apart. Since I'm replacing a bunch of gears and bearings and synchros I didn't bother to take any measurements since I'm going to do that later once the new parts are on. Honda isn't very consistent with how tight their press-fit parts are, some transmissions come apart easily with no tools and others need a puller and/or press. I used the 7 ton rental puller from Autozone and it worked pretty well.


I wiped the parts off with a microfiber towel then cleaned them with brake cleaner before reassembly. The shafts are hollow so its a good idea to give them a good blast with brake cleaner down the middle to clear out any gunk, as everything will be covered in a layer of black goo.

The service manual lists a couple of shaft diameter measurements you can check now that the shaft is mostly apart. If you have a mic go ahead and check them but it's probably not necessary unless there's lots of obvious wear.

Starting to assemble the JDM mainshaft. The fit of the hubs wasn't very tight so I was able to do it mostly by hand. At the end, I taped up the nose and gave the 5th gear hub a few light taps with a hammer and big socket to make sure everything was properly seated.



I don't really have many pictures here but the countershaft process is essentially the same but it has a REVERSE THREADED staked locknut at the top and almost definitely requires a puller to separate the first couple gears since they are a tighter press fit.

EDIT: So far both [MENTION=35590]Big McLargeHuge[/MENTION] and [MENTION=18194]Honcho[/MENTION] are reporting that a 40mm socket is too small. I guess I mis-measured or made a mistake converting my inches mic reading to metric. Sorry about that. Since 1 5/8" sockets are equivalent to metric 41.28mm and fit and Honda uses metric sizes I think its actually a 41mm nut.

I started off by removing the locknut with my impact gun. Theoretically, you can do this in a vice with a breaker bar but it's bad enough torquing a new locknut on where you only have to get to 112lbft instead of whatever the breakaway torque needed is for a nut thats been on there for 25 years. IMO you'd be better off taking the countershaft to the nearest lube shop and asking one of the techs to zip it off with his impact gun than you would trying to do it in a vice, but you do you.

My puller was only 6" so I couldn't reach 3rd gear, so I hooked it onto the bottom of 4th gear and pulled off 4th and 5th, then turned the shaft upside down and tapped it on a piece of wood to dislodge 3rd gear, you might not be able to get away with this, but if you try, make sure to be ready for 3rd to fall off at any given moment. There are thin sheet metal bits at the ends of the shafts since they are hollow, you can't use a puller against these, you need a socket on the end of the shaft or you will destroy it.

If you turned it upside down and tapped it to remove 3rd gear the rest of the parts probably came off at the same time. If you're using a press, do it the right way mentioned in the manual. But basically 1 and 2 are more of the same stuff as the mainshaft with the exception of the 2nd gear synchro which is a dual cone synchro and you need to make sure the tabs line up with 2nd gear during reassembly.

Again, check the measurements of the shaft as per the service manual if possible. Mine was perfectly within spec at 109k miles.

Putting the JDM gears on my countershaft proved rather difficult, the right way is a press, but I used a hammer. A normal sized hammer with reasonable force, not a 4+ pound sledge. With 1st and 2nd gear on the shaft, slide 3rd down and make sure the splines are lined up and push it on as far as possible by hand. Then slide 4th on and line up the splines, then slide 5th on. 5th won't engage with the splines yet but you need it as a spacer since most sockets aren't deep enough without it.

Here is what the stack looks like when hammering. I put a big washer over 5th to protect it a bit more, and also taped up the nose to keep the socket from rattling against it. It will take a bit of movement before the 5th gear splines engage so you constantly need to be checking. Go slow.


At this point, get out your feeler gauges and do a preliminary check of the clearances covered in the manual. You'll do it again after torquing the nut down but it will save you a bit of time if something is way too loose or way too tight at this point.

Once the gears are all on, put the two bearings on. Put tape over the bearings so you don't get any debris that may be on your hammer/press or socket in the races. Do the bearings one at a time.


Once both bearings are on, clamp the shaft in a vice with blocks of wood like the manual states, put the spring washer back on, and put a new locknut on, torqing to 112lbft. At minimum, you need a 4.5" vice and 2 scraps of 3/4" plywood. A 6" vice and scraps of 2x4s would be better but 4.5" worked for me.

Now do a final check with the feeler gauges.

2) The differential

1991-1994 NSXs as well as the NSX-R use a LSD with clutch disks whereas the 95+ non-R NSXs use a torsen diff which I've never messed with. One fairly cheap and easy mod is to swap out the preload springs in the differential to bump up the breakaway torque to match the NSX-R at 110lbft or so instead of the 40lbft in the non-R models.

Make a mark on the ring gear and on the case with a permanent pen so you can put it back on the same way as it came apart. Opening it up is pretty easy, just use a small impact gun (or a big one with the pressure turned way down) to zip the case bolts off in a criss cross pattern with a 12 point 10mm socket.

Once it's open you'll see this. I didn't mess with the clutch disks but I suppose you can inspect them or replace them if you want to. The part we're concerned with is on the right, there's a big snap ring holding the spring washer in, you can pop it out with a screwdriver by working your way around. Keep a hand over the case while you work so it doesn't pop up and hit you. There's not enough force to actually send anything flying across the room, but the snap ring itself is pretty light and springy so wear safety glasses. Or you can do it the right way with a press and compress the spring washers before removing the snap ring.


Once you've popped the snap ring out you can take out the retaining ring and spring washer, and replace them with the NSX-R retaining ring and 2 new spring washers. Some people reuse the original spring washer and only buy one additional one but the parts are a couple bucks so might as well replace it.

Getting the snap ring back in is a bit trickier, again, the right way is with a press. Once you have the spring washers and retaining ring in the case, slide the snap ring down as far as it can go. Instead of a press I used a big C clamp, various large washers, a couple blocks of wood, and a socket. Cranking down on the clamp compressed the washers enough to easily seat the snap ring back in it's groove.


After that, put the case back on in the same place it came off using the mark you made, thread all the bolts in hand tight, then go in a criss cross pattern with a ratchet snugging them down. I did 1/2 turn at a time and made a couple passes to get all the bolts snug, then I held on tight with my left hand (you'll want gloves) and used the torque wrench in my right hand to torque them back to 29lbft in a criss cross pattern. A vice with wooden blocks would be ideal, but if the taper bearings are still on the diff then that's not really an option. If you're having trouble getting the bolts to 29lbft have a friend wear thick gloves and hold the diff while you operate the torque wrench, or if they won't go for that, let them hold the wrench.

Getting the transmission housing diff bearing out the Honda way is terrible, they either tell you to pry it out with a screwdriver (seriously WTF how does that even work) or heat the housing to 212F and remove it that way (again WTF). Pop the oil seal out, flip the housing over, and tap it from the back with anything that fits, in my case, a PVC pipe fitting I use for whacking various things. In my case it took almost no effort to get it out, and no fire or prying on aluminum was involved.


Removing the clutch housing taper bearing race was about the same process, pop the oil seal out, flip it over, and tap on the metal oil control ring or whatever it is that's behind the bearing race. It came out fairly easily with just light tapping.


The bearing race came out before the oil control ring thing so I flipped the housing back over and lightly tapped it back in.

Getting the differential bearings off is beyond the capability of the tools I have in my garage. It might be done with a good bearing splitter, but I don't really care to buy one so I had a local machine shop remove the bearings for me. They ended up cutting them off since there isn't really a good way to latch onto them with any sort of puller. Machine shops tend to have metal shards flying everywhere so wad up paper towels and stuff them in the axle holes before you hand it off to anyone.

Putting the new taper bearings on was more of a pain than I expected it to be. The bearings have wildly different inner diameters so you can't use the same tool for both, so after thinking for a bit I decided to make my own tool. You can't use a socket because any socket big enough to clear the shaft of the differential will smash the roller cages of the bearings since they stick up, so what is needed is a skinny tube. I used a piece of 1/8" thick aluminum stock I had laying round and rolled it around a socket using vice grips to hold the end to the socket and a hammer to beat it into shape. I did the bigger bearing first and used a socket slightly smaller than needed so after it sprang back a bit it would be close enough to bend into shape. After doing the large bearing, I cut the end off the aluminum piece to make it smaller and saved myself a lot of work since I didn't have to start over. The bearings went on fairly easily with just light taps from a hammer and a block of wood wrapped in tape to keep splinters from getting in the bearings.



To check the preload on the bearings you have to lubricate the bearings with transmission fluid, bolt the case halves together, and see how much force it takes to spin the diff. Honda sells a special tool to stick in the diff to allow you to attach a torque wrench but rather than buy another tool thats probably been discontinued I just got a 10" long bolt, some nuts, and some washers and made my own. This also makes the diff easier to install and remove in the clutch housing since it's fairly heavy and hard to get your fingers around and sharp so definitely something I'd recommend doing before you even separate the case halves in the first place. I put the two nuts on the end so I could easily put a torque wrench on it without worrying too much about how long the bolt was.


Finally, bolt up the case halves, give the diff a good spin with a ratchet to seat the bearings, and attach a torque wrench and see what it takes to spin it. In my case it was about 20 inch pounds, the factory spec is 17-26 so IMO 20 is perfect.


3) The housings

If you want to remove the countershaft bearing, start by getting your favorite flat blade screwdriver and pry the roller retainer like so. Honda has a special slide hammer attachment for this but why would you want to spend money on that when you could just use the free one from Autozone?


Once the bearing is adequately destroyed, put the slide hammer in like this and tap the bearing out of the clutch housing.


Put the new one in using a socket and the outer race of the old one to tap it in. But I prefer to leave reassembly until the very end after I've had a chance to clean everything.


Undo the 3 screws at the bottom of the housing to expose the tiny little baby oil pump so you can check clearances with a feeler gauge here. Behind the spring is a ball, so make sure not to lose it.


Removing the shifter arms from the housings is easy, just remove the 12mm bolt with a low power (low pressure) impact gun and slide it out, then use a screwdriver to pop the oil seal out.

Here is the clutch housing with the guts removed.


4) Mainshaft Thrust Shim

Unfortunately, there is no ghetto cheap way to properly set the mainshaft thrust shim thickness, you need a dial indicator and a micrometer/caliper. To put the bottom mainshaft bearing on I just stacked up a bunch of circular objects of roughly the correct diameter and tapped it on with a hammer. The important thing here is that whatever is in contact with the bearing only be touching the inner race, you don't want to apply force to the outer race. I used the 3/4 hub from an Integra transmission since it fit. It's fairly tight but doesn't take too much force.


I didn't have a dial indicator holder that would work for this, so I made one out of this billet battery tiedown from a Subaru WRX I had laying around. (long story, I've never owned a Subaru)


Install the mainshaft with the shims at the top but NOT the conical spring washer at the base of the mainshaft and bolt the case halves together, then set up your dial indicator at the end of the tip of the mainshaft. Once everything is set up, push in on the mainshaft and zero the dial indicator while holding force on it, then pull out on it and take the reading. In my case it was 0.0380" give or take a few tenths, a 0.0001" precision dial indicator is absolutely overkill for this but it's what I have and it works. This measurement is too tight by 0.001" so in the interest of perfectionism I'll be ordering a shim one step looser to compensate. Honda sells them in increments of 0.05mm (0.002") so that should put my clearance right at 0.0400 which is perfect. (factory specification is 0.039-0.041") The only way to know which shim(s) your transmission has from the factory is to measure them with a micrometer or caliper.

The FSM process for this first uses feeler gauges to find the play in the mainshaft without the spring washer installed (which is what we measured above with the dial indicator since the FSM method is terrible), then you install the shims and spring washer and check the final clearance using a special mainshaft holder tool to compress the spring washer. You don't need this special tool, you can get around this by measuring the thickness of the spring washer using a ball end attachment on a micrometer. In my case it was 0.0334", so subtracting that from 0.0380 gives us a final mainshaft play of 0.0046" which is just shy of the factory specified 0.006-0.008" clearance. This is just a way to confirm that our previous measurements are correct since the only additional work it requires is measuring the thickness of the spring washer, the measurements differ by 0.0004" which is essentially nothing in this case.

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5) Reassembly

Reassembly is basically the reverse of assembly. Once you've got all of your shafts rebuilt and clearances double checked, you're good to start putting it all back together. I'd recommend you start by cleaning the insides of both cases if you haven't already since they will be covered in a black oily film from decades of sediment. I wiped all the easily accessible gunk up with microfiber towels, then took them outside and finished with brake cleaner.

First up is the seals. The service manual wording and diagram do not match, it tells you to install the input shaft seal from inside the transmission, but the diagram shows it being driven in from the outside. I've always installed oil seals from the outside, so I flipped the clutch housing over and did the input shaft seal and the intermediate shaft seal at the same time. The diagram in the manual shows the input shaft seal being the same width as the transmission case, but it's actually thicker, looking at the clutch housing from the engine side, the seal should be perfectly flush with the face of the clutch housing, it will protrude a bit inside the transmission but this is normal. (Honda doesn't do this, but I like to put a very thin layer of Hondabond around the outside of oil seals before installation, more as a lubricant than anything, but it will also help the seal in case there are scratches on the inside of the seal bore or anything like that.) You always want to drive seals around the outer edge, I use a combination of washers, sockets, and PVC fittings depending on how big the seal is but I should probably just buy the harbor freight seal driver kit.


The driver side axle seal is a bit tricky since it has a dust lip thing sticking out very close to the edge, but if you take the outer race from the 2nd countershaft bearing (the races just slide apart since it's a needle bearing) it's a perfect fit to slip around the lip and push on the outer edge of the seal and drive it perfectly flush with the housing.



Install the shifter seal in the transmission housing next, this is pretty self explanatory, just install the seal then slip the shifter arm back in. You can use a small socket to drive it in with gentle taps from a hammer. After that, flip the clutch housing back over and install the shifter seal and shifter lever here as well.

Next install the countershaft bearing, I tapped it in with a hammer and a big socket. The factory screws on the bearing retainer plate have 8mm heads/M6x1.0 threads and are staked, I forgot to buy new bolts for that so I used some regular bolts of the same length with blue loctite. I would recommend you just replace the bolts and stake them properly, that said, there are a bunch of these normal 10mm bolts in the transmission and none of them are staked, so using non-staked bolts here isn't going to hurt anything. Use a torque wrench for everything, small bolts flying around inside the case will wreak havoc on the gears


After that, apply a bit of transmission fluid to the differential bearing races and drop the diff in, then install the oil pump gear.


Now it's time to do the shafts, oddly enough, getting them back in is easier than getting them out. Lay them out on a clean surface like this, then get some big zip ties and strap them together. You can try to wrangle them in by hand but the zip ties will save you a ton of time and frustration.



With them all strapped together, and the input shaft taped up, you can slowly lower the whole assembly into the clutch housing, once the input shaft and countershaft bearings start to seat you can wiggle the shift forks with your thumbs a bit to get them lined up with their respective holes. This can be done without the zip ties, but if you haven't done it before the zip ties help quite a bit IMO.


From here, go to the other side and install the reverse shaft and shift fork. Put the shaft and fork together, then drop the fork into it's hole while still with the shaft. At this point all you have to do is gently lift the mainshaft and countershaft, and rotate the reverse shaft assembly around the shift fork into it's hole. This is much easier with a helper since one person can lift the shafts and the other can seat the reverse shaft, but it's possible to do it alone by lifting the shafts then sticking out your finger and pushing the reverse shaft into place. Once reverse is in you can reinstall the shifter assembly


At this point you're pretty much done, all you have to do is seal up the case, however, sealing up the case is probably the worst part of this job.

You're going to want to practice putting the case on, there's a trick to it that can't really be explained you just have to figure out what works for you. The difficulty stems from the fact that you have to slip the transmission case shift lever into the fork on the shifter assembly as you drop the case down. Basically start with the lever rotating as far clockwise as it will go, then as you seat the transmission case you have to flip it into the fork on the shifter mechanism. I found it easiest to just hold it away from the mechanism (as far clockwise as it will go) until the case seats (it won't go down all the way until you spread the snap ring so you don't have to worry about pulling apart sealant), then lift the case back up a bit and flip the lever into position. I spent over an hour just practicing putting the case on so I could be sure I could get it on quickly and properly on the first try after applying sealant.

My transmission case differential bearing race had a tendency to fall out since it's a VERY tight slip fit so I put the bearing and washer on the differential then lowered the case down on top of it instead of putting the race in the case. If the bearing falls out while you're lowering the case down you'll have to pull the case off and start over, as well as you risk damaging the race.

Now that you're a transmission case installation pro, since you already scrubbed the transmission case flanges with a brass bristle brush they should be free of old sealant and other gunk but will probably be oily from reassembly, so go ahead and wipe them down with acetone on a clean rag several times. You want to be absolutely sure the flanges are free of grease, oil, grime, or any other contaminants. If you screw up here, you'll have to remove the transmission from the car to fix it so take your time and do it right the first time.

Once both flanges are perfectly clean and you've wiped them down an extra time or to just to make sure, apply Hondabond to the clutch housing as shown in the service manual and close it up. Since you've practiced, it should be a quick process to drop the case on, get the shifter assembly lined up, and pop the snap ring so the case seats fully. Before you torque it down make sure the shifter mechanisms are working correctly. You might not be able to select all of the gears since the shafts are vertical and applying pressure to some of the synchros, but you should make sure you can shift into one gear on each of the 3 positions of the clutch housing shift lever. Drop in the bolts (don't forget wiring harness the bracket on the bolt near the starter) and torque them to spec in the order specified in the service manual. I didn't get any pictures of this process since I was in a rush.


After torquing the bolts, go around one more time to make sure you got all of them and let it sit for an hour or two so the sealant can set up before you flip the transmission. Once you start turning the transmission around oil will start to pool at the bottom so you don't want to do that until the sealant has started to set up.

Once you've waited, you can flip the transmission over to seat the snap ring and once it's on its side you should be able to row through all the gears and make sure everything is working as it should. From there put the various sensors and spring ball bolts back in and you're done.
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Once again I admire your courage!
I must have opened the NSX gearbox at least 3 times: once to install the OS Giken diff, then again to add the helicoidal gear that drives the speedometer takeoff ( that doesn't come with the OS Giken diff) and the third time to revert to the OEM NA1 NSX-R diff.
No fun at all in my view!
Once again I admire your courage!
I must have opened the NSX gearbox at least 3 times: once to install the OS Giken diff, then again to add the helicoidal gear that drives the speedometer takeoff ( that doesn't come with the OS Giken diff) and the third time to revert to the OEM NA1 NSX-R diff.
No fun at all in my view!
curious as to why you went from the OS back to the R diff?
curious as to why you went from the OS back to the R diff?
In tight corners such as roundabouts, I hated the sound of crushed marbles coming from the OS Giken diff.
Beyond that with my turbo setup I didn't need the shorter end drive ratio.
Of course on track in the hairpins the NSX-R diff will allow the inside wheel to spin if I apply too much power...
MotorMouth93 said:
Anyways I'll post more as I finish it up. I encountered an unexpected delay in that I need to replace one of the mainshaft thrust shims with one 0.10mm thinner.

Interested to see how you are measuring the mainshaft thrust clearance. [MENTION=12356]Mac Attack[/MENTION] mentioned that the service manual measurement method will not yield accurate results. I'm holding off on doing the mainshaft bearing because I don't want to mess with this.
I dabbled with the service manual method a bit before reaching the same conclusion as Mac Attack. I ended up installing the mainshaft in the transmission without the spring washer but with the two factory shims and measured the play using a dial indicator, then used a micrometer to measure the shims to figure out what the best one to order would be. Unfortunately, there's no good way to get around needing a dial indicator and mic for this, but you don't need super high end tools for the level of precision needed here so cheap stuff from Amazon should do just fine. I'll post pictures when I get around to adding more. Movement should be 0.039"-0.042" according to the service manual and I doubt you could get accuracy better than maybe 0.01" using the FSM method.

If you're swapping the mainshaft to do the JDM gears then clearances should probably be checked.
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Thanks for taking the time to clearly document this process.

I figure you'll touch on this again later anyways, but, final mainshaft clearance with everything installed should be 0.0055 to 0.0083 inches for a '91 5-speed per page 13-42 of the SM. Not a whole lot of range there. I figure with my method and reassembly process that I have in reality a tad over 0.0045 in. clearance. So, a bit less clearance than allowed by spec but I wanted to get it back on the road within a certain window and figured how much could 0.001" make?

Well, it's probably just my anal compulsive inner self, but I'm pretty sure I have a "tad" more gear noise after the rebuild than the previous 70k miles I've driven in it. Again, it's probably my imagination because the transmission shifts fine and has taken a good 400 ft-lbs of torque, but I probably should have tried to get the final clearance somewhere mid-range.

Having said that, I do not have plans to ever touch the transmission (or engine) again. If either one goes then I'll be swapping in a Tesla drive unit!

Here's my thread for reference:

I recommend you go ahead and get a clutch shift fork or throwout bearing collar if yours are worn!
I recall from reading [MENTION=10201]goldNSX[/MENTION] 's transmission thread that Kaz recommended replacing the 5th gear shift fork with the "later spec" version, even if the old one is within wear limits. [MENTION=25737]Kaz-kzukNA1[/MENTION] can you confirm the part # for the later spec fork is 24201-PR8-000?
The revised version of the 5th gear shift fork has thicker 'arms' (double). Not sure if you still supply the old version as it doesn't make sense. Kaz mentioned broken shift forks in some cases in Japan. Pretty sure they threw them away and completely replaced them by the stronger version.
I didn't check the part no. now but I've ordered the no. listed in my thread and got the later version. Hope this helps.

Setting the mainshaft clearance correctly is actually not rocket-science, it's just time-consuming but I understand if someone wants to skip it. But if you have the time I'd not skip it. Most of the clearance is set at the birth of the gearbox itself. If you change parts with Honda OEM parts there shouldn't be much variation. From that point of view, re-using the old shim will not kill the gearbox. On the other hand the tolerance is set quite tight to 0.14-0.21 mm with shims with steps of 0.05 mm.
Added photos to the bottom of the original post of the mainshaft thrust shim measurement process.

I'm not super worried about my 5th gear fork, the 5th gear synchronizer is tiny and weak as it is so if you're shifting into 5th hard enough to break a fork you're doing it wrong. Shift fast into 1-4, but slow it down for 5th if you don't want it to start grinding on you.

The worst part about the mainshaft clearance is that you don't know what shim you'll need until you take it apart. The measurements themselves aren't bad at all if you have the tools. I'd say most people are probably fine to just leave it as is because changing parts out shouldn't push you out of range by more than 0.001" or so but if you're a perfectionist, check.
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I recall from reading @goldNSX 's transmission thread that Kaz recommended replacing the 5th gear shift fork with the "later spec" version, even if the old one is within wear limits. @Kaz-kzukNA1 can you confirm the part # for the later spec fork is 24201-PR8-000?
Hi, Honcho.
If you order the fork using that parts no., you'll receive the latest design with much thick/stronger fork despite the parts no. stayed the same from the original design.
You may already have the latest design so just compare it with the photo on several web sites.
I think someone posted in nsxcb forum as well.

I'm not super worried about my 5th gear fork, the 5th gear synchronizer is tiny and weak as it is so if you're shifting into 5th hard enough to break a fork you're doing it wrong. Shift fast into 1-4, but slow it down for 5th if you don't want it to start grinding on you.
There are more parts involved while shifting into 5th. Since ever I had my NSX the 5th gear needs a little bit more force (+50%) to engage when the gearbox is cold. After 3 miles and a slighly warmer Honda MTF oil it shifts into 5th like into any other gear. Even after my extensive rebuild with more new parts than needed esp. around the 5th gear it's that way. So I believe there IS an obvious reason why some people in Japan faced broken forks and why Honda modified the part without chaging the part no. itself.

First pic is the old one. The new one is double the size in each dimension.

You might be right about the small synchro but fact is the more parts are involved while shifting into 5th. More parts - more resistance. I'm not talking about the fork - synchro but about the shift lever (outside of the gearbox), two shafts and the collar (3/4) which has a special function.
Oh definitely I don't mean to disagree with you. Some transmission designs just lend themselves to certain quirks like being a bit more difficult to shift into some gears when cold.

With how small the synchro on 5th is and the not uncommon reports of 5th gear grinds as well as excessive wear on 5th gear components noted by people here I think it's wise to be be careful on the 4-5 shift though, especially with the shorter JDM gear ratios where the synchro has to work even harder.

Also, I've found that the mainshaft thrust shims (as well as a few other O-rings and such) can be cross referenced with modern Honda transmissions such as a 2017 Accord 6MT, having a local Honda dealership order those parts tends to be a lot quicker than waiting for an online retailer to order it from Honda then ship it to you. Even with the holidays I've gotten a few obscure parts in less than a week that way, and the Honda dealer near me doesn't charge shipping on small parts either.
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My 5th gear hub/sleeve and new mainshaft shim showed up from Japan today, so I checked the mainshaft clearance again (this time using a much more appropriate 1" travel 0.001" dial indicator that my wonderful girlfriend gave me for Christmas) and found that it was perfect at 0.040" without the spring washer, or 0.007" with the spring washer compressed. I checked again for good measure using my 0.05" travel 0.0001" Mitutoyo indicator just to be sure and it was correct, then I was able to get everything put back together except for sealing up the case because I was feeling lazy. Anyways, the original posts have been updated again.

At this point the DIY part of this post is pretty much complete, I'll probably add another photo of the sealing process but that's self explanatory. I'm thinking I'll make a PDF version in case my personal web hosting ever goes offline.

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Resurrecting this thread since I'm planning on using it as a guide in the future. Some questions that I'm hoping can be answered..

-Is the countershaft lock nut 40mm or 32mm? Every parts site I look at for 90201-PR8-000 says it's a 32mm nut.
-Is there a flat on the countershaft to clamp with the vice to torque the new nut on? With the hollow shaft it seems iffy to clamp on the thinner parts.
-The factory SM says the torque sequence for the countershaft nut is 116->0->116 lb-ft, so it's meant to be torqued fully, taken off, then torqued again and staked? Didn't see that mentioned here.
-For your improvised "diff inspection tool", is it just some 10"+ all-thread that's double-nutted with washers on either side to clamp against the inner diff shaft inside of the tapered bearings? I assume the washers are doing the clamping since the diff splines aren't engaged to anything.

-Reading through this and some other threads, I'm a bit unclear on the mainshaft thrust clearance procedure. This is what I gleaned for the procedure without the mainshaft holder tool:
1) Fully assemble mainshaft with both ball bearings, oil guide plate & original thrust shim(s) into housing (assuming it is the proper thickness, can be changed after first clearance measurement)
(OPTIONAL: leave thrust shim out, then subtract shim thickness from final clearance in last step to get a more accurate total clearance reading)
2) measure spring washer with micrometer, leave OUT of case
3) install mainshaft into housing, torque the case halves together
4) room temp, mainshaft oriented vertically, dial gauge on the mainshaft, zero, and pull shaft in & out to get a clearance reading (this clearance - spring washer thickness = 0.14-0.21mm according to FSM)
5) if (measured clearance - spring washer thickness) is NOT = 0.14-0.21mm, take mainshaft out again and upsize/downsize the thrust shim as needed
(i.e., if clearance without spring washer is 1.08mm, spring washer measures 0.82mm, final clearance is 0.26mm, so upsize thrust shim one size (+0.05mm) which would give ((1.08mm-0.05mm) clearance - 0.82mm spring = 0.21mm final clearance, on the upper edge of 0.14-0.21mm? Could go up another +0.05 thrust shim to land at 0.16mm)

Just looking for a sanity check of the above. I'm not planning on getting the mainshaft holder tool, and I read you can't measure the clearance with the spring washer installed without it since the washer needs to be compressed properly.

Thanks in advance.
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