• Protip: Profile posts are public! Use Conversations to message other members privately. Everyone can see the content of a profile post.

Replace just synchros or buy SOS tranny rebuild kit?

Joined
18 October 2015
Messages
1,197
Location
Cleveland area, OH
I have had a minor grind with very fast upshift to 5th gear since I took possession of my '99 NSX over two years ago. It just turned 72k miles this week. It's nothing major but I am now considering improving the transmission as a winter project. I switched to GM Synchromesh before NSXPO in early September, which is helping considerably. (This thread is not meant to be a debate on whether Synchromesh damages NSX trannies, thanks!)

In searching Prime, I have found this to be an occasional issue with the consensus being that this is an issue with the 5th gear synchro, and the solution is to replace the 5th gear synchro.

However, it seems to me that one doesn't know what's really going on in the transmission until it's opened up. In that circumstance, what if you've purchased a new synchro and the problem isn't that?

So that brings me to my question. Is it better to just do a transmission rebuild using, say, the SOS 6 MT transmission rebuild kit ($2250 for my 1999), to replace just the 5th gear synchro, or to replace all the synchros? I can get the 6MT NA2 synchros for $150 from The NSX Shop. I don't want to spend more than is necessary, but if it's better to just replace the entire tranny with new parts, I may consider it. If I'm spending $1500 for labor in opening and closing the transmission, it may be worth doing it right.

Opinions?
 
Last edited:
Wow, that's a lot of money to address one synchro.

If the GM fluid helped, maybe you could do another transmission fluid flush to nurse it along at least until you need another clutch? Fresh fluid always helps, and who knows how bad your old fluid was before you changed it in September? Unless you completely disassemble the transmission or do a real fluid flush, old residual fluid was left in there with the GM stuff you added. If you did the FM cocktail blend, what was your ratio? Maybe a fresh fluid fill with a modified ratio could help even more, and it's relatively cheap compared to what your true fix would be.

Anyways, I would call the real experts like LarryB and Nick at Applied Motorsports to get their recommendation. Once it's opened you can measure the synchros for wear and replace as necessary. 72k miles really isn't that many. Depends how it was used by previous owners, but I would still think it's in good shape overall.

Also, your estimates are low. The $2300 is just for parts to do a complete rebuild. Figure an expert will charge another $1k in labor just to work on the transmission once it is removed from the car (that's basically what SOS charges, neglecting shipping costs if you were to send it to them). Then, another 10 hours to remove and replace the transmission. Probably $4500 when it's all finished.

Just sayin'... If the GM fluid helped a lot, I would try another $75 drain/fill with a potentially-revised cocktail, and try to nurse it along until you need a clutch replacement.
 
Open it up and see what it needs. No need spending $2300 in parts if you only need a couple of syncros. If you can like Mac said do it when you do the clutch.
 
Thanks for your opinions! Much appreciated.

I’ll almost certainly first take the suggestion to repeat the transmission flush with more Synchromesh. It’s improved the shifting of my car significantly already. Not to mention, as stated above, I don’t want to spend more than necessary. If I can nurse the situation along for now by flushing the tranny, I’m happy to spend not much money doing that. :)

The labor guesstimate starting at $1.5k I quoted came from Source 1, my semi-local NSX specialist.

Clutch was replaced by the prior owner at 41k miles and in my estimation is a long way from needing done given I don’t abuse it.

Thank you again.
 
Open it up and see what is needed. I would anyway do the bearings and the gaskets. Considering the labor price compared to the synchros price, I would do at least the 2nd and 3rd gears which are the most molested if not all of them. Inspect the gears and sleeves.
 
I'd say leave it as is.

You didn't describe under what circumstances it grinds.
- while the gearbox is cold?
- while shifting as fast as you can?
- does it also grind when you shift slowly?
- while shifting from 4th to 5th or from 6th to 5th?
- while shifting from ANY gear to 5th?

If you find a way to NOT open the box (oil or shifting skills) it's a little bit overkill to open a gearbox to replace just one synchro just because it grinds slightly under certain circumstances that can be avoided.
 
[MENTION=10201]goldNSX[/MENTION] , The grind only occurs if I am upshifting, fast, from 4th to 5th, occurring once the shifter makes it into the 5th gear gate. My response has been to shift just a tick slower into 5th, so it is intermittent, perhaps one shift in 100 or less. It doesn’t matter whether the transmission is warm or cold. The GM Synchromesh has greatly improved this behavior.

No grinding with slow shifting. I don’t skip-upshift, so I don’t know whether shift from any gear other than 4th, to 5th, is involved.

I’ll be changing the transmission fluid again at the end of the winter, just before I start driving regularly, to see whether I get further improvement.

Again, thank you!
 
Last edited:
As long as it is in an early state I'd try an oil change as Mac Attack suggested. As long as you can live with it I'd not open the gearbox. To reduce further damage I'd shift the gears very slowly until the oil is warmed up (5 miles driving). Synchros don't like operating with cold oil in general.
It's good that you don't skip gears, synchros are not designed for that in the long run. The national speed limit is responsible for the synchro wear out in the higher gears if you'd like to interpret it this way as some people WOT in 2nd and shift directly into the highest gears as soon as the speed limit is reached. It's good you didn't/don't/won't in the future but maybe the former owner of your car did and that's what contributed to the problem. Ruined synchros due to skipping gears are well known in the S2000 world.
Your 5th gear synchro now has a reduced capability and a special oil can help you with that but it won't cure it. Maybe you can go several tenthousend miles with. But weak is weak. The more it starts to grind and the higher the revs are when it does the more the material on hub/sleeve and the gear itself is eaten away (rounded off). IF you have to open the gearbox for whatever reason you most likely have to change these parts too IF they are still available by then...
As soon as it grinds more frequently even when shifted slowly it's time to open the gearbox and give it a full overhaul. But right now I'd leave it as is.

You're in good hands because all of the three people who have responded to your thread have opened a gearbox within 2017 and we're not educated professionals but maybe with more than average skills. :)

Merry Christmas!
 
I’ll be changing the transmission fluid again at the end of the winter, just before I start driving regularly, to see whether I get further improvement.

Again, thank you!

Do you remember what you put in it last time? Was it all GM Synchromesh, or did you add one or two quarts of the Friction Modified? If you just used 1 qt of FM in the past, maybe try 2 qts of it this time?
 
I'm not looking to derail this thread, but do we know the long term effects of GM Syncromesh / Friction Modified cocktails on the NSX differential?

Thanks,
Brian
 
For $600 I'd bite. I've been wondering why they charge so much on their website for just the parts... unless they are sourcing OEM parts only.

This has been on my mind as well but as I've stated in previous threads start with the cheapest solution first and changing fluids is probably the cheapest one. For me, my clutch change fixed many of my shifting issues, but then again my mechanic used Honda MTF. If you can be patient, I would wait until the clutch change before taking any major action such as a rebuild. If not, follow the advice of all the experts who pretty much just say "find the right transmission fluid cocktail."

If you have an extra $4000 to throw around you can always go to a rebuild. But that's a last resort option which is overkill for a 5th gear shifting issue that isn't debilitating your car.
 
Edit: After SOS' response, I added in the sleeve and hub kits so the total is bumped to $900.

The price I listed was for original parts, Amayama is a distributor for JDM Honda parts (and a bunch of other Japanese makes) based in Japan and shipping to the US is very reasonable. SOS puts an insane markup on OEM parts because plenty of people aren't willing to dig through OEM parts catalogs. Here's the list:

Note that this is for 1994 5-speed manual transmissions, it is for reference, do your own research or just buy the SOS kit.

Seals

91206-PR8-005 Oil Seal (40X68X9) Bottom Axle Seal
91207-PR8-005 Oil Seal (42X63X9) Top Axle Seal
91216-PR8-005 Oil Seal (32X46X7) Input Shaft Seal
91215-PR8-005 Oil Seal (17.5X27.5X7) Shifter Seal
91215-PR8-005 Oil Seal (17.5X27.5X7) Shifter Seal

Differential Bearings

91121-PR8-008 Bearing, Taper (55X90X23) (Ntn) Differential Bottom Bearing
91122-PR8-008 Bearing, Special Taper (45X75X24) Differential Top Bearing

Shaft Bearings

91103-PR8-018 Bearing, Needle (44X72X23) Countershaft Bottom Bearing
91003-PR8-008 Bearing, Special Ball (34X68X20) Countershaft Top Bearing A
91102-PR8-018 Bearing, Needle (34X68.2X22) Countershaft Top Bearing B
91002-PR8-008 Bearing, Ball (63/32) Mainshaft Bottom Bearing
91004-PR8-008 Bearing, Angular (31X82X22) Mainshaft Top Bearing

Gear Bearings

91108-PR8-008 Bearing, Needle (52X57X33) 1st Gear Idle Bearing
91105-PR8-008 Bearing, Needle (54.5X60.5X33) 2nd Gear Idle Bearing
91104-PR8-008 Bearing, Needle (43X49X29.5) 3rd Gear Idle Bearing
91104-PR8-008 Bearing, Needle (43X49X29.5) 4th Gear Idle Bearing
91107-PR8-008 Bearing, Needle (42X47X29) 5th Gear Idle Bearing

Sleeves and Hubs

23619-PR8-010 Sleeve Set, Hub (1-2) 1-2 Sleeve/Hub
23621-PR8-010 Sleeve Set, Hub (3-4) 3-4 Sleeve/Hub
23624-PR8-020 Sleeve Set, Fifth Hubs 5 Sleeve/Hub

Synchronizers

23641-PR8-010 Ring, Blocking (78MM) 1st Gear Synchro
23651-PR8-000 Spring, Synchronizer (78) 1st Gear Synchro Spring
23646-PR8-020 Synchronizer Set (79) 2nd Gear Synchro
23651-PR8-000 Spring, Synchronizer (78) 2nd Gear Synchro Spring
23641-PR8-911 Ring, Blocking (66SS) 3rd Gear Synchro
23651-PG1-000 Spring, Synchronizer (66) 3rd Gear Synchro Spring
23641-PR8-911 Ring, Blocking (66SS) 4th Gear Synchro
23651-PG1-000 Spring, Synchronizer (66) 4th Gear Synchro Spring
23642-PG1-912 Ring, Blocking (55S) 5th Gear Synchro
23652-PG1-000 Spring, Synchronizer (55MM) 5th Gear Synchro Spring
23642-PR8-010 Ring, Blocking (44MM) Reverse Gear Synchro (Probably unnecessary)
23651-PB6-960 Spring, Synchronizer (44MM) Reverse Gear Synchro Spring (Probably unnecessary)

Other

90602-PR8-000 Ring, Snap (68MM) Countershaft Snap Ring
90201-PR8-000 Nut, L Hex (32MM) Countershaft Nut
91309-PX4-003 O-Ring (16X2.1) (Nok) VSS O-ring
91307-PR8-005 O-Ring (29.7X2.4) Oil Pump O-ring
94109-14000 Washer, Drain Plug (14MM) Drain Plug Crush Washer
94109-20000 Washer, Drain Plug (20MM) Fill Plug Crush Washer
90471-580-000 Gasket (8MM) Seals bolt on top of trans, no idea what it does.

transmission_parts_list.jpg
 
Last edited:
nice leg work...
 
SOS puts an insane markup on OEM parts because plenty of people aren't willing to dig through OEM parts catalogs.

You are not including a number of costy components which we include in our kit inluding the syncro / sleeve kits, and bearings which in our experience, we deem necessary.
 
The price I listed was for original parts, Amayama is a distributor for JDM Honda parts (and a bunch of other Japanese makes) based in Japan and shipping to the US is very reasonable. SOS puts an insane markup on OEM parts because plenty of people aren't willing to dig through OEM parts catalogs.
Not sure why you are beating SoS. I'm pretty sure their pricing reflects the general US pricing for Honda parts and that's where they get the parts from. So if you really want to create a headline then it should be why the US pricing is different from the one in Japan. One thing is that the US pricing rose quite a lot after the financial crisis. So we see maybe some inflation. On the other hand the YEN is a very strong currency while the $ sucks.
Customer service in the US is very good. Not sure if you can return parts if you've ordered the wrong one as long it was not mounted. Japan: absolutely forget it. Amayama doesn't accept returns. The other thing is that I'll better spend some more money to get a full kit with everything included and the right parts. Some people don't have the time to spend hours in the parts catalog, they have high opportunity costs of their time, maybe you don't have if you do that for fun. Believe me, I'm also ordering in Japan and am pretty versatile doing so but I also did two mistakes in the past and ordered the wrong non-returnable part. Period.

Amayama in particular: they are good but their ordering system is some kind of complicated compared to a US dealer.

I didn't went into details of your list but you have to add shipping and depending on the state also taxes.

BTW some parts are more expensive in Japan like brake rotors for example.
 
Last edited:
I'm not beating on SOS, they perform a valuable service for our cars that we don't get anywhere else, and I've ordered from them in the past and will do so again in the future since they ship fast and answer questions faster. I'm just pointing out that, for people such as myself who like to get their hands dirty and tear into the guts of their car themselves as a hobby (and are willing to accept the consequences of doing so, expensive as they may be) can save a good chunk of cash by shopping elsewhere for some things. If you'd rather just buy a kit and not worry about it then by all means, buy the SOS kit and the excellent support that comes with it.

As for the markup, it's still a large markup even compared to US market prices. You're paying for the support and ease of access.
 
I'm just pointing out that for people...........can save a good chunk of cash by shopping elsewhere for some things.
We came to the conclusion that the OP (neuronbob) should try it with an oil mixture before throwing parts at the car so he saves even more...:)

If you really do a revision of the internals buy whereever you want. :)
 
Last edited:
Do you remember what you put in it last time? Was it all GM Synchromesh, or did you add one or two quarts of the Friction Modified? If you just used 1 qt of FM in the past, maybe try 2 qts of it this time?

Back on track....I believe the dealer used the Synchromesh with FM, but I am not sure of the ratio and the service manager wasn't sure when I asked about it. This is the reason I wanted to do another fluid change, this time using a quart of the FM. I will probably do this myself. This is so I can be sure of what's going into the car rather than depending on what the service manager says.

Thanks for the listing of parts. Also, thanks, Chris, for the explanation of what goes into the SOS kit. My hope is that I can delay a parts change altogether now that I have a solid plan for dealing with the consumable fluids.
 
I just looked in our parts catalog and the diff's are the same. The clutch plates, disks and case are the same the only difference from 91-94 and 95-05 is the final driversion gear. The 95-05 all have the same final drive gear.

Sorry they changed the planetary gear in 95-05MT but they never changed in in the automatic it is the same as the 91-94 MT
Hope that solves the great mystery of the difference'so in the diff's.
 
Updating my thread a year later.

I was never 100% sure which GM Syncromesh my Acura dealer used 15 months ago. I therefore bought my own fluid (the recommended 2:1 GMSM:GMSM-FM) and had the dealer redo a full flush and fluid replacement this summer, in prep for NSXPO.

The shifting is tremendously improved overall, the 5th gear grind is not gone, but less, and my occasional stuck gear from 3rd to 2nd is essentially gone. I also baby the shifter for the first couple miles of travel. I am pleased with the result. I’d like to avoid work on the transmission unless necessary, and the fluid change appears to have been successful.

Thank you again for your advice. I have other uses for $4k.
 
Updating my thread a year later.

I was never 100% sure which GM Syncromesh my Acura dealer used 15 months ago. I therefore bought my own fluid (the recommended 2:1 GMSM:GMSM-FM) and had the dealer redo a full flush and fluid replacement this summer, in prep for NSXPO.

The shifting is tremendously improved overall, the 5th gear grind is not gone, but less, and my occasional stuck gear from 3rd to 2nd is essentially gone. I also baby the shifter for the first couple miles of travel. I am pleased with the result. I’d like to avoid work on the transmission unless necessary, and the fluid change appears to have been successful.

Thank you again for your advice. I have other uses for $4k.

Another update a year later? Still enjoying the new fluid in the transmission?
 
Another update a year later? Still enjoying the new fluid in the transmission?

Lots of change in the last year!

The clutch deteriorated over the last year and since I was planning to go with a SOS TT kit, I purchased a new SOS Sport 350 clutch. Shift fork was apparently bent, which is what was causing the stuck gears. I now note no jamming on shifts, and try as I might, no 5th gear grind, either. I suspect the latter is a function of not needing to get to 5th during acceleration anymore after TT kit install.

I have a very long writeup on the long trip it took to get here, in the Forced Induction part of the forum.

The shifting is the best it’s ever been since I bought this car almost five years ago, and the pedal effort from the Sport 350 is not at all onerous.
 
Back
Top