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5-Speed Gears & Mainshaft Replacement

8 February 2019
Atlanta, GA
Greetings everyone,

I'm in the middle of my 5-speed transmission rebuild. It already has the JDM shorter gears for 2-4 installed, and I'm putting in the NSX-R 4.23:1 countershaft, ring gear, and oil pump drive gear as well.

I've completely disassembled both main & countershafts for inspection, and some of the gear dog/engagement teeth are pretty worn, plus the JDM mainshaft has some discoloration (burn marks?) where the input shaft bearing is press-fit on and where the pilot bearing sits.

I'm looking for some opinions about whether or not some of these parts can be reused without causing issues. I'm already replacing all synchros/blocking rings, bearings, seals, etc. but judging by the obvious wear on all of the selector sleeves/hubs I will be replacing those as well. At some point, this won't even be the same transmission save for the cases...

Here's the mainshaft. Top arrow is the input shaft bearing surface, bottom is pilot bearing. I measured the input shaft bearing surface diameter according to the service manual and it seems to be above the service limit from 95% of the micrometer measurements I took. Not sure if I can reuse this. I didn't feel any grooves or pitting on the bearing surfaces, seemingly just discoloration.

The 1-2 selector sleeve. I don't remember if this was the 1st or 2nd teeth but both sides are pretty rounded, especially 2nd since it presumably got beaten on more. I'm already planning on ordering the 5th, 3-4, and 1-2 sleeves & hubs due to similar wear, I'd say 5th is a bit better than this but not great.

2nd gear on the countershaft. The main gear teeth all look good, not much visible wear on the gear's synchro cone (covered by the synchro in the pic), but the dog teeth aren't looking the best. This is part of the JDM gearset but this one seems to be available.

This is the mainshaft fourth. Most of the teeth still have somewhat pronounced points, but not the best looking. The problem is that this one is on a very long backorder and I probably won't be able to get one before the end of the year....is it possible to have a machine shop put on new dog teeth or am I stuck reusing this? I'm hoping with a new selector sleeve and synchro that I don't have any issues with this gear, and the previous owner never said they had problems with it before.

Judging by the wear on the mainshaft 5th gear, 3rd gear, and countershaft 2nd and the fact they're available, I will likely be replacing those. Pretty much every gear with engagement teeth except for 4th and 1st.

Since the mainshaft 4th gear is effectively discontinued at this point, I don't think it will be feasible to replace, though it would probably be best-practice to do so.

Still not sure about the mainshaft. If I do end up replacing it, I might as well have bought the entire JDM gearset all over again..

Ultimately I'm dropping another $1000+ I didn't plan for so that this gearbox lasts for as long as possible, but would like to hear opinions on the 4th gear and the mainshaft to see if I can salvage something. Thanks in advance.
Discoloration: Press a new bearing on and it's ok. No worry about the flywheel bearing either. Your mainshaft is fully ok.

How many miles on the car? Whoever drove the car before really 'used' it from what we can see. :)

4th gear: I guess you mean it's backordered in Japan (Amayama, isicar). Even though the angle of perspective is not ideal it's not completely worn down (from what I see). But if you could get a better one... You might consider looking for a used one. If you reuse the old one you could try to flatten out the harsh irregularties a little bit, so the sleeve can slide over 4th with less resistance (avoid deadlock).
Thanks gold! I happen to have been looking through your gearbox thread back on NSXCB for some extra reference so thanks for that too.

Glad to hear for the mainshaft. I bought the trans used, the car it came out of had ~80k miles but the trans was a snap ring replacement unit that had probably been taken apart before for the JDM gear swap so no telling exactly. It was used by Cody at Lovefab for much of that time so maybe that has something to do with the wear and tear :rolleyes:? I'm having to replace a lot more "non-consumable" parts than I expected.

Yes, the JDM 4th gear doesn't seem to be available from the normal sources, people say they've waited up to a year to get one so I could only buy a new one as part of a full set. I will post some WTB's for one but I don't expect much luck. Here's some better screenshots of a vid I took during disassembly. From this perspective I think it looks more serviceable.

Maybe I could gently file down any small abnormal protrusions but I am very wary of making something worse. Might go real slow with some 6-800 grit on an eraser head for the high spots on the dog teeth so that the sleeve has less of a chance of snagging. I have a dremel with a small sanding wheel but it might not be gentle enough for me to avoid mistakes.


From your new pics I think I'd go with the old 4th gear. It doesn't look that bad. Of course, I'd favor a new one too but regarding the delay...no way.

As you now can see how it works I'd remove the biggest 'obstacles' or hickups but as you say very conservatively. I don't see many of them in your first picture but the one at about 320 degrees is catching my eye. But still...

What's that green fluid? Gearbox oil or cleaning fluid?
Yeah, I think the majority of the engagement teeth are okay besides some of the tips, so I think the replacement sleeve and synchro will help with good engagement even if the tips aren't as sharp as new. I would assume in the worst-case (not necessarily what will happen here) that the slider might have a bit more trouble slotting onto the gear while ramping off the blocking ring which might increase shifting effort a bit.

The ones you're referring to at 320 degrees looks to be the worst on that gear. I might try some techniques on the old gears I remove to see what the best way of removing protruding metal is. I say that instead of "sharpening" the engagement teeth since I want to modify as little base material as possible to avoid weakening or misshaping the teeth further.

The fluid is Redline Shockproof Lightweight oil which is an off-green. Apparently good for aging & racing transmissions but I will be going back to Redline MTL after the rebuild.
Try a good nail file (ask your wife/gf :)) and maybe try on one of the gears you intend to throw out anyway first before using a dremel or the like.
IMO the brass syncro's should be replaced, fairly difficult to determine wear visually.

I agree that the mainshaft is good. The pilot bearing shoulder should be checked for pitting and diameter: as long as it fits snugly in the pilot bearing you are good to go. A sloppy main shaft fit into the pilot bearing can destroy the whole transmission.

I have only rebuilt a few manual transmissions so I tend to err on the side of longer service life and spending my way out of knowledge or ability deficits.
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I have all the synchros already, they will be replaced.

I will look at the mainshaft pilot bearing surface a bit closer to check for damage, but what are the specs for diameter? The FSM has no specs for that part of the shaft. I can add a new pilot bearing to my parts order I'm about to submit since I was planning on getting one when I get my complete clutch kit.
I'd buy a new flywheel bearing and check if it slides on the mainshaft 'easily'. Means: it should slide on the mainshaft end with no resistance but only in the correct angle.
Not sure but it looks like dirt or minor corrosion (not of the mainshaft itself but transferred material from the inner race of the flywheel bearing. I'd try to remove it if it's excess material if it helps in the slide test.
If you already have the clutch alignment tool you have a benchmark for the diameter.
I personally prefer a tiny 'shade' of greese on there too.
That bearing is the reason why it's quite hard to finally attach the gearbox to the engine. Make sure that the parts are perfectly aligned. Brute force is not the answer, it's more about wiggling and moderate force to find the right angle.