My gearbox revision thread

[goldnsx]

The input shaft of my gearbox (totally 190k km but revised with short gears/4.23 at 80k km, so untouched for the last 110k km) makes some noise at idle with the clutch pedal up and the 5th gear was making some kind of the sound of marbles at 1500 rpm cruising around.

First taking the gearbox out. Special thanks to Sylvian for borrowing me his lift.



Opening the box, at first sight:
- snap-ring ok
- countershaft bearing (the one at the snap-ring) ok

Not sure what to think about the gears. There are spots and even rust ??? on them? I didn't lend my car to 007 for diving in the sea...



5th gear: mysterious signs of wear. I've to admit that it popped out on the German Autobahn 2-3 times but that was back in 2008 or so. Something is wrong here. Of course, I use the 5th gear a lot (a lot of cruising). Opinions?



I already have new main bearings and all synchros for sure to go in. Will do a lot of measurements by the SM tomorrow.

Expert advice warmly welcome, thanks in advance!

[goldnsx]

Reverse gear synchro teeths. While the synchro ring looks good the reverse drive gear looks strange. The sleeve looked ok. I've compared it to another gearbox we did in the past and it looked about the same.

[Kaz-kzukNA1]

Hi, goldnsx.

As you went for the final ratio change, you were forced to replace the countershaft, final driven and oil pump. I presume you replaced the countershaft bearing at that time.

With the short gear ratio, you were forced to replace the mainshaft (main 1st, reverse, 2nd directly machined on it), counter 2nd, main/counter 3rd & 4th gears.

I don’t know the base gbox no. of your NSX but looks like there is no washer between the main 2nd/3rd gears so I believe you got the later spec mainshaft which is good.

I'm afraid that main 5th gear needs replacing.
As it looks like you are replacing all of the synchro related parts, not much to say but that damage is the typical sign of driver accelerating to the limit in 4th gear and then quickly shifting up to 5th.
As you opened the gbox, you will notice that compared to the 3rd and 4th synchros (both using the same capacity), 5th one is much smaller for some reason and that is the main reason for the damage.
Addition to this, the inertia of the CL will play the big part against the synchro capacity but I'll leave it for now.
Just a matter of interest, what CL were you using when you first noticed the 5th gear issue?

I can't tell the spec of your 5th shift fork.
Could you take the photo of it to show the shape of the craw/fork?
Honda did change the shape/spec of this fork as there were some reports of broken 5th fork in the past.
Since you opened the gbox, best to replace it if you are using the old spec one.

I believe you selected the proper shim thickness for the new mainshaft but may want to double check it.

What MTF were you using?

Personally, I don’t like the Honda own synchro design without the synchro key but no options…..


Kaz

[goldnsx]

Kaz, thanks for your reply.

Back in 2005, my tranny (1003042) of my 1991 was built with new bearings, FD4.23, new mainshaft, new countershaft and short gears 2-4 (1st and 5th not changed), new synchros and main bearings and new snapring. I always had the OEM twin-disc clutch kit, being the first one, so will replace it now too. Always filled with MTF III, changed every 20k km.

I only have a bad picture of the shift forks but they never have been changed so far. EDIT: I've measured them and they are about 0.05mm thinner than the standard. The clearance between the shift fork and it's corresponding sleeve is still within specs (around 0.60 mm). Service limit would be 1.0 mm, so I think they're good to be used again.

Thanks for the hint for the 5th fork failures. Is the newer design interchangeable with the old one?

I just got home and am still upset (and depressed too) because something is wrong with the countershaft I've been working on today. I'll have no sleep tonight...


I've changed the 1st/2nd synchro/hub/sleeve as a complete set. Before, I've measured the clearance between the 2nd and the 3rd gear (according to 13-30) and found no play at all but the 2nd gear was turning. The standard clearance is 0.04-0.10 mm. I've pressed the 3rd gear onto the shaft and now the 2nd gear is NOT turning, neither so the synchro ring (this are expensive new parts!). The synchro also looks too far of the gear. Now, it looks like the 2nd gear synchro is pressed and it that's the case it's damaged I think. If I'm lucky the spacer collar is just a tiny bit too small.
In the first two pics is the 1st/2nd gear hub/sleeve. One side is 6.5 mm deep and the other is 7.5 mm. I've installed it the way the old one came out and backed up the the installation with the pics I've taken before. The tiny groove on the outside (pic 3) is towards the 1st gear. The SM doesn't mention anything about that.

I'll need some motivation before I go on working on it...

Thanks for listening.

[ozon02]

Hi,

Honda 5th Fork change



Left Old
Right New

[britlude]

Hi,

Honda 5th Fork change:

Attachment 12780

Left Old
Right New

bad pic link.....

[goldnsx]

I'm afraid that main 5th gear needs replacing.
As it looks like you are replacing all of the synchro related parts, not much to say but that damage is the typical sign of driver accelerating to the limit in 4th gear and then quickly shifting up to 5th.
As you opened the gbox, you will notice that compared to the 3rd and 4th synchros (both using the same capacity), 5th one is much smaller for some reason and that is the main reason for the damage.
...
Kaz

I've found a different possible explanation for the wear on the 5th gear: long gearing, WOT in 2nd and speed limit. I was on the German Autobahn only a few times. But what I've done in the early years on normal roads quite often is shifting from 2nd at redline to 5th directly. I did this with a small delay but who knows the speed of the shafts in the gearbox by that time? Anyway, the 5th gear has to do all the synchro work, and being the smallest it wears out. Since several years I don't skip gears anymore as I've heard of the problems people/Honda had with the 6speed gearbox in the S2000.