9 years since I've posted here...

I've been wracking my brain on this. I've searched and read through every post I could find relating to the Main Shaft on Honda-Tech and every other Honda forum and I can't find anyone with a similar issue. I'm going to give as much detail as I can think of, so thank you in advance for even reading this, regardless of whether you have a suggestion.

Nothing appears to be wrong, but my measurements are implying that I need a .50mm shim, while the lowest Honda lists is .60mm. I also probably shouldn't need a new shim, as I haven't replaced the Main Shaft or the transmission case halves.

What I'm doing: Rebuilding a 91 Si transmission I purchased in unknown condition. Previous owner claimed it worked but was "what you would expect from a 300k transmission".

I purchased a rebuild kit with bearings, seals, and carbon syncros, as well as any OEM replacement items I could find, though the only items applicable to the main shaft that I replaced are:
Both bearings
Input seal
Oil plate
Thrust washer
Spring washer
Syncros

I followed all instructions in the FSM to the letter on how to determine thrust shim thickness, though the process seemed overly error prone, so I worked backwards from the clearance present when assembled. With no thrust shim installed, I measured .64mm of clearance, which minus the middle of the .11mm to .18mm specification, .14mm, would call for a .50mm shim. That can't be right... The shim that was in the transmission was 1.11mm, which I assume would have damaged the transmission to the point that I would find tons of wear and metal shavings in the case, which I didn't. I can't even get the case halves to close with the 1.11mm shim I have. The transmission case is stripped except for the main shaft and it's bearings, nothing else to cause interference issues.

In my confusions I've verified:
Both bearings dimensionally match the bearings that were in the transmission.
Both bearings are installed in the correct direction, verified by closely examining orientation from multiple how-to sources
The clutch housing side bearing is seated fully, verified by measuring the bore depth and bearing thickness
The transmission housing side bearing is capable of seating fully, verified by measuring the bore depth and bearing thickness
The transmission housing side bearing is fully seated and flush with the end of the main shaft. This seems to imply that all components on the main shaft are assembled in the correct orientation.
Old sealant was cleaned up gently with a very mild abrasive wheel, it's impossible that .6mm of material was removed...
Assembled clearance measurements were taken using a dial gauge and clamps/spacers simulating the OEM holders.

Any ideas? What else could possibly cause the main shaft clearance to be tighter than what Honda offers a shim for?

 

Unless you change the case, the shafts, or the differential, there should be no reason to have to reshim it.

 

You check the clearance with the shaft bare of any gears, not assembled. The thrust washer needs to be installed as well.