39 Knuckle Trans 12N34 Rebuild

[Holeshotbob]

I have a 39 Knuckle project and need some advice on rebuilding the much maligned 12N34 transmission. I would like to keep the original 12N34 layout. I have found a lot of good information on rebuilding the traditional 4 speeds, but have come up empty on how the rebuild for a 39 transmission would differ. I have rebuilt 5 speeds but this would be my first 4 speed. Can anyone help me out with what to do differently with the 39 transmission vs. the traditional 4 speed layout? Or given the oddities of the transmission, would I be better off assigning the job to an expert ? Thanks. Bob

[Rubone]

Nothing unusual about the transmission Bob. It is essentially the same as any '39 up gearbox with the exception of a sliding 2nd gear (very much like a 3&R without an idler gear). All bearing fits and specs are common to 40 up 4 speeds. And shifter fork spacing is dictated by the correct top (same as 3&R) so it is all very straightforward. Not a bit "special".

[Holeshotbob]

Thanks, Robbie. I have been able to make it all the way down to the bear box without any issues.

[Holeshotbob]

One surprise I found was 45 needle bearings in the 4th gear end of the mainshaft. My understanding is 4-speeds have 44 needle bearings at 4th gear. Thinking I miscounted or somehow mixed in one from the countershaft, I double checked and confirmed that 45 needles came out of 4th gear. The story I got on the transmission is that it came out of its bike in the early 60’s and it looked the part, no obvious signs to me that anyone had been in the box. Is it possible the factory put an extra bearing in there? Or might it be a long-ago mechanic who thought one more was better?

[Rubone]

The only way 45 will fit is if the race is quite worn. A standard race and 44 standard rollers do no allow enough room to force in an extra. Something is well worn. Most often when OS rollers are fitted you can end up with one less due to roller size with correct shaft to race clearance.

[larry]

Did you mic the rollers?

[Holeshotbob]

I did, they are .1250, which I believe is standard size. I also mic’d the race on 4th gear, it is 1.629. I am now thinking the race is way out of spec. I have to get a dial bore gauge and check it. For what its worth, the roller bearing on the opposite end was quite worn.