Diagnose SM-420 problem

Dozer

I have an SM420 in a '66 Chevy C60 that I pulled partly to reseal the oil leaks and partly looking for a high speed squeal noise that I was getting at 50 mph and above. I didn't find anything major wrong with the trans beyond the low gear synchronizer ring was a little loose in the gear. I re-staked it to the gear.

After reinstalling the trans I now get a noise in 3rd & 4th gears that's about the speed of the main shaft. Sort of a clunking sound. Worse in 4th gear. No noise in neutral except when lever is over above 4th gear where I get a slight gear ticking sound. Feels like just the weight of the shift lever is moving the gear a little. If I go ahead and shift into 4th, clutch down, the noise gets louder and if I drive it I get noise in both 3rd and 4th.

When I had it apart I didn't pull the 3rd/4th gear synchronizer hub and gears off the front of the main shaft as the synchronizers seem to have plenty of grip on the gears.

The trans noise doesn't seem to have anything to do with the initial high speed noise I was trying to locate plus there were no issues shifting before I worked on it.

I pulled the trans back out and tore it down again. No obvious assembly mistakes. The only odd thing is there was a fair amount of "fuzz" on the magnet and there was a little metallic slime on the inside of the 3rd/4th shift ring. No other obvious signs of wear.

Any idea what I could possibly changed or done wrong?

Thanks in advance

YJSam

Dozer Thank you for the question. Our transmission builder has some ideas on this, but would like you to send some photos of the 3-4 stack-up and pieces. If you would please.

Dozer

Here are a few pic of the trans parts. Nothing obviously bad except corner wear on countershaft gear (cause?) and a somewhat worn 2nd gear syc. I was able to locate another trans from a '53 pickup that's in very good condition (my brother saved it back about 40 years ago) and I will probably end up using the internals from it in my case. I'll have to reuse the top main shaft because it's a different length.

Dozer

I still don't know what the problem was, but after swapping the countershaft and all the parts on the main shaft the noise seems to have disappeared. My only guess is that one or more parts were worn enough to cause a problem, even if they visually look good.

YJSam

Dozer Thank you for the photos. The builder said the parts look real good for a SM420. Not sure why it was giving you issues, but we're glad it was figured out and working fine now.