I just swapped in a sm420 transmission into my 67' CJ5 and I am having some problems. The transmission performs flaylessly under power, however, when under extreme compression braking (going down steep grades) it sounds as though gears are rubbing against each other, very noisy. Yet, as soon as I give it a little gas while compression breaking, the noise goes away. Does anyone have any suggestions? Since I rebuilt the trans. before installing it, which included new synchros and bearings, the only thing I can think it would be is the reverse idler gear coming into contact with other gears. After rebuilding, I did not touch the eccentric bolt on the side of the trans. to adjust the position of the reverse idler gear. Has anyone performed such an adjustment, and if so, how was it performed? Lastly, I don't know the history of this trans. and I don't know if I have the correct shifter assembly for it. I suppose the whole shifter assembly may need to be adjusted for the trans. If anyone has any suggestions, I would appreciate hearing them.

Sounds like you can tell me a few things and maybe some solution might arise. Which adapter are you using and how is the mainshaft retained at the outboard side of rear bearing?? How are the bearings in that tranny? Also the 420 can realy talk to you (whine ) on compression in 2nd speed and also 1st. I do suspect bearings however.
Dave

Mine does the same thing, I just dont worry about it!

The 420 is just a loud transmission. If the noise is really bugging you, I'd look at the needle brgs in the input shaft, and or check the input shaft main brg for excessive play. the good news is that you can check them both without having to completely dissamble the transmission. Do you have an adapter that may have wear in the endcap of the upper shaft, which bolts to the rear of the transmission, holding the upper shaft rear bearing in place?

:confused: