420 Trans & D18 slipping out of gear?

About 6 months ago my son and I completed an sm420 swap into his 67' cj5, we used a novac rebuild kit in the trans before we installed it, but made no other mods. After the install, it would occasionally slip out of 2nd when coasting downhill. Recently on a local ride the xfer case slipped into neutral in the same situation.... I don't think they are related, other than the wear factor, but they both make for an exciting brake test.

anybody want to take a stab at what might be the most common wear parts that could cause this in either the sm420 or the D18??

craig

 

My experiences with the SM420 are that when worn excessively they will slip or pop out of 2nd under power. As far as the transfercase goes, you may have worn bearings, gears or both.

 

Typically (on any transmission), when the internal bushing on 2nd (or whatever gear) wears enough, this allows the gear to skew on the mainshaft. This skew pushes the synchro hub away from the gear and the transmission may then pop into neutral. Excess end play on the main shaft could also cause this (I presume)

Re your transfer case, if the shift rod detent springs are weak, that may allow the transfer case to come out of gear.

 

The SM 420 2nd. speed assembly isn't like a regular tranny synchro at all. The gear itself has the clutch splines in it and if it has a tapered wear to it it will pop out of engagement.

 

the 420 main shaft had some scuffing but no wear groves, 2nd gear "looked" ok, but then I didn't have a new gear to compare it to. I will try to get the books out today and noodle through it, and xfer case.

thanks, craig