I have a 1949 4 speed that sometimes drops out of 3rd gear at low/medium speeds. I've seen several posts about this on 3 speeds, but none on 4 spd. the transmission has never been rebuilt, and the truck had been running since new.
Steve - A quick search of previous SM420 topics came across a couple of possibilities that may cause your popping out of 3rd gear. This reply from 1 Ton tommy mentions that "Jumping out of 3rd gear is usually because of worn clutch-gear or rear mainshaft bearing or both".
There's another thread that includes a reply from Busta1 that mentions "For jumping out of gear, check the detent ball spring." which are supposed to help hold the shift levers and gear in the proper positions.
Other should be along offer further advice soon.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
Thanks for your input!! I'm not sure which part is the "worn clutch gear"? Is their a schematic of the tranny available that would help identify the gear they mentioned? I don't drive the truck that much and it is just annoying when it occurs.
The shop manual doesn't really display the whole thing broken down, but here's an exploded diagram of the SM420 from a 1929-57 Chevy Master Parts Book.
The "3rd and 4th gear clutch" is Parts Group 4.380. It works together on the main shaft with the synchronizers, synchronizer hub, keys and snap ring (all Group 4.384) for shifting the gears between 3rd and 4th.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
I just got elbow deep in my 3 speed. Far enough that I'm suspecting the mainshaft bearing is what's causing the noise in 2nd gear. Parts on order.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
I had something similar happen to me on my SM420 4 speed. Mine kept popping out of 4th gear. It turned out that the roller type pilot bushing (bearing) came apart and the clutch gear wasn't properly supported and created a miss-alignment. Replacing the pilot bushing with a solid brass bushing solved the problem. I will not longer use the roller bearing type pilot bushings for this reason.
