The Stovebolt Site Forums The Shop Area Engine Shop jumping 4 speed Muncie trans-what gives?

I have a 54 chevrolet 1 ton truck with a 235 engine and Muncie 4 speed transmission. (Transmission casting number is 591953.) Definitely Muncie as everything matches in vintage chev shop manual. The trans was mfg from 62-67 and bolts up fine.

Rebuilding the 235 so everything is apart. Transmission has been 'jumping out' of second when you hit a bump in road, let off gas, etc. Not every time, but often enough. All other gears, including reverse, are fine.

Trans is out, with top of case off. Shifter forks position gear so drive/second gears are only 50% meshed.

What is wrong/worn and how do I fix?

The synchroniser slider assembly is worn.it will eventually get worse

The detent in the rail for second is either worn ,,or the spring isnt holding it in... or also could be the shift lever ball is worn where it goes into the fork...its probably the detent tho... those 420's thats the first thing to usually go is second gear...typically its the collar and gear thats worn or tapered...rather than the fork not going all the way in... all a sychronizer will do is stop the gear from spinning or slow it down to allow it to mesh into gear without grinding....the sychronizer usually wont cause it to jump outta gear or not allow it to go into gear all the way.....its a gear brake is all it is.... as in puttin it in low where it doesnt have a synchronizer it will grind a little sometimes.....same with reverse...