Jumping Out Of 3rd Gear

Jumping out of third gear can be a problem with 1940-54 Chevrolet 3 speed transmissions.

If everything else has been checked out misalignment could be the problem.

The first and second pictures show the tapered wear on the teeth caused by misalignment of the transmission.

The alignment process involves shimming between the bellhousing and transmission at one or more of the four mounting points until the rear bearing bore is inline with the front.

Note: If you were to install a .010" shim at the 4 o'clock position for example you would want to put a .005" shim at the 2 and 8 o'clock positions to prevent strain on the transmission case.

Tapered teeth on 3rd gear

Tapered teeth in synchro drum for 3rd gear

This is the tool I made up for aligning the transmission.

It consists of a 3/8" rod Loctited in the bore if a main drive gear.

A dial indicator is then attached

Testing setup

Rod without dial indicator

Indicating off rear bearing bore

Starrett "Last Word" indicator

I turned the clutch splines off so I can rotate the shaft with the clutch in place, although the clutch pedal could be held down for this job

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Another option I made up but have not used is this simple to make thumb screw tester.       I feel this setup would work fine for most people

Thumb screw aligning tool

Closeup of thumb screw

Thumb screw tool in place

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One trick I do to insure maximum tooth engagement and reduce the chance of jumping out of third is to install a shim on the main drive gear before the bearing is installed. This moves the gear back the distance of the shim so the synchronizer drum engages further onto the teeth.

Shim

Shim sitting in place

Shim and bearing going on

Shim and bearing in place. Note: If the shim is tight against the bearing face as pictured bend it slightly away from the bearing so oil can enter the bearing