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MG MGB Technical - problems with a 4-synchro transmission

I’ve taken out the first motion shaft of my 4-synchro, overdrive transmission in order to have it turned down to use with an MGB 3-main engine. I didn’t disturb anything on the main shaft except the 3-4 gear sliding hub. It fell out when I unwisely turned the transmission on end. The 3-4 sliding hub also came apart when I played with it, so I reassembled and replaced it. The lay shaft was removed. All is now put back together. The synchro rings and gears are all in excellent condition. The sliding hubs click smoothly back and forth and the transmission turns easily when I rotate the first motion shaft by hand.
I put the selector shafts and forks and the shifter mechanism back in place. In neutral, the transmission still turns easily by hand, and it shifts smoothly into all 4 forward gears and reverse. The sliding hubs move back and forth with each gear change like they are supposed to.
The problem is that in first, second, fourth, and reverse, the transmission no longer turns by hand. It moves through small part of a revolution and then stops with a metallic ‘clunk’. But in third gear it turns smoothly.
Can anyone tell me what might be wrong? Thanks in advance for your advice!
M.E. Whalen

The symptom indicates that it is IN 3rd gear, or at least the 3rd synchro is engaged.

These things are so reliable that despite all the work I've done on these cars, the only 4 synch box I ever saw apart was in 1972 (we couldn't get parts even then!), so I'm going by the WSM here. There should be a special nut retaining the 3/4 synch hub on the 3rd motion shaft; if that is in place the 3/4 synch assembly could not fall off the shaft. If it is not in place all sorts of evil things could be expected.

I think you have the 3/4 synch hub backwards on the shaft, or the outer sleeve is on the hub backwards, if it is assymetrical. Or the cutouts in the outer sleeve splines are not aligned with the cutouts in the hub. The WSM does not have a drawing of this to work from.

If it continues to puzzle, call somebody who is more familiar, like John Esposito at Quantum Mechanics.

FRM
FR Millmore

Sometimes the synchro cone can stick quite tightly onto the gear cone . especially if it has been assembled dry. A quick flisk with a plain screwdriver blade will free it. Then squirt some oil into the gap. Worth a try before you dismantle again
jim soutar

Jim's correct, but it doesn't explain how the synch assembly fell off the shaft. This could cause other problems, since that nut appears to clamp a bunch of stuff on beside the synch unit.

FRM
FR Millmore

Page Fb5 of Workshop manual AKD 4957 available from Moss shows nut being tightened which holds on the synchro hub centre etc, Whilst the outer can still slide off on its own, if the inner came off then as FRM points out, I afraid you have missed something and only another strip down will show the cause.
jim soutar

I took a 4 speed "apart" last Christmas out of curiosity, but did not remove the parts from the main shaft as all seemed well. Not sure if the attached will help. Nothing fell apart when I pulled the main shaft

Michael Beswick

It could just be that the 3rd gear synchro ring is stuck on the taper -- maybee a little poke in there with a scredriver to check that it is free and a little twist with the same weapon to free it if not --Willy
William Revit

The problem seems to have been a stuck synchro ring on 3rd gear, as several of you pointed out. I freed and oiled it, and all seems to work smoothly now.

As a note, the 3-4 sliding hub is not held on the main shaft by anything except the rear end of the first motion shaft pressing against it. When the first motion shaft is removed, the hub slides right off of the main shaft if you are unwise enough to tip the transmission sharply forward.

Thanks very much to everybody who shared their knowledge!
M.E. Whalen

This thread was discussed between 14/04/2012 and 17/04/2012

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