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MG MGA - gearbox experts

I'm about to remove my gearbox as part of a general oilsear and gasket overhaul (The car has been leaving oil marks of Torrey Canyon proportions all last year).

While I am at it, I would like to solve the problem where the car often jumps out of 4th gear when cold, so would appreciate any suggestions of what the cure should be. I shall be replacing the 2nd Synchro anyway, so a delve into the box is not a problem.

I fitted new bearings and detente springs/balls about 7 years ago, and have to replace the synchro as it's going again after the 40k odd miles the car has done in that time. The box makes no strange noises and shifts fine into 4th, but until the bix heats up, it keeps jumping out of 4th. All other gears are fine.

dominic clancy

Could be worn shifter fork not pushing the sliding member far enough or broken/worn spings in the ball detents in the sliding member or the shift linkage shafts.
John H

Dominic

Your jumping out of 4th gear reminds me of my 90 year Father-in-Law in his last year who had a similar situation with his Fiat 127. He solved the problem by hooking a rubber bungy round the gear stick to hold it in place!

He also had no power in his left arm to pull the gear stick into reverse, so he also had a rope tied round the stick that he would pull on with his right hand to pull into that gear.

Not much help to you though, I am useless with gearboxes.

Steve
Steve Gyles

I always vote for the worn detent balls and springs for no other reason as that's what it says in the Haynes manual! I've read on bbs's like this one that it could also be a perished rear gearbox mount.
Paul Hanley

Hi Dominic

Look for fore and aft play in the first motion shaft bearing.

Cheers
Ian F
Ian Fraser

Dominic, check for wear on the 4th gear hub, jumping out is often caused by this. If you are stripping it completely it would pay to renew the bearings and synchro's. Check the dogs on 4th gear for excessive wear also the nose of the main shaft


Terry
Terry Drinkwater

Wear in the 4th gear hub would be my guess as well. What happens is that the internal splines which are a keystone pattern are designed to hold the gear in mesh. However with use and wear they tend to loose this keystone pattern and as a result the hub can easily slide out of mesh. Just check however that the gear is fully engaging and that the selectors dont have deep grooves in their sides.
Iain MacKintosh

There is only one external cause I know of for popping out of 4th gear, interference between the shift lever and boot and the tunnel. This could be caused by misalignment from a badly deteriorated rear mount and bar engine mounts (both at the same time). If that's not it, then the box will have to come out for disassembly and inspection and repair. Meanwhile we can only line up a variety of possible causes to look for when it's apart.

Wear on sliding hub and mating teeth is found occasionally on 2nd or 3rd gear if run for a long time with a bad synchro, but very seldom on 4th gear.

Popping our of gear implies that the small teeth on the sliding hub were not fully engaged. This could be a single problem or a combination of a few problems. Wear on shifting forks is usually not so severe to cause the problem alone.

A loose lever on one of the shift rods could cause insufficient travel. The most common one to be loose would be the rear lever on the remote shift rod at the bottom of the hand lever. Next in line of suspicion would be the front lever on the same rod, the one with the split bronze bushing, or a missing split bushing, or a nylon replacement type split bushing being broken. Going forward from there the levers on either end of the selector rod may be loose. Any of these items can cause sloppy motion of the hand lever with excess free play, especially in the fore/aft direction.

Another problem I encounter often is, about 3 out of 4 gearboxes I have had a hand in disassembling have been missing some or all of the shims for the input bearing inside the front cover. The Book says to be sure to replace the original shims, so many people don't buy the things. But original shims may be damaged, or some DPM just left them out, or maybe the front cover was changed, or a replacement gasket is (very often) a different thickness than the original gasket. In any case the shimming should be carefully measured and adjusted any time the front cover has been removed. If there are insufficient shims here the front bearing and input shaft can float forward reducing engagement of the sliding hub teeth on the input gear, which can cause it to pop out of 4th gear.
Barney Gaylord

I seem to recall that if the spiggot bush that goes in the end of the crankshaft is worn it may contribute to jumping out of gear when on the overrun!...JB
J Bray

Thanks for the responses.

The box is now apart - no wear apart from 2nd Synchro. The detente springs and balls seem fine but will be replaced anyway. Shims are present at the front cover, but I will measure here on reassembly.

The rearmost bolt on the output shaft was loose, so I suspect that this may have allowed some float, but curiously the box is fine when hot, and only jumps out of gear when cold.

The boot and lever and all mounts are fine, and there is plenty of space at the lever when in 4th, so it's not just the boot pulling the lever back out of gear.
dominic clancy

This thread was discussed between 08/01/2007 and 12/01/2007

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