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MG MGB Technical - gearbox jumping out of 4th gear

my 1978 mgb roadster jumps out of 4th gear in to neutral when lifting off the accelerator from around 60mph. It does not jump out of any other gears and the gearbox is quiet and shifts easy. It does not do it as much at slower speeds but does sometimes.
Is it easy to fix or do I look for another gearbox ?
David

David,

Does "jump" out, or just quietly slip out? My '73 GT did this in third. I'd be engine-braking down a long hill and suddenly notice the engine had dropped to idle and I was going faster.

Synchro rings.

Not easy to fix, but it's not really hurting anything. If this is an o/d box, rebuilding is in order and worthwhile. If it's a non-o/d box, there are cheap used replacements lurking in the garages of most of the people on this BBS. You could probably get one for about $50-100. But you wouldn't want to go to that much trouble without making sure your replacement box is good - so even then, an expert examination and/or rebuild is a very good idea. Or take it to Twist's transmission rebuild seminar and have a lot of carefully-supervised fun doing it yourself.

If you don't have o/d, now might be the time to consider it. Or maybe converting to 5-speed.

Outside of the annoyance, take your time to think about what you want to do, while you continue to enjoy the car.

FWIW,
Allen
Allen Bachelder

Check that the rubber gaiter is not out of sorts, thus putting pressure on the shifter lever. I'd tend to think its the detent balls or springs that are worn if its not the gaiter. Cheap parts but gearbox must be removed from the car to replace.
Paul Hanley

thanks for your comments guys.

I did check first to see if it was something easy like the gaitor and I took this off and tested the car again with no improvement. The gearbox jumps out of gear fairly gently, certainly not with any noise and as you say you can only tell because revs go down to neutral. It is a non overdrive box.

I am thinking about going to University Motors session in February as this looks like an ideal way to learn how to overhaul the gearbox properly, and Grand Rapids is not too far away.
David

Hi Folks:

Another source for jumping-out of 3rd and 4th gear is the loss of the 1st motion shaft cover/input bearing shim clearence. Both 3 and 4 synchro gearboxes have had this problem, especially not setting the clearences correctly during rebuild/overhaul. I have seen many gearboxes missing this shim, and other completely chewed-up.

Good Luck: Rich Boris 67 B.
Rich Boris

David- I've got 2 good 4 synchro RB boxes 58 and 75k, either one of which you can have for shipping and crate. Vic
vem myers

Before you do a lot of work, take the 4 screws out on the gear shifter rubber vover inside the car. The rubber may be pulling you out of gear - If so, it is in backwards

Just something that happened to me

See ya
Frank Baker

David, I'd go to the University Motors session there can be no doubt you will learn a lot about transmissions. At the same time your box may be expensive to repair as you will no doubt have to replace the 3rd/4th synchro hub and cones. In addition I would never rebuild such a box without replacing the layshaft and bearings plus perhaps selector springs and plungers and you are looking at quite an expense. I'd take Vem up on his kind offer as a box with 58k should be in pretty good nick as he confirms.
Iain MacKintosh

David,

Vic's very generous offer gives you a pretty good idea of what these non-o/d boxes are worth. If you are happy with a non-o/d box, by all means, take him up on it! If you want o/d, this is a perfect opportunity to look for a used one. Condition is not all that important if you're planning on Twist's seminar. You might end up spending some serious money on parts, but if you value o/d, it's money well-spent. Twist will take you through the o/d at the seminar; I took mine a couple years ago, expecting to leave the o/d unit for him to rebuild and send to me later, but he was happy to take me through it as part of the seminar. Added bonus!

Allen
Allen Bachelder

This thread was discussed between 17/10/2005 and 24/10/2005

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