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MG MGB Technical - Gearbox woes

Put my new engine and rebuilt gearbox in and it worked perfectly, for two miles. Then it jumps out of second and will only go into second without massive graunching at slow speed.
Bit ystified about this as I replaced all the baulk rings and as far as I can be sure, the gearbox was perfect before install.
If I had made a mistake assembling then surely it would have shown up right away and not two miles later.
have now done 100 miles and it has not got any better, or worse.
What is likely to be wrong here!
Neckieman

I wonder if the 1st 2nd hub is back to front?
mog

That wouldn't explain why second gear is in the correct place when it actually goes in and why it works fine for two miles.
Neckieman

3 or 4 synchro box?

When I first got my B, it had a "rebuilt" 3 synchro box. It had the same symptoms that you have. Frustrating, so I did a 5 speed swap. Sold the 3 synchro and the guy who bought it opened it and found that a ~4mm spacer was missing.

Herb
H J Adler

4 synchro. I am sure there are no parts missing as I had the box apart and replaced every seal, bearing and shim plus some of tne gears to bring it to factory specs.
If it hadn't had worked for the original two miles on initial test I would assume that I had assembled something wrongly but since it worked initially I have to assume it is correct.
My feeling is almost that the synchro on second is worn, however its brand new!
Neckieman

Overdrive or non?
If non-overdrive, did you position the mainshaft correctly via the shims at the rear? 2nd & 3rd mainshaft gears should centre on laygear counterparts. The pic shows incorrect alignment - the two yellow lines should line up.
If not, the gears will slip out. In this photo, 3rd gear was slipping out as the shaft was too far back.

Paul Walbran

4 synchro O/D
Neckieman

Another explanation could be synchro fork out of place due to grub screw not tight or done up while not located in its hole. You can remove the side cover and check these and the gear positioning with the gearbox still in the car.
Paul Walbran

Grub screw....that's a great guess...and all you would have had to do...is not tighten enough and it backed off just enough to cause the issue described.
S

I was very careful getting the fork screws and lock nuts lined up but it would explain the change in operation.
Its worth a shot to have a look as I need to drain the box to get it out anyway!
Neckieman

Another thought...

IIRC the 4speed boxes used steel synchro rings, whilst the 3 synchros used bronze. You didn't use bronze, did you?

Herb
H J Adler

Does it work OK for 2 miles at the beginning of each trip or just the first trip you made after getting it back on the road?

If the former then it could be an inadequate clearance somewhere, tightening up as things warm up. If the latter then I'd also opt for something like S D Devine's grub-screw.
paulh4

Baulk rings are all steel and correct.
It was just the initial two miles then it stopped engaging and started jumping out of gear. I am away for work the next couple of days but will get it onto the stands at the weekend and have a look in the side cover.
Neckieman

I had the car up and took off the side cover last night. Its not easy to see with the gearbox in the car but I can see witness marks on the selector fork for 1/2nd. Second gear also does not look properly aligned.
Unfortunately as I can see clearly into the box to see what is being contacted the whole lot will need to come out again, bugger!
The selector forks are tight on the rods and have not moved.
Neckieman

Many years ago, after mechanic performed some work on 3-synchro box, I experienced the same jumping out of gear (forget all the details). It turned out that the gear lever had not been properly inserted into a bush (?) which engaged with the selector rods. Sometimes the simplest of things can create serious difficulties!
D C McLean

Its not the bush unfortunately, neither is it the selectors after removing the gearbox today.
There seems to be a fair amount of play between second and third in the gearbox which wasn't there on assembly, the 1st/2nd selector fork is hitting the layshaft gear for second and throwing the car out of gear. The noise is the selector hitting the layshaft as it goes round.
Frankly I am mystified at the moment till I can get the gearbox apart and find out what has happened.
Neckieman

Any chance of a phot through the side cover before you pull it apart?
Paul Walbran

I don't understand why yet but the input shaft has been rubbing against the main shaft and worn off some of the retaining nut, this has allowed the play in the mainshaft bearings and explains why it was fine for the first few miles.
Since these parts are not supposed to actually touch I am unsure what has happened as yet, more investigation needed.
At least I know what happened now, if not why.
Neckieman

The input shaft shim pack tends to get eaten up. This may have led to your excessive clearance issues. RAY
rjm RAY

It sounds as though your mainshaft was too far forward. The only location is the rear bearing in the tail housing. Sometimes the mainshaft is very tight in that bearing and can get pushed out of place. Refer the photo in my earlier post showing incorrect rearwards alignment, (the yellow lines should align) I sustect your has probably the reverse situation. That would bring the mainshaft nut into contact with the first motion gear.

If you get the chance take a photo before dismantling.

Paul Walbran

To clarify my last post:

When the tailhousing is fitted its rear bearing has to slide over the mainshaft as a light interference fit. This usually pushes the shaft forwards a bit thanks to the roller bearing in the back of the main casing not providing any location. The flange nut is then tightened up and that pulls the mainshaft back into place, its final position being determined by the shims just ahead of the bearing.

If the mainshaft is tight in the tailhousing bearing (we have seen up to a thou variation in mainshaft size, a lot for an interference fit) then it can give a false torque reading on the flange nut, it will feel tight before the mainshaft is fully home onto the shim pack.

If this has happened, the top gear syncro ring can also split thanks to being driven hard onto the first motion gear, so be sure to check that out.
Paul Walbran

I agree with Paul concerning the bearing fit. Also, the shaft can accumulate a varnish sort of layer. If the varnish layer is not removed at the end where the bearing goes, prior to putting it back together, it can give a false impression of the fit.
S Akai

This thread was discussed between 25/07/2016 and 26/08/2016

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