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MG MGB Technical - Rebuilt gearbox won't shift correctly

In 2010 I rebuilt the engine and had an MG shop rebuild my gearbox on a 74.5 then I let the car sit until a few days ago. In the last week I have put approximately 45 minutes on the engine and 30 on the gearbox. The engine is running pretty well but when I first started driving, the gearbox wouldn't go into 3rd or 4th, only 1 and 2. I found the gearbox oil was quite low, due to 4 yrs of leaking at the speedo gasket. Upon refilling, the car went from 1st to 2nd and then into 3rd and 4th. However, from that point forward I could no longer upshift or downshift into 2nd. If I forced it through the considerable initial resistance in 2nd then I hit gears and it would just grind so I had to back off. I probably drove it another 20 minutes like this but it never changed even though it would shift nicely into 2nd from a stop and if I started out in 2nd then I could upshift to 3rd but never back into 2nd. Additionally, although it is going into and out of 3rd and 4th reliably now I do hear a minor "nick" sometimes when changing between these gears. Any ideas?
Tom Gillett

How high is the "bite" point on the clutch?

Can you change gear if you "pump" the clutch pedal?

My guess is that the clutch is not disengaging fully or there is drag on the spigot bearing.

If the bite point is low then perhaps bleeding the clutch will sort it.

Pull back the rubber boot on the clutch slave cylinder and check for black fluid behind it. Check the master cyl too.
Chris at Octarine Services

If it grinds when trying to select reverse then the clutch is dragging. In my experience a dragging clutch baulks (not grinds) worst in 1st, progressively less for each gear after that, and only when stationary. When rolling any gear can be selected, although how easily depends on how much it is dragging. This can be irrespective of the bite point, if the problem is in the clutch itself and not the hydraulics.

If the problems are when the car is underway i.e. at normal gear-change speeds it sounds more like a problem inside the gearbox than anything else.
Paul Hunt

I am able to shift into any gear when at a stop and the clutch was replaced less than 50 miles ago. I can start from a stop in 2nd and then upshift to 3rd but after that I can not ever again downshift to 2nd or upshift to 2nd. If I start out in 1st I can not upshift to 2nd although I can skip 2nd and go right into 3rd but never into 2nd from above or below. I have tried pumping the clutch to no avail. THoughts?
Tom Gillett

How about with the engine stopped and the clutch pedal down?

Whilst you can get baulking at 1st and reverse in any BMC gearbox, once you have selected each gear once you should be able to keep going through all gears many times. If you can't do that then it's mechanical inside the gearbox or the remote change.

If the baulking only occurs when the engine is running, and particularly if it grinds when trying to select reverse, then the clutch is dragging. If this started happening after the clutch was replaced that has to be suspect. What's the biting point like i.e. is is fully bled?
Paul Hunt

Hello Tom and all. I am no longer a regular contributor to this BBS as I no longer have my MGB, but I always read it as it is a mine of information on classic car problems.

I had a similar problem after I rebuilt the gearbox on my '64 B. Search the archives for

2nd Gear Type and 2nd Gear Surface Finish.

I used the wrong type of synchro ring (brass instead of steel) when I did the rebuild and had a very similar problem to yours.

Hope this helps

Tony
Tony Oliver

I was thinking the same thing as Tony....

This guy does a good job of explaining how it all works. Ive had both the early and late boxes apart and though this isnt an MG box the components are very close to the same. This guy gives a great
overview.

http://www.youtube.com/watch?v=MXsRfbOiBhE
Steven Devine

This thread was discussed between 24/02/2014 and 11/03/2014

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