MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - MGB 4Synchro Gearbox Problems

We have a 4 synchro gearbox, with straight cut, close ratio, gears. After it's last outing, at a track in the US, the clutch appeared to be slipping, and no gear seemed to be engaging properly. Reverse would not engage at all. So, we prepared to replace the clutch - but there was noting wrong with the clutch. We took apart the gearbox, and discovered that there was no glaringly obvious problem. The reverse gear was showing wear, so we replaced that. The synchro rings were also showing wear, so we replaced those. The first motion shaft bearing had a glitch in it, so we replaced that. The third motion shaft washers were replaced, and the shims on the first motion shaft.
The problem now is probably exactly the same as the original problem. It will not engage reverse gear, although the shift fork moves into the correct position, it clearly is not 'in' reverse, because the shaft will not move at all. You can clearly see that the teeth of the gears are meshed correctly. Nor will it 'go' into neutral, because, although the position of the shift lever indicates neutral, and the gears appear not to be engaged as they would if in a gear, the shaft continues to turn in it's entirety, rather than being able to hold the input shaft still whilst the output shaft turns.
We have had this thing apart more than once now, and checked everything that we can think of. The darned thing continues to to be unco-operate. Any suggestions would be of great value.
At this point, we'd probaly try just about anything. We have another race this coming weekend, and at this point it looks as if we may even have to resort to a stock gearbox.

Elizabeth

Hi Elizabeth,
You do not mention it, but is it fitted with overdrive ? If it is, then I would check there. (sprag clutch ?)
Good luck Ric
R.A Willis

Thanks, but no overdrive. This thing can't even send my hair grey, it already is, so I'm just pulling it out.<G>
Elizabeth
Elizabeth

I would pay serious attention to the internal shift linkage. Something could have worked loose and be allowing the shift lever to appear to engage a gear but only partly be doing so. Keep us posted. Ray
RAY

Thanks Ray. The gearbox is sitting on the bench, with the side cover off. The linkage seems to be in good order, and as we move the shift lever through the gears everything appears to be in order, and moves correctly. It isn't, of course. the forks are sliding as they should, but ............

elizabeth
Elizabeth

WE HAVE LIFT OFF. You probably won't believe any of this, but .......... I mentioned that we'd replaced the shims ...
We measured the bearing surface, and replaced two 4 thousandths shims with one, two thousandths. A new bearing, a new shim, right? Wrong. We took out the shim, by a method of elimination, and, she moves, smoothly in every gear, we even have neutral. It is hard to believe that something as thin as this could possibly have been the problem, but it certainly seems to have been the case. When we disassembled it in the beginning, one of those 4 thousandths shims appeared to have slipped, and become pinched in situ. Maybe that caused the problem that we had at the very beginning, but it would have been hard to see that as the only problem. Well, we shouldn't need to change the reverse gear, or the synchros, for another couple of years.
Thanks for your suggestions. It really helps to know that there are knowledgeable people out there who are willing to help with the problems we encounter with these great old cars. Long Live BBS.
Elizabeth

Just an FYI. Another source suggested checking the clutch pressure plate for 'true'. When we took the clutch out it looked to be in good order, but, just for chuckles, we measured the pressure plate last night, and as you might have guessed by now, it was distorted. So, we'll be including the new clutch in the reassembly. It is really amazing how so many little things can come into play. At this point, I believe that the bearing, and the pressure plate, were the two main culprits, but the other stuff that we replaced would have needed to be done at some point in time, so we've probably saved ourselves so time, in the long view.
elizabeth
Elizabeth

Hi:

Had similar problem with my early B,: clutch slippage; caused by deterioated (internally) clutch hose. Acts like a check valve.

Good Luck: Rich Boris
Rich Boris

Tony doesn't need reverse! In fact, from the track map, he may not need 4th!

Good luck,
Glen
Glen Popejoy

This thread was discussed between 26/11/2006 and 29/11/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.