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MG MGA - gearbox crunchy shifting

I had gone over my 1600 Trans last year during an engine rebuild. Since then it has been stiff to shift geting worst as it warms up even grinding during quick shifts from 3rd to 4th. Take it easy and it is fine just not smooth and silky as I was hoping for. I have it out now to replace the clutch plate slipping due to a front seal failure. I have been told that the problem I have can be due to a dragging clutch not fully disengaging even though I never had any trouble getting into 1st. I hope that is it and replacement will solve the trouble but while it is out I want to make sure I didn’t do anything wrong during the rebuild. I had replaced the bearings and synchros, the lay shaft was fine showing almost no wear, all other clearances were within specs. Any ideas Chuck
Charles O'Brien

Chuck, what oil did you put in it? The gearbox should have the same oil as the engine, but if heavy hypoid oil is put in, it will stop he synchro from working properly and eventually ruin the gearbox. My car had hypoid oil in the gearbox when I got it and it had the same problems you have. I put the correct oil in and it worked much better. After I changed the oil for a second time, it worked perfectly.
Lindsay Sampford

I used castrol 20w-50wt. I changed it once after rebuilding it helped a little but did not cure it. I did notice small brass flakes in the oil during that change, not a lot but more than I expected, Now that its apart it looks as if the 3rd 4th shifter fork showing wear. The thrust washers and baulk rings still look like new.
Charles O'Brien

These gearboxes really were not great, the internals being essentially from the "family" saloon range. The factory persevered with them until the introduction of the MGB Mk 2 (the far greater torque of MGB eventually found them out). They were probably alright when they were new (although a friend returned his early MGB to the dealers for gearbox attention when the car was new!). When overhauling mine (many years ago!), the spacers needed to restore the recommended end float between the gears were only available to factory tolerances (ie they were all too "thin") so I had some made to my own specs (they are fairly simple to make--out of phosphor bronze from memory). I think that the "looseness" between the gears does not allow sufficient pressure on the synchro cones? It is necessary to carefully examine (and measure?) each component of the gearbox. I dare say that no-one thought that we would be still driving (and enjoying!) such cars as MGA's this long!
Barry Bahnisch

I have checked the end float on the layshaft and 3rd gear. Layshaft was good, 3rd gear a little loose at 0.008" not the .002"-.004" Barney states is desired. I already have the largest thrust washer at .161" and the interlock ring shows no wear. Everything on the mainshaft seems to work as it should i am starting to wonder my problem may be in the selector shaft assembley
Charles O'Brien

The mild steel selector shafts tend to get sharp on the edges after a while and they catch. When I had my box apart I gently rounded off all the sharp edges and the engagement became much smoother. You could try that first before getting into more serious stuff.
AR Terry

This thread was discussed between 30/05/2011 and 31/05/2011

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