Welcome to our resource for MG Car Information.



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 MGA - Urgent Gearbox Problem

Hello all, I have a race coming up in a couple of weeks and my gearboc just went out. Before I tear the whole thing apart, I'd like to get some input as to possible cause.

While driving I suddenly lost 3rd and 4th gear. It acted as if the shift fork had come loose from the shift rod. Unfortunately that's not the cause. I can select 3rd or 4th just fine the appropriate parts move to engage but the input shaft and 3rd motion shaft are able to move completely independent of each other, even opposite directions as if in neutral. I'm guessing at this point that splines may have sheared off the 3rd motion shaft? The laygear appears fine. There are some smallish 1/8" chunks in the bottom of the box.


J Bries

If you can hold the 3-4 sliding hub still while you turn the output shaft, it has to be sheared splines. You have to take it apart anyway, which only takes an hour, so why guess?
Barney Gaylord


Turns out it is a broken main shaft right where the spring loaded pin is. Once broken it pushed against the input shaft and destroyed it too. The splines actually melted where they were forced into the input. I'll send you some pix if interested.

J Bries

I have a spare 1500 mainshaft if you're interested.
Mark J Michalak

The pics might be neat for my Used and Abused part page.

Been there myself, just not quite so bad. When the spring loaded pin breaks the front thrust washer can rotate and slip up the splines. This allows the gears to float end wise on the mainshaft (moving forward on overrun). In my case it was immediately noticeable as popping out of 2nd gear on overrun, but all was otherwise working well for vigorous driving (for a short time).

I opened mine up the same day before there was too much damage. It had buggered up the front face of 3rd gear and the back face of the input gear. I had to replace 3rd gear, the thrust washer and the spring loaded pin. Back side of the input gear normally doesn't touch anything, so that could continue in use, and my mainshaft was not damaged.

Pushing 2nd gear forward will dislodge the bronze thrust washer between 2nd and 3rd gears. This will likely damage the internal keying bits on the bronze washer and the two bushings. Pushing the 3rd gear bushing forward and/or rotating it on the shaft will cover the radial oil supply hole in the mainshaft. This cam stop oil feed to the 3rd gear bushing and the front thrust washer. Oil should continue to feed through the front restrictor orifice to the needle roller spigot bearing.

If you need a mainshaft I have lots of spare gearbox parts (and a couple of spare gearboxes). There are three different mainshafts, so which model of gearbox?
Barney Gaylord

I've got local help with the parts (Sports Car Craftsmen) and hope to pick them up today with any luck. If this falls through I'll let you know. I'm using a mk2 box with the 23 or 32? spline input and flange main shaft. I believe this is the same as an early B box.

Thanks for your help.
J Bries

This thread was discussed between 29/04/2009 and 01/05/2009

MG MGA index

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