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MG MGB Technical - Gearbox dimensions

I need to replace the breather on my 4 Synchro/LH overdrive gearbox, the one that screws into the top of the gear change remote housing. As it is totally inaccessible from under the car I am planning to make a small hatch way in the left hand side of the tunnel. If someone has a similar gearbox/overdrive setup out of their car at the moment I would really appreciate knowing the straight line the distance from the centre line of the gearstick to the breather.

If anyone has used an alternative method of replacing this breather with the gearbox in the car I would very much like to hear about it.

Many thanks.
Graham Gilmore

Nor normally needed, although I suppose if it is blocked heat expansion could force oil past seals.

You can see the OD switch with the small removable panel removed as in the 1st attached and the 2nd shows where the breather is in relation to that switch which may help, this hole was cut to enable replacement of the switch but I've done them without that. Removing the gearbox cross-member bolts will allow the gearbox to drop down a bit and be pulled across, but be careful of the speedo cable where it goes past the lower edge edge of the tunnel.





paulh4

Fortunately, I have a spare gearbox in the shed.

Measured from the centre of the gear lever when in neutral.

Dave O'Neill 2

I like your idea of dropping the cross member, but do you not also have to drop the exhaust in order to a) get the gearbox to move and b) to get your hand on to the top of the remote. I have a three branch manifold which would make access more difficult I believe.

Thanks for the diagram for the position of the breather, that may be the way to attack this problem. Do you any idea of how far below the level of the face of the gear lever hole the breather is please? Maybe that would allow me to cut a large diameter that hole could then be sealed with the 'bung' that goes in the gearbox filler access hole on the top of the tunnel. Just a thought.

Thanks for your help Paul.
Graham Gilmore

I found that even with the front strap in position the gearbox still dropped OK, it doesn't move much, but it's only the bolts at one end of he strap that would ned moving anyway.

But I was thinking more in terms of you cutting an access panel using the position of the overdrive switch viewed from above as a guide rather than accessing the breather from underneath, and Dave has supplied the dimensions you requested. Again, the photo of the relative positions of switch and breather should allow you to gauge how far it is below, and I'd be using a cutting disc rather than a drill and jig-saw. Whilst a hole the size of the rubber bung may be enough for the breather, while you were at it and for 'future-proofing' maybe something bigger to access both switch and breather?
paulh4

Sorry Dave, but I did not twig that you did the drawing of the remote, it is very useful. Any chance you could measure how far below the level of the face of the gear lever hole the breather is please? Maybe that would allow me to cut a large diameter that hole could then be sealed with the 'bung' that goes in the gearbox filler access hole on the top of the tunnel. Thanks, Graham
Graham Gilmore

Difficult to be precise due to access restrictions, but it’s approximately 50mm from the top face to the top of the breather, or 2” in old money.
Dave O'Neill 2

Thanks for looking Dave, that gives me some better parameters to start working with.
Graham Gilmore

Many thanks for that Dave. I will see where that brings me to.
Graham Gilmore

This thread was discussed between 16/01/2026 and 18/01/2026

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