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MG MGB Technical - Leaky gearbox

Where would a gearbox be more likely to be leaking from.
Checked the oil level a couple of months ago as the overdrive was reluctant yo engage. Had to put at least a litre in to bring it up to the mark, that cured the overdrive problem. After having done about 500 miles it needed another half litre., so it must be leaking from somewhere. 71 roadster with overdrive.
Trevor Harvey

First choices are probably the rear oil seal, or the OD sump, or the solenoid cover, or the front oil seal and that can contaminate the clutch. Relatively easy to determine which with those. Shouldn't be the drain plug as that has a tapered thread - if it has been tightened. It can also leak from the switches surprising as it maybe that oil can get up that far. Could also be where the OD bolts to the gearbox.

Normally if they leak they drip where you park, but that will be from the lowest point so may well not be adjacent to the source.
paulh4

Thanks for your reply Paul. If it’s the rear oil seal is that an easy refit. The o/d sump is that the filter cover. I previously noticed every bolt on the filter cover had a bubble of oil on the head and I was able to get almost a half turn on each one. But was reluctant to over tighten.
If it’s the front oil seal, that I assume is an engine out job. Will have to get under and have a good look. Trev
Trevor Harvey

Oil coming from above can also be due to wear in the remote housing where the shaft exits. There is no seal at either end, and as Paul notes the oil reaches that far.
Reclaiming via bushing is the only solution we have come up with, would be interested in simpler solutions! Unfortunately it's engine-out to fix it.
But in this instance I suspect that there is too much oil loss for it to be that.
If coming out the solenoid cover, renew the 3 o-ring around the solenoid as well as the gasket and wire seal.
Paul Walbran

Put a clean piece of cardboard down to see where it's leaking it's directly below the filter housings as the photo shows.No oil from the propshaft oil seal or from the sides of the overdrive unit, so it looks like a new filter gasket. Is the new filter also the gasket.

Trevor Harvey

I thoroughly cleaned all round the bottom of the OD, put a sheet of blue hand-wipe paper on a slab of polystyrene on a piece of stiff board and lightly jacked that up underneath. Next day removed it and I could see exactly where it was leaking from. Several have reported that replacement gasket/filters leak from where the ends of the filter wires have been cut off at the edge of the gasket.
paulh4

I changed my gear box oil a couple of weeks ago & changed the overdrive filter & cleaned the low pressure filter as well. At the time i also replaced the fibre washer on the low pressure plug & the two O rings inside.When putting the overdrive filter / combined gasket back i smeared a small amount of grease on both sides on the gasket surround (as suggested by John Twist). I carefully nipped the bolts up , with the intention of revisiting after a short while. I have covered approx 100 miles since the change & have just taken the attached photo which shows a couple of drips on probably two of the bolts which might benefit of being nipped up a bit more. i have no drips from the low pressure plug. (Before i removed anything i had slight drips from all).
Ignore the blue wire - this simply holds the overdrive solenoid wire from dropping lower than the gear box support & getting snagged - my car has the o/drive switch in the gear lever , so the wire has to follow this route. As the solenoid works & there are no leaks from its' gasket i am not disturbing it or the old insulation tape i wrapped round the wire (circuit protected by a separate line fuse put in by me) until i have to.
Cheers
Charles


Charles9

FWIW the solenoid wire on dash and column lever cars normally follows the same route, with the other wire to the OD switch, under the gear lever.
paulh4

Watching some videos on YouTube about the pressure release valve oil seals ,could this be part of what might be the problem I’m having with the oil leat from my overdrive. If the pressure relief valve is not good does this increase the oil pressure in the overdrive. I don’t want to fit a new gasket and find I still have an oil leak problem. Thanks
Trevor Harvey

There shouldn't be any pressure in the air-space inside the gearbox and OD, there is a breather on top of the 4-synch as in the attached same as on the axle.

Any leak from the O-rings in the valve, pump or solenoid would be internal and more likely to affect the operation of the OD. The pressure relief valve plug and solenoid cover do have seals to the body of course.

paulh4

Thanks once again Paul, very helpful.
Trevor Harvey

This thread was discussed between 27/05/2023 and 13/06/2023

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