Just wondering if this is normal, or if more investigation is required…
I have an overdrive Moss box in my 140. This was fully professionally rebuilt last year, including all new gaskets and seals, at the same time as the engine.
The box works beautifully, with (relatively, for the period) slick gearchanges, no graunches, no noise in use (apart from first, obviously). No complaints at all there.
But it leaks. After putting the car away following a 100-mile drive, the next morning there’s a stain 3-4 inches across, and when viewed from underneath the box has oil down the sides, including the overdrive. It’s presumably only leaking when the car’s running, as if the stain is cleared away it doesn’t come back until the next time the car’s used.
It’s hard to tell which actual joint(s) the oil is coming from. I’ve changed the O-ring for the o/d adjustment lever, which helped greatly, but obviously anything else will mean engine and box out. I haven’t removed the transmission tunnel yet, but that comes next depending on advice. Is there a breather somewhere that could be blocked? Or do these old boxes just leak? I’ve had o/d boxes before on Triumphs, Healeys etc. but don’t remember this degree of leakage. Obviously these were not Moss boxes, and weren’t seventy years old back then. If it’s to be expected, that’s fine, I’ll live with it - I’m no stranger to leaks, owning a Cooper S and having had the mother of all leakers in a Series 2A Land Rover.
Any advice gratefully received, as ever!
Other than making sure that all is mechanically tight: What type of oil are you using?
I had a lot more leakage from synthetic than I do from any mineral oil.
In case you have MTL, this may very well be the cause.
Hi LLuis, the oil is straight mineral 30. Is there a breather? The T5 in my Mustang once had a blocked breather, resulting in oil pumping up the speedo cable, so the first hint of trouble was oil dripping on my shoe from the back of the dash panel… that took some working out, I can tell you
Hmmm cannot help you there. In my 120 i did not notice a breather as such. Seepage comes from the o rings of the fork shafts.
Ps your leake is much much more thab mine: after 100 km or so i get just a couple of drops (or nothing at all(
I had the same problem. The bolts to the top cover were not tight. During running the oil leaked past the gasket and down the sides of the tranny The wind would carry it over the OD. Had to remove the tunnel to tighten everything down.
The parts book does show a breather, on the rear of the top cover. No mention of valves or springs but I think I’ll have to get the trans cover off. It should be obvious if it’s coming from there then, or wherever.
Hi Roger yes they have a breather plug…not sure what models do…just googke moss gearbox breather…Steve
Yes, there is a breather on the top cover, that brass round thing.
Plenty of other places for it to leak if it was not done right.
Shifter rod seals are a common leak source, but your amount of leak suggests oil splashing up, so one of the two top cover gaskets.
Thanks Rob - that would be a good result, as I can sort that without pulling the gearbox.
Roger,
There’s another leakage area which are the felts of the striking rods at the rear end of the top cover.
My XK 140 JLE gearbox (as shown by Rob) only leaked after longer drives at these 3 positions (although the felt washers had been replaced).
The breather on top could be the culprit but it is posioned very high and also covered at the inside.
Cheers
Bob K.
Good tip, Bob. At some point I’ll get the tunnel out and take the top off, so I should be able to inspect all these areas.
With the breather, it’s not so much oil getting up to it. If it’s blocked, or incorrectly assembled, when running the gearbox can become pressurized and blows oil out past the gaskets. At least, that’s what I’ve had with other boxes, so I don’t imagine the Moss is any different. And it must be a risk or it wouldn’t need a breather.
Hi Roger, having just reached out to you on your valve guide hold down topic, here we are with leaking gearboxes…. Curious to hear what you learn as I am sorting out the same issue here… I went over my box and tightened down all hardware… this a is high miler with no resto history… I don’t remember the numbers but I recall the box and OD hold about 2 L oil? I always refill about 500ml and then after a drive or two find 500ml on the floor. Always takes the same amount (give or take) to fill it back up regardless of the amount of driving. I think it is the OD lever o ring but have yet to attack that. I did take the tunnel off, and crawled under and over to remove all traces of oil with a hope of making it obvious where the leaks were originating, but no luck, it’s everywhere! If you find a likely leak spot place share! Cheers!
Hi Dave,
slightly different as mine is a fully-rebuilt 'box, so I wasn’t expecting too many leaks.
I have now rebuilt the selectors in the top cover, and replaced the two gaskets either side of the overdrive spacer casting with thicker quality ones from Coventry Auto, which are much nicer than the thin paper items (or you could make some yourself with decent gasket paper). These were sealed with a thin bead of Würth RTV, which is the best I have found. The gearbox breather and the overdrive breather were both removed and thoroughly checked (nothing found, but worth doing). I also changed the three solid old felt seals for 3.5x13mm Nitrile O-rings.
So far, so good, but it hasn’t had a proper trial on the road yet. I am very suspicious of the selector seals as most of the leaking oil was all over the overdrive unit, and there isn’t much there to leak. I also changed the small O-ring on the adjustment lever of the overdrive, which was easy enough, but you have to check adjustment of the lever when you refit it. Again, pretty straightforward.
Hope that helps!
As a previous owner of this SH box did. It is a page from the Service Manual listing the accessory dealers in a lot of foreign countries. 
Good grief.
Better check the one I just took off, it might be an original Magna Carta.
In the US the traditional home-made replacement gasket was made from the front cover of a J C Whitney catalog. The paper had a high clay content so it wouldn’t wick and was highly calendered. There were always plenty of them around since they sent out a new catalog every month. Actually, any magazine cover would work, but using the cover from an automotive parts catalog was de rigueur.
Just an update for future readers:
A new, slightly thicker gearbox top cover gasket (two, is it has overdrive) and a set of O-rings 0.5mm undersize to the selector shafts has cured it. Not a drop on the floor, and looking up from the pit reveals the gearbox and overdrive castings to be nice and dry now. The big finger is pointing at those felt seals on the selector shafts.