AJS & Matchless Owners Club

I have been trying to get to the bottom of the oil seepage from the primary chaincase of my 1954 model 20. I am now convinced that the oil is coming from the join between the back half of the chaincase and the crankcase. The oil comes out even when the bike is standing - it isn't overfilled (just below the level of the chain). The oil is definitely from the chaincase and the level drops both sitting and when used (much more so).

I am assuming that if I want to fix this that I need to take off, and refit, the back half of the chaincase.

However, before i decide whether to do that, I just wanted to check that what I am assuming seems plausible.
Any thoughts would be grateful received.

Thanks Spridder but it's not leaking from the join between the two halves - fitting and refitting that seal many times has convinced me of that.
I am now as sure as I can be that the oil is coming from the join behind the back half of the chaincase where it is joined to the crankcase.
It's not a major leak but just enough to be annoying, especially when coming back from a ride.

Brad M wrote: ↑Sun Nov 12, 2023 6:35 pm Thanks Spridder but it's not leaking from the join between the two halves - fitting and refitting that seal many times has convinced me of that.

Shortly after replying to suggest the improved seal I realised that as a fairly long-standing member you'd already know about it so deleted my contribution. Sorry.
I've never had a twin so can't really offer any help other than even if not overfilled the oil will get to the interface twixt the back of the chaincase and the c/case by splash. I know that on the singles there's no gasket at the joint so maybe yours needs checking to see that there's no distortion to the back of the chaincase at that mating point and maybe a wipe of your preferred sealant.
Out of my depth here with a twin but could the oil be coming from inside the c/case rather than the chaincase? Could wet-sumping be a possible cause?

There is no gasket fitted between the chaincase inner and the crankcase with the tin chaincase and the chaincase should be a close fit over the crankcase boss and is further secured by three small screws so an oil leak from this joint should be unlikely but not impossible but I would have thought any leak here would be minimal unless the chaincase aperture is warped or the whole case is loose allowing the case to fret against the boss.
If the leak is however from this joint then you’ll need to remove the whole clutch and inner case and use something like blue hylomar to give an elastic seal between the boss and the rear of the chaincase.

Many thanks Spridder and Reynard. I'm pretty sure this is where the oil is coming from. Although the bike does wet stump, the oil seeping out is fresh and clean - I.e. the oil I just put in the chaincase - and the level falls in the chaincase. Although it is a fairly small seepage, it's enough to be a neusance when back from a ride.
It took me many hours of refitting the rubber band (with appropriate levels of swearing) before I realised the leak was actually coming from the crankcae join .
It's very useful to confirm that there is no gasket - I couldn't find one on the parts list. Is there one behind the clutch? I just want to be prepared.

Brad M wrote: ↑Sun Nov 12, 2023 5:27 pm I have been trying to get to the bottom of the oil seepage from the primary chaincase of my 1954 model 20. I am now convinced that the oil is coming from the join between the back half of the chaincase and the crankcase. The oil comes out even when the bike is standing - it isn't overfilled (just below the level of the chain). The oil is definitely from the chaincase and the level drops both sitting and when used (much more so).

I am assuming that if I want to fix this that I need to take off, and refit, the back half of the chaincase.

However, before i decide whether to do that, I just wanted to check that what I am assuming seems plausible.
Any thoughts would be grateful received.

When you say that oil is coming out even when standing, do you mean just after the bike has been running? Unless it has been standing a long time there should be very little oil in the crankcases - certainly below the crankcase/chaincase joint, and you've already confirmed that your chaincase level is below that joint. After a run there would be oil splashed all around that are that could then be finding its way out. Could splashed oil perhaps be coming out from the gearbox aperture end and running down to the lowest point? My twin has an alloy chaincase with a gasket where it fits onto the crankcase and I don't have any personal experience of your chaincase design I'm afraid.

Malc

Thanks Malc.
The oil is definitely from the chaincase and not the crankcase. It is worse after coming back from a ride with oil dripping from the underside of the chaincase. However if I clean it and leave the bike to stand, there is continued seepage from the chaincase/crankcase join - and none from the join between the two halves of the chaincase itself.
I expect that this continued seepage while standing could either be where the oil sitting in the chaincase is finding a way out or, as you say, some oil that has been flung about on a previous ride having collected somewhere and finally getting out. It is certainly worse when going along. I guess I could see which one it is by checking for a lowering of the oil level in the chaincase over time while sitting - but the seepage while sitting is slight and life is too short to wait . Either way it really seems to be from the chaincase/crankcase join.
I think that I will bit the bullet and take off the rear half of the chaincase to have a look at it.

Bearing in mind there is no seal on the clutch shaft/gear box shaft exiting the primary chain case, the primary case will always leak. I find after a run the oil drips on to the garage floor then stops, put down clean newspaper and the paper stays clean until the next ride.

Just to add some sort of conclusion - it turned out that the lower of the three screws fixing the back half of the chaincase to the crankcase was loose. This is below the level of the oil, hence the seepage. Now tightened, everything seems oil tight (at least for now) and I'm grateful it was that simple.

good job you caught it before the screw started catching on the sprocket