Finding the source of an oil leak

[Phil C]

I'd like to get this oil leak sorted at some stage, maybe when the summer is over. But I'm not sure how I'll go about it. Stopping a leak is one thing. But finding where it's coming from is clearly not always easy. And I guess it's hard to stop it if you don't know the source, and not knowing the source is frustrating. I just took the bike (1965 B40F) out for about two miles, and took this photo when I got back. You can see there is an oil leak from somewhere. I have been thinking it might be coming from the alternator wires grommet in the primary chaincase, but am not sure.  Maybe, maybe not. I think I'll need to buy a mirror on a stick to perhaps get a better look (been thinking of getting one for a while now.) Maybe it's from the primary chaincase gasket (gasket has been replaced and the allen bolts are done up very tight, but I'm not sure if they should have to be so tight.) It doesn't appear to be coming from the sump plate - that looks fine.  And maybe not from the gearbox plug, but not certain. Any advice please on how to locate the source of an oil leak? And any ideas on where my leak is?  Phil

[AWJDThumper]

It's always difficult to find the source of this type of oil leak. What would narrow it down is if you can tell whether it is engine oil or gearbox oil. One of the classic sources of oil leaks in this area is from the seal around the drive sprocket which becomes especially noticeable when you leave the bike on its sidestand.

[Phil C]

I think I probably replaced that seal some time ago when, with much guidance from, I think, you and others on this forum, I went in search of the leak and ended up doing quite a bit of work inside the primary chaincase, eg taking the clutch off. This was, I guess, a couple of years ago. I learnt a lot, but still have a leak. Is there a way of telling which oil it is?  Phil

[Phil C]

Checking back to summer 2018 on this forum, I replaced the seals behind the clutch and at the gearbox sprocket. Is that the one you mean, AWJDThumper?  Phil

[Phil C]

 I think I might do something which I think you suggested back then: drain the primary chaincase and take the bike for a run, and see if the leak is still evident. Would doing that cause any damage, for example to the clutch?  Phil

[AWJDThumper]

That test would be worth doing. The oil is mainly there to lubricate the primary chain but I don't think going for a shortish ride will cause any harm. In order to determine whether it is chaincase oil or gearbox oil, I would be tempted to add a dye to one of them:

https://www.demon-tweeks.com/ring-uv-dyes-250947/?sku=RINRLD4&istCompanyId=a2904180-3a7d-4e56-b876-cf81c9512180&istFeedId=6fbc4b04-fd28-4ce1-8513-835c8f118690&istItemId=wptrtapap&istBid=t&gclid=CjwKCAjwxqX4BRBhEiwAYtJX7ZFuCvbAJGgCyyv-IRsrFyIEnfRCBm8NiXzyzdwTX_WOhNLg0zmdYRoCeg8QAvD_BwE&gclsrc=aw.ds

[Phil C]

Thanks AWJDThumper. I had thought a dye would be useful, but didn't know they existed. Anybody used this dye?
In the meantime, I'll drain the primary chaincase and go for a short ride.
Phil

[AWJDThumper]

I believe the dye is designed for this purpose and is therefore harmless to engine. If you add it to the chaincase oil for the test then it is easy enough to drain and change the small amount of oil afterwards. Slightly more involved if you need to test whether it's gearbox oil versus engine oil!

[Pete A10]

I know when I took a previous bike for an MoT it had a leak. The guy doing the test used powdered chalk.
Totally clean the area of the leak then dust the area with powdered chalk over it. Take it for a run and then as well as seeing where the oil is dripping from you should be able to see track through the chalk back to the actual leak position.
Obviously you need to have eyes on the whole area which will mean a mirror or lying in the mud under the bike.

[AWJDThumper]

You would have thought that chalk powder would do the trick but the problem sometimes is that you get so much spread of oil from the leak that it doesn't generate a clear track. This is likely to be particularly true if the leak is around a rotating part as is probably the case here and also when it happens when the bike is being used rather than when it is parked up. I've used the technique on a cylinder head leak and it was impossible to see where it had originated from the oil soaked chalk powder due to slipstream spreading - in the end I had to replace the engine! That all said, it certainly does no harm to try the technique - it might work in a particular case.

[Phil C]

Interesting. Drained the primary chaincase oil. Went for a ride - just about a mile or so.  Had a look when I got back. No evidence of the leak. So, that they d progress. It looks like it was coming from the primary chaincase. Just need to establish exactly where from. I did find, when I took off the footrest, the chain oiler Allen screw was a bit loose, so I've tightened that ( some time ago I replaced the oiler with a nut and fibre washer.) I'll now put some oil in again and see what happens. Maybe I'll use some dye, as a double-check. Trouble is I'm colour-blind!  Haha. Seriously, I am. But colour blindness does not necessarily mean unable to see any colours, so the dye might be useful.    Phil

[Mike40M]

You could try using ATF oil. Already red. Though there are conflicting opinions on using it in primaries.

[Phil C]

I thought I had just posted an update on this, a few minutes ago, but it seems to have disappeared. So here goes again. Refilled the primary chaincase (pc) with 1/4 pint of oil, went for a spin down the road and back, and had a look. Sure enough, there were a couple of dribbles of oil running down the inboard external wall of the pc. That supports the theory that the leak is coming from the pc, but I need to know where exactly. I noticed the Allen screw which used to hold the chain oiler ( now has a nut and fibre washer instead) was loose, despite having been tightened earlier today. Presumably the vibration of the ride had done that. So I need to think about how to stop that happening. Could it be the source of the leak, I ask myself. The oil didn't appear to be coming from the gasket area. But could it be working its way along the thread, past the fibre washer and nut, and creeping out that way? Hmmm... I wonder. The oil looks to be a bit too spread out for that, but I'm not sure. Need to look again. I guess. Any thoughts much appreciated.    Phil

[AWJDThumper]

Your original picture showed oil on the bottom of the gearbox - it's hard to see how chaincase oil could have found its way across while at the same time not leaving similar evidence on the bottom of the chaincase?

[Phil C]

When I took the original picture there would have been oil on the bottom of the chaincase as well. But after cleaning it off and a short ride today it was on the inboard face of the pc but not on the underside of it. Not enough of it produced on such a short ride, I guess. Anyway, I think the ex- chain oiler bolt is no longer a prime suspect. I've just been looking again, and the vertical dribbles on the pc wall are too far forward for that. They are about in line with the clutch. I see them running down where the bottom half of the circular plate is, but the upper half of the circular plate looks clean ( I think). So I'm now thinking it might be coming from the clutch/ shaft area. I had the clutch off two years ago ( hardly any miles since then, and the leak's been there for ages.) I replaced what I thought needed replacing, but can't recall exactly what. A seal was one thing, and a fibre washer which I think was missing. I seem to recall the felt washer was loose, and am now wondering if that is an issue. Do you think pc oil could be leaking via the clutch/shaft to the outside?  Phil