Oil leak

[Phil C]

I'm looking for advice again, please. Having recently changed the oils on my 1965 B40F, this morning I took it for its longest run since I've had it - about 8 miles. It was great fun, much lighter than my 2011 Bonneville T100, albeit a bit slow on a long straight (even though I'm probably the slowest motorcyclist in Devon!) Anyway, I got home, put the bike on its sidestand, and when I came back to it later I noticed an oil leak.  I wiped the oil up off the floor and from the underside, and couldn't see much if any oil coming out anywhere, but thought it was coming from the primary chaincase, so I topped up the level and took it for a run up the road and back. When I got home the oil appeared to be coming from the bottom of the primary chaincase, a couple of inches or so  behind the drain bolt, so maybe an inch forward of the the bolt next to the brake pedal.  The leak seems to slow or even stop when the bike is left to cool down. 

I haven't had this leak before, so I'm guessing it's because I've disturbed the seal by taking out the drain bolt and the oil level bolt? I've checked the tightness of both those, and the one behind the drain bolt, near the brake pedal.

Three questions:

1. Am I likely to be right about the reason for the leak?  If so, does this mean I'm likely to have the problem each time I check/top up/renew the primary chaincase oil level?
2. Why does the leak slow down when the bike is left standing? Is it because the oil thickens a bit as it cools?
3. Do I need to take the casing off, clean the joint, and re-fit it?

Thanks in advance again.   Phil C.

[AWJDThumper]

If the leak is coming from the bottom of the primary cover or one of the bolts or screws then it probably won't make a lot of difference to the amount of leaking on whether the engine is running or is switched off. However, if the leak is coming from the seal behind the clutch then you will only experience this leak with the engine running because it requires the clutch to drag the oil up and over where the seal is..

[AWJDThumper]

It might also be worth checking if you've got a rear chain oiler fitted to the rear most chaincase screw as these are designed to leak oil when the engine is running.

[Phil C]

My apologies, the leak is not where I thought. Here is a photo taken from the other side of the bike. You can see three vertical runs. Any ideas?   Phil.

[ducati2242]

Could be either from the seal behind the clutch or the gearbox sprocket nut . They now make a nut with a built in oil seal to help seal said nut . This is the one I use on my b34 . https://www.ebay.co.uk/itm/BSA-Gearbox-sprocket-nut-with-oil-seal-67-3053/272521755139?hash=item3f738ffa03:g:Cq4AAOSwGvhT4Phe

[AWJDThumper]

Unfortunately, it's difficult to tell whether its chaincase oil leaking past the mainshaft seal behind the clutch or gearbox oil leaking past the seal on the gearbox sprocket. Either way, to solve it might require the clutch to be removed together with the inner cover plate and gearbox sprocket to get at both seals and to inspect for a problem.

[Phil C]

Crikey. That sounds a bit major for someone like me with no previous experience and no specialist tools?   Phil.

[AWJDThumper]

I would see if you can live with the leak for the time being provided the oil doesn't get on to the chain and start to coat the back tyre. A few drips of oil on the floor after a ride isn't a big issue and, if it's from the gearbox or chaincase, you're not going not going to be loosing that much oil.

In the longer term, if the problem is a failing oil seal and the engine hasn't been looked at for a long time, it might be due for a complete overhaul.

[Phil C]

It was more than a few drips - maybe half a cup full.

Couldn't be anything simpler, could it? Anything connected with me changing the oils?

If it is a seal, is that beyond me, would you say? (I know you don't know me, but we've conversed a fair bit on this forum and you might have a very rough idea of my capabilities?) I don't have any special tools, just ordinary hand tools, a garden shed, a small work bench and a vice.

Phil.

[AWJDThumper]

I wouldn't expect it to loose that much oil from the clutch and sprocket seals unless they have failed completely, especially the latter. Given that you were going to put EP90 oil in the gearbox, can you tell whether it is a cup full of engine oil or gearbox oil that is leaking out from its viscosity?

It's not a difficult job to get at the seals. You need the correct size of Allen key to take the primary cover off and the correct size sockets to undo the LH crankshaft nut, the clutch centre nut and the gearbox sprocket nut. You will also need a clutch centre extractor tool and a clutch locking plate. Unfortunately, BSA designed their unit engines so that changing the gearbox sprocket was a tedious job which is essentially what you will effectively be doing.

[Phil C]

I've just checked the levels in the gearbox and the chaincase. The gearbox oil ran slowly from the hole as soon as I took out the bolt. o that must be full. The chaincase ran out of the level hole after I put in about an egg cup full of oil. So maybe the oil is coming from the primary chaincase? If it were the seal behind the clutch, would the oil leak from the primary chaincase, and would it show as the three vertical runs of oil are on the photo (attached again below)?    Phil.

[AWJDThumper]

In the case of either of the two seals, the oil levels should be below the level of the seals and therefore it would only be oil splashed upwards by the moving engine parts that finds itself near the seals. Unfortunately, the gearbox seal has more work to do and is more likely to have failed.

That said, the rotating gearbox sprocket and chain is in the way of gearbox oil getting to the back of the primary chaincase. However, the worst case scenario is that the gearbox oil is leaking out between the clutch shaft (gearbox mainshaft) and the bush in the output pinion on which the gearbox sprocket is mounted. This leak would be effectively next to the back of the clutch seal. Therefore, what you see on the back of the chaincase could be still either engine oil or gearbox oil.

Until you take off the clutch and the cover plate behind it, you won't really be able to tell where the problem actually lies. It might even be the cover plate leaking.

[Phil C]

So if I were to take off the clutch and the cover plate behind it, could I reasonably hope to find the cause?  For example, if the problem is the clutch seal, would I be able to see that?  Phil.

[AWJDThumper]

Once you've got the clutch and cover plate off, you can pop out the seal and slide it on to the clutch shaft to see whether it's a snug fit or not. If the gearbox output pinion bush is badly worn and leaking oil you will be able to check this via excessive sideways movement of the clutch shaft in this bearing. To check the gearbox seal, you will need to remove the sprocket. Normally, it will be pitting of the sprocket journal under the seal that is the cause of this seal leaking.

[Phil C]

Thanks very much, AWJDT. Much appreciated.  Phil.