CC003 POST-WAR OIL LEAKS/CHAINCASE

I have very recently purchased a 1960 Venom; the engine, gearbox and clutch were restored by a reputable Velo restorer about 5 years and 800 miles ago ( I have receipts for the work) and it runs sweetly. Except that oil has been escaping in large quantities from the outer primary felt oil seal area, and I noticed upon investigation that there is a gap of about 1/8 to 3/16 inch between the felt and the outer clutch face! In effect, no seal at all!
I am puzzled because the inner case is attached firmly to the gearbox and the outer case in neither displaced outward nor distorted, so that the final drive chain alignment is fine and does not foul the case. So, the case looks OK
So I turned my attention to the alignment of the clutch mechanism itself. A check with a straight edge shows that the clutch chainwheel teeth are perfectly in line with and parallel to those on the crankshaft sprocket and everything here appears to line up well. The chain run is perfectly aligned and a clutch 3/16 inch out of alignment would, of course, create havoc here.
I have the engineer's notes on restoring the clutch and the only mod he made is to replace the nearside thrust race with roller type.
So, clutch alignment and chaincase alignment seem fine. So, where does the 3/16inch gap come from, and what can I do about in, apart from using a thicker felt?
I really am stumped. Your advice would be very much appreciated; I have owned the bike only for three weeks and am on the steep part of the learning curve! - Roger Hinton 14/3/2002

Very interesting.....
Firstly, if there's lots of oil coming out it may well not be just because of the gap you describe (unless the chaincase had more than the required eighth of pint - or is filling up from the crankcase breather).
It could be coming down the mainshaft, particularly if the gearbox is a bit too full and the sleeve gear bush is a bit worn.
However, let's assume it is all from the chaincase via the clutch outer plate.
How far is the case from the outer plate - i.e. the rim of the channel into which the felt seal goes rather than the seal itself?
Although in theory it only needs to be a 40 thou gap because this is all the lift needed on a perfect clutch, it needs about twice as much to allow for the tilt during the first revolution, and will widen as the linings wear a little in use. So this gap can certainly be 3/16. This is the gap that the felt supposedly seals. As you'll have seen in the other Q&As it is quite a sensitive point - too stiff or thick a seal and the gearchange is affected.
The club's silicone ring would bridge 3/16 OK, but it is delicate and might pucker and be damaged unless fitted with lighter grease etc than according to the instructions supplied with it. Or the previous version in neoprene that was a bit too stiff for some gaps could be fine at 3/16 - the spares section probably can still supply on request.
Is yours a screw together or strap together case?
There was some variance in their dimensions - and a screw type can be lightly nipped up with a fresh cork gasket (i.e. just right) or over time retightened as the gasket compresses by over a 1/16th.
One distinct possibility is that the chaincase distance piece (around the bolt in the middle) is too long. The two halves get squeezed together by the bolt quite appreciably - if the distance piece is 1/8 too long, then you will get nearly 1/8 more clearance at the front side of the seal - less at the rear, and little difference on the rear chain clearance. I think the length of the distance piece is in another Q&A.
Otherwise your setup seems fine - a Venom rear chain should almost rub on the chaincase, and the other main thing that matters is the primary chain line, and yours sounds fine.
So there'd be nothing wrong with making up a thicker new seal (evostick and part of another seal), or use a neoprene or silicone one.
If oil leakage then reduces (it won't stop completely) forget about it. If it doesn't but you can see the seal is working, move on to the mainshaft leak by testing with only half a pint of oil in the gearbox for a few miles of careful riding, etc (after checking the chaincase oil level isn't the cause of course) - Tony Wright 15/3/2002

Many thanks for your prompt reply, which alerted me to the fact that a minimum gap MUST exist between the felt seal and the outer clutch plate, to accommodate the outward movement of the plate during clutch operation! I will endeavour to tune things so that the new felt minimises oil loss here without restricting clutch operation
I have no reason to believe that oil is entering the chaincase from the mainshaft, not least because the gearbox has been rebuilt, but I will check. When I acquired the bike a few weeks ago and drained the primary chaincase, it was full of the sort of emulsified goo that can only be found in a chaincase in a British winter - no sign of bright, clean stuff
The rim of the felt-retaining channel is between 3/16 and 1/4 inch from the clutch. The case is, incidentally, the screw-type.
I have no means of knowing whether the distance piece is the correct length - it measures 28mm (1.125inches) Does that seem about right? It looks OK inasmuch as the gap to which I refer appears to be of about the same size right round the perimeter and the outer case shows no sign of bowing in or out
I will now try packing out the channel within which the felt seal sits with a 1/8inch thick rubber gasket, before sticking the felt in place, in order to bring it into closer proximity with the clutch, but am now, thanks to your reply, aware of the danger of closing the gap entirely and hampering clutch operation. As you say, some fine tuning is needed here and I will, in the meantime, enquire of the spares section about obtaining a neoprene seal, which I can then try if the new felt doesn't do its job entirely, but is a move in the right direction.
I have now learned that some degree of oil leak is probably inevitable, that careful tuning of the cork gasket and/or felt arrangement can probably minimise it, if not cure it. But most importantly, your note has confirmed that the gap which I see is not evidence of some profound transmission alignment problem!! - RH

As discussed in Q&A CC002, the distance piece FK205 is 1.125 inches long, 1.250 inches outside diameter and 1.125 inches inside diameter.
So yours sounds right.
Ideally, the felt seal should just kiss the clutch when the clutch is in, and it should compress easily enough to accommodate the movement of the outer plate, which would be about 1/16" except that owing to the tilting action of the Velo clutch it is about twice this initially until a revolution has been completed but it won't be a problem if the seal is pressing on the plate for this revolution.
The problem is that for a long period the felt seals were made of too stiff a felt, which barely compressed.
Last year the club had some made from the original Velo spec felt, which is soft and compresses easily.
However, if you only put 1/8 pint in the chaincase and got some back out when you drained it, it doesn't sound like too much is leaking out from there.
A little oil does spread a long way, and looks worse than it is - look on it as an automatic rear chain oiler, and try and ignore the rear tyre oiler that comes with it. - TW