Good morning all. A couple of years ago, Julian suggested getting a plug screw which has a baffled breather, or drilling a small hole in the existing one. (My chain oiler is blocked off.) Any comments on this, anyone? I suppose the reason is to release pressure which might otherwise force a leak, but what I don't really understand is: what is the difference between a breather hole and a leak? Phil
The problem is that if the hole is needed to release built up pressure then oil mist will find its way out through it. Don't forget that there's a lot a fast moving parts within the chaincase and these will splash oil all over all the internal surfaces. To avoid oil leaks, it needs any breather to vent into an expansion chamber with a drain hole back to the sump or to vent back to the oil tank (or to the engine carb inlet).
The BSA B50 breathed through the chaincase because the original timed breather was not able to cope. However, this incorporated a relatively large vent pipe out of the top of the chaincase back to the rear mudguard to prevent oil from seeping out.
Any advice re a 1965 B40F?
If you're worried about chaincase pressure build up, I would be tempted to refit the rear chain oiler if you still have it and connect a suitable rubber pipe to allow it to vent under the engine for test purposes.
My advice same as before - noting that the 1969 season on unit single B25 and B44 were fitted with such a plug - part 70 9800.
Photo below shows both sides of such a plug.
How much oil comes out of it? It is a deliberate leak, isn't it?
Releases pressure, not oil.
Easy enough to try with simple drilling a 1mm hole in the plug.
Below is what the US BSA importer recommended for excess oiling of rear chain for the A65 back in 1963. Same theory for the unit singles.
I used such a plug on a Starfire and a 1961 B40 with good effect.
Currently using one on my A10 primary chaincase.
What it will not do is cure an oil leak due to worn seals, damaged bush/seal joint or poor joint surfaces.
Haha, no, I wasn't expecting it to stop my leak. Thanks for the information, Julian. Phil.
As said by AJWDThumper, "To avoid oil leaks, it needs any breather to vent into an expansion chamber with a drain hole back to the sump or to vent back to the oil tank "
The 69 and -70 B44 breathed plug does exactly that. The three holes on the inside venting into the small chamber in the plug. The lowest of the three acts as a return for oil. And the hole to the outside releases any internal pressure. Normally there is very little if any overpressure in the primary.
I don't remember noticing any oil coming out there on my first B44VS. But that was a long time ago.
So you agree Julian's plug is a good idea?
I had to search all my unit singles to find a vented chaincase plug but in the end found one on a 1971 B25. As can be seen, it is quite a sophisticated design with a cavity on the back with 3 small holes on the inside and one small hole to the outside. Presumably designed to prevent oil from escaping. It's not a breather in the usual sense but probably designed to release a small build up of pressure over time.
Interestingly, I found another plug without the cavity but the small hole had been plugged by a brass insert. Probably found it weeped oil on to the front of the chaincase!!
You sound less than convinced, AWJDT.
Phil
I plugged up the oiler on my A50
I bought a vented cap from one of the usual suppliers and as said it is not a cap with a hole drilled into it. There is a baffle with 3 holes drilled in it. It works fine - no oil escapes from it or the chaincase.
Cheap to buy, widely available, and works.
I think that, even though I didn't feel any pulsing/pressure with my existing cap off and my fingers to the hole whilst the engine was running, I'll probably get one. Adrian, when you say it works, I guess what you're saying is it doesn't leak oil but must allow pressure to pass. Presumably you don't actually notice any difference? Phil
FOR REFERENCE.....should this thread be googled.
Beware:a baffled vented cap ( as shown in picture upthread) if fitted to the clutch inspection hole is likely to foul with the clutch centre adjustment screw..... with potential to leave swarf in the chain case. Remember (as I didn't) that when you pull the clutch this screw moves towards the cap.
This type of cap was fitted post 67, apparently, on cases with a 3rd (higher) inspection hole.
So follow Julian's advice ( upthread) on 1mm drilling of cap, for earlier chaincase covers.