I don't think you have missed a lot here, I assume and of course I maybe wrong but isn't that the little flap valve in the crankcase breather? The originals were make of pen nip steel I believe. How well they work is any bodies guess.
Richard
Number 104 in this drawing,(air breather disc) and allows a small vacuum to build up in the crankcase when the piston is on the up-stroke. Ron
Aha!
That's what I thought. And I suppose that the vacuum then in turn helps to prevent oil from spitting out the breather? Which would explain a thing or to.
Are there any more serviceable parts in there that I might need to order while I'm at it? Been having a hard time finding a proper exploded view of the mechanism (and I live in a different city than my bikes so I can't take it apart and check).
The breather valve disc is made of 'tufnol'...a type of fibre board. It is held in place by a (theoretically) removable fitting which is screwed into the inside of the breather body....
Check first that you can unscrew that fitting...They frequently seize into the body. The theory is the flap valve is forced off its seat by air pressure on the downward stroke of the piston, thus venting crank case pressure to atmosphere...On the upward stroke the valve is sucked onto its seat, creating a partial vacuum in the crankcases and reducing the amount of air sucked back in....
The purpose of the mechanism is to reduce mechanical 'pumping' losses...ie. the loads imposed by the piston having to compress the air in the crank case on the downward stroke if there was no venting mechanism....Ian
Ok, well of course. Now i get it! Thanx Ian.
Didn’t pay much attention to it before. Actually thought it was just a straight open vent pipe. Being used to the more complex disc breather on my Bonneville, and also knowing how Triumph later just vented the engine through the primary.
Anyway, I have had a bit of excess oil coming out of the breather during the short test runs I’ve made. But I think that may be due to the rather heavy wet dumping taking place in between test runs. I.e. that the problem will go away once the crank is not filled up with oil time and time again (se other thread on that).
On engines that do not have oil seals you need to be sucking air in through the scrolls & slingers, otherwise when the crankcase gasses are compressed by the down ward stroke of the piston crank case gasses would be blasted out, in an M20's case, past the dive side main and into the chain case.
The unsprung flapper valve works to around 3500 rpm when it gets out of time with the piston due to the mass inertia of the crank case gasses.
This is why they never drip oil when sitting idleing but will blow oil when reved hard.
So you are saying that a bit iof oil blowing out of the breather during driving is to be expected?
Won’t this then risk getting on to the rear tyre, with a hazardous result?
The Gold Star had the flapper valve replaced with a mechanically driven, timed breather when maximum revs on late models got to more or less 7,000rpm....
At speeds of approx.. 6500-7000rpm the flapper valve ceased to know which direction it was supposed to be going in...M Series engines, the B Series B31/33 models and early Goldies coped OK with the flapper valve in place....
In my experience engines have a tendency to blow a little oil from the breather straight after a rebuild, or more specifically a rebore....After around 200-250 miles this tends to stop as the bore/rings become 'bedded in'...
Wet sumping aside the breather should pass little oil under normal use...For example, I completed a 2,600 mile trip on my 1951 B33 and didn't have to put in any oil, though the level in the tank had dropped a little...Approx. 900 miles of that was motorway driving at around 60mph...
I've done 1000 mile plus trips on the M20 on many occasions and also not had to use any additional oil...
The 600cc tuned B33s I have built that produce maximum power at approx. 5,850 rpm have also all run on the standard breather...The only one I did have problems with regarding leaks/excessive oil from the breather was my 720cc B33 and I suspect that was an air volume issue...The addition of a second flap valve resolved that...Ian
