If you want the full explanation then go to Rex's blog. He has left it on the web for the benefit of other Pommie bike owners.
The whole story goes like this.
He bought a job lot of unit singles and kept a B44 & B50 for himself.
After a rebore & two attempts at seating the rings he found oil emerging from under the head nuts ( B50's are through bolted ).
This is because when BSA made the studs thicker they did not move the centres out, just drilled bigger holes which end up being too close to the mouth of the crank case.
They oft leak due to failure of the cylinder base gasket, failure of the studs to make an effective seal agaist the crank cases or the case itself cracking between the mouth & the stud hole.
The usual trick is to pull the studs , drill a small counter sink & fit some really thin O rings. or acorm nuts on the head with copper washers underneath.
Rex was not interested in pulling the motor apart again and wondered why there was enough pressure to force oil all the way up the stud and then out from undeer the nut.
What he larned was breathing theory had never been investigated properly from the days of oil slingers and while it still stood up for slow reving bikes with no oil seals, it did not stack up for modern bikes with modern seals.
With oil seals fitted there is no reason for having negative pressure in the crank case in fact it robs the engine of power, up to 2 Hp on some engines. ( Got your attention Musky ? )
He then went off on a crusade to fix this problem, not treat the symptoms as my "fixes" did.
Thus he found out the best practice was to allow air in then allow it back out and by doing this blow by from the rings was almost eliminated as most of the blow by happens at or near BDC as the piston rises but not enough to have compression force the rings to seal against the bore and of course on exhaust stroke, the piston rings do not make an effective seal and any reduced pressure under the piston, sucks the gas past the rings into the crank.
Even worse for timed breathers, because air actually has mass, it has a resistance to being moved and will compress slightly before it starts to move so the pressure pulses inside the crank end up being out of phase with piston movement so timed breathers only work properly over a speciffic range of crankcase speeds and these will change with bore : stroke ratios.
Flapper valves ( as fitted to the singles ) had the same fate. Up to around 2000 rpm the M series valve works very well however by the time you get to around 4000 rpm it is "fluttering " 100% out of phase with piston movements.
This is basically what the strokers found out with transfer ports & why they spent a fortune developing variable port engines.
Rex tested every available check valve ( brake valve to the USA readers ) to try and find one that worked from 500 rpm to 7000 rpm in an engine . ( guess who was a guniea pig for the preunit singles )
Every one on the market ( the complete market, not just automative ones ) failed miserably.
So thus the design of the various valves and eventually the entire kit.
The hope was to make a simple drop in replacement unit but that just did not work.
Further more the exhaust tube should be treated ( and thus tuned ) exactly the same as an exhaust pipe ( listening Musky ) to gain the maximum efficiency.
At one time we actually fitted some clear tube and watched the pulse nodes traveling up and back down the tube.
Now if you are thinking of making your own reed type valve, the teflon coated silk fabric that they make the impulse pump diaphragm valves out of would be about the best material you could find without going to the ultra expensive stuff that he used.
