I am all for supporting vendors, But dont get so wrapped up stepping over a penny to spend a dime. There is a variety of breathers that work quite well and motorsports compatible without buying a Binford 1000 model. The unit that came stock on later model HD sportsters and Buells works quite well, and used on a variety of cars and motorcycles if you cross reference the part number. The metal one on the link Musky provided is a common variety as well and IIRC is specced for lawn mowers and Yamaha ATVs???
Rex was commisioned to fix a HD breathing problem.
Not sure if their breathers are part of his work or not but he was getting royalties from quite a few companies.
As for auto PCV valves, brake check valves, duck bill valves, even BSA's own preunit flapper valve, they all work to some extent and some better than others.
But none of them work all the way from idle to valve bounce.
Just like end fed cranks, you don't have to do it & for most BSA owners the side feed through the main bearing will be good enough.
So weather you need one or not will depend upon your particular bike, your riding style and your ego.
If some oil leaking out the rocker box covers and a lot more from the various orrifices does not bother you, and it does not bother me then the bike will be fine just as it is.
After all it is your bike & you are entitled to fit or not fit whatever you please.
However they do work and they work much much better than anything else on the market.
The research, facts & figures are all there and if you can't be bothered to go through the 500 pages of blog, then you can buy the book.
The early one has all of the details & research results & to be quite frank is a difficult read
The second was a distilled version of the first with the information most 1/2 competient owners would need to fabricate their own.
The only thing he did not give away was the brand of the only material tested that did not suffer from resonance.
The M20 for instance is fairly well oil tight to about 4000 rpm.
After that it covers everything behind it with oil.
But peak torque is around 3600 so there is little use in valve bouncing it.
There are a lot of things that can affect both breathing & the amount of oil that is expelled.
A customer had course to run his new Triumph at & over redline for extended periods.
When it was safe to continue on, he got about 5 miles before the engine locked up solid
Why ?
Because ring flutter had pumped the entire contents of the sump out the exhaust pipes.
Indeed as we were pushing it up into the van, oil poured out of the muffler.
We thought it must be a cracked head, but the head was fine, the gaskets intact but the head was coked worse than a WWII tractor running on kero.
BSA knew the flapper valve did not work particularly well which is why the Gold Stars got the timed breather off the magneto drive and eventually the tacho drive as well
However right up till the B31/33 series got replaced by the C15/B40 the flapper valve remained on the road versions.
