What is it (breather on tappet cover?)

[BSA_54A10]

I'm still not going to mess around with the breathing on my bikes. 46 years of good service and hardly an oil drip is good enough for me.

Oh so you got the good one did ya ?

[Butch (cb)]

Mine has developed a slight leak from the rocker covers this last year so is due some re-sealing over the winter.

[hdawson]

I've seen some bikes with an additional breather drilled from the top of the crankcase and been told that the engine 'spins more freely".
Whatever that means.
I think it would have to have significant benefits to mutilate my case.

[Black Sheep]

"Oh so you got the good one did ya ?" Yes. 2 in fact.

[JulianS]

Like Black Sheeps bike my A10 keeps the oil in - and it gets used good and hard and often.

Back along Webco in the USA and Eddie Dow in the UK produced finned rocker covers with a breather outlet. Tried them but not convinced they had either positive or negative effect on breathing. A simple experiment with a freezer bag sealed to the rocker breather tube showed that more air was sucked in than expelled.

(Damaged) Webco rocker covers below.

[Sluggo]

I will not argue with the "Stock is good enough" crowd, or the "Factory knew best" as if it works for you, then thats all that matters.  But Trev is correct, There is a lot on these bikes that can be improved.  Breather is one of them.   
No lack of testing on other motorcycles and cars and science and technology have evolved since the 1950s,, Point in case, what was thought to be ideal Port and Polish techniques on intakes and cyl heads has  changed 180 degrees.  (Smooth & polished is not the ideal anymore)

Many other brands tend to have their own ideas (Dont go arguing with the Norton nutters!)  But here is a design that has proven to be ideal.   Buell came out with it on production machines and most modern brands now use a variation or something similar, Although, MOST run the venting INTO the intake as this is now mandated in most jurisdictions.  I have updated my older V twins to this Buell type spec with their PCV valves.  That Eric sure did know a thing or 2 about performance engineering.

In high RPM applications, there is proven DYNO verified performance gains with a driven air pump evacuation system, Not totally practical on a old BSA... but the test results are out there if you care to review..With such a small engine a pump design would benefit it, But the HP & Torque would not be that big of a number but the leakage problems would go away for sure..

Here is a typical Auto system... See: https://www.jegs.com/i/Moroso/710/25900/10002/-1  (no pump)
                                                   
 https://www.ebay.com/p/Moroso-22640K-Original-3-Vane-Vacuum-Pump-Kit/15017008969?iid=172256694429&chn=ps                   (With vacuum pump)

I have seen over the years many dyno tests and race car engines can see between 15hp to as much as 40 HP using a vacuum pump design,
Here is a Big Block V8 test (Many youtube videos out there as well)
See: https://www.hotrod.com/articles/ccrp-9903-moroso-vacuum-pump-test/

"  Peak to peak, the pump in its as-run configuration on this engine was worth about 14 lb-ft of torque and 10 hp over the no-system configuration. It was also up about 17 lb-ft and 6 hp compared to the exhaust evacuation setup."

More on the tech,,,,See:
https://www.dragzine.com/tech-stories/tech-how-external-vacuum-pumps-free-up-horsepower/

[bsa-bill]

Correct me if I'm wrong but to some extent will the engine breath via the oil pump and the breather on the top of the oil tank

[muskrat]

G'day fellas.
 I doubt that Bill.
My use of the Bunn system was wholly to do with performance and it did that very well. The motor spins much quicker (quicker throttle response) and it's expelling all the burnt and unburnt fumes (that can eat seals). A much welcomed side affect was the reduction in oil leaks. The swing arms rockerbox leak isn't as bad but will always be prone due to the stupid head steady design, plungers and rigids don't suffer as much due to the steady below the rockerbox. That's a different subject that has been discussed before.
Cheers

[bsa-bill]

Well don't like to boast but at this moment in times my Flash is oil tight

[muskrat]

  don't let her hear that!
Cheers

[BSA_54A10]

The simple physics of it is compressing air requires energy
Sucking air out to make a vacuum or just lower the pressure requires energy.
Moving air requires energy.
Of the 3, just moving it requires far less energy than the other 2.
So a breather that is a flow through design will sap less power from your engine than one that tries to run a low pressure in the crankcase.

What we always need to remember is a lot of the design work that was done pre-computer age was suck it and see because they knew the theory & principles but lacked the measuring & calculating power to optimize .
Now days we can map the flow of oil inside the engine by measuring the difference in the heat on the outside then correcting for radiation & conduction.
Back when A series engine were being designed, the best they could do was cut a hole in the engine and shove a lump of pyrex glass in there and hope to see what was happening before the window got too dirty to see through.

So a breather that was good enough to prevent the entire contents of the oil tank being blown out the overflow or exhaust on one model , got transferred to the next model.
Also research cost money, lots of it, so winning races or running for a long time , things that were major selling points get the money spent on them.
Back in the 40's & 50's a lot of the roads were not sealed kerb to kerb, if they were sealed at all so oil dripping from a motorcycle was not considered a problem, or even a selling point till the Japanese made motorcycles that were bone dry underneath.

Also as I have tried to illustrate, fitting a breather that actually works is a lot more complicated than just making a hole and shoving any old pipe on it.
Thus a lot of the kits of the day did more harm than good.
HD never solved their oil leaking problems till they got Rex on the job and started fitting his modified breathers as standard.
The same goes for RE

[Black Sheep]

Enfield 250 singles had a very small one way crankcase breather that was almost adequate (mine was oiltight up to 52 mph) but when they went for a bit more performance with the Continentals they opted for a great big drainpipe open to atmosphere to do the job. It seemed to work so presumably they drilled the hole in the right place and made the breather pipe the right length. Presumably it freed up a little bit more power at high revs - just before the con rod snapped.

[kiwipom]

Hi guys, difficult to see how sucking air in and blowing air out can require energy
as the piston going up and down does that normally anyway, `pulse` fuel pumps
do that on the likes of some lawn mowers/atv,s and the like, cheers

[kiwipom]

pulse Pumps.

[muskrat]

If you breath in it takes energy (muscle movement), if you breath out it takes energy, if not you die. The piston going down pushes (uses energy) air out of the way. Piston going up creates a vacuum (slows it down, uses energy). 
In a 650cc twin it's breathing in and out 650cc's of air 6000 times a minute at 6000rpm.
Try doing that yourself without using any energy and not passing out!
Cheers