If even more than that is needed, then there are SU carburetors with a 1/4" brass tube located just before the throttle plates, and these are neat because you don't need a PCV valve, and it draws more vacuum the higher you rev the engine (so you get more breathing when the engine is cranking up the most pressure).

These were used on the '71 ~ '74 Midget A series engines as a way to help reduce engine leaks. The system used a 9/64" restrictor hole in the oil fill cap, as a regulated air supply. (probably used on the mini for a lot of years too, but I don't know which years, off the top of my head)

Norm

My thought is more venting is better.  I have three (four if you count the small hole on top of the rocker cover cap), and gave up on air filters and steel chips inside vent cans as they clog up too fast.

One is coming from my clutch housing.  It wasn't drilled, so I drilled and tapped it and fabricated a stainless breather fitting.  The hose just vents under the car.

Another comes from my valve cover near the top.  I installed a simple stainless hose nipple bulkhead fitting and the hose vents under the car.

The third is a stock Cooper S tappet cover breather with the canister removed.  The canister is filled with the metal turnings (chips) which clogged with oil and gunk.   This also vents under the car.

Even with all of this venting, when I rev the motor I can feel a stream of air coming out of the vented rocker cover cap, so i know that letting these motors breath is important.  I also eliminated oil leaking when I added the breathing.  

pixie , thanks. This is a lot more detail. Much better explanation of the physics and mechanics involved.

I have run a PCV valve on a Weber manifold for 20 years. Initially I had it into one port only, but this mucks up the idle mixture and general part throttle running a bit.
So then I used some 1/4 BSP brass fittings and 3/8 bore hose to make a low flow balance tube. It works fine. The PCV valve is a Toyota inline one. My only breather is from the rocker cover, as I found the drop gears chucked oil up the flywheel housing breather on long LH bends at high rpm. Redline is 7500 currently.
[edit] There is vacuum present with the filler cap removed and my hand on the filler hole, 1360 engine has done 12,000 miles since rebuild. I have no oil leaks.

Recently I have also water heated the manifold to cure carb icing when freeway cruising @ 110KMH and above.

Here's a current pic of the plumbing. Note the vacuum hose goes into the bulkhead, as the brake servo is mounted under the dash.

the gears act like big oil pumps, I had a trani full of oil on the bench with a  dc electric motor and amp/volt  meters   hooked so I could determin  how much  power i was loosing with various  gear settings and clearances  ,when I turned the electric motor on  all the oil in the  trani left in a few revolutions  covered my shop with oil every where.  so all that oil gets foamed around and is in the air   as the engine expells gasses due to valve  and ring leakage   its full of oil mist.  i ran total seal top rings  and did a lot of oil control to learn how to limit the oil foam ,  first was design and  have mini spares make a windage tray, next was  plug the  oil bypass outlet on the bottom of the  block, there is 80 percent of the oil you pump goes out the  bypass  which is aimed right at the crank after it hits the angle on the trani  ,  i drill a hole in the  block to redirect the bypass oil to the  transfer gear case.   pluged teh bypass in the oil  filter holder,  made a oil limit slosh panle in front of the spedo gear to help keep the oil form sloshing around.   ps I used a lexan cover next time and did learn how to half the  power the trani takes, with  gear design and  clearances and bearings .    its in my mini race book at dmrpcb.com . 

IMHO it has a lot to do with the age of the design.  Don't forget the A series started out around 70 years ago at 700 odd ccs.  A top flight A series is now twice as big and pushing out something like 5 (five) times the HP of the original design - in naturally aspirated form!!..but still using the same basic envelope/architecture..  Even a fairly unremarkable road motor can have 3 to 4 times the original output..

Block/bore rigidity probably has the most to do with it... witness the move to thick flanged blocks and then the ribs on the A+ (although I'm told these have more to do with material specs).  It doesn't matter how good the rings are if the bore surface is moving....

I don't see the gearbox having anything to do with crankcase pressure (unlike other issues).  It may impact the amount of oil being carried by the escaping gasses but baffles and large breathers, that reduce the velocity of the escaping gas, can mitigate this.

Years ago I used to feed my breathers into a catch tank but found this filled up with scummy water so now I use the vandal approach - just dump it on the road.  (For track use they usually demand a tank..)

Cheers, Ian

1.  Long stroke

2.  Shared gearbox/oil pan

Check out online images of Swiftune engines and notice the huge vent outlets.  They were not the first, btw.  I'll bet Mini Miglias from 25 years ago utilized various mods designed to allow off-the-chart crankcase venting.  That and they were dealing with hot spots in cooling sytems. 

so someone please educate me as to why a little old mini engine has too little capability to handle crankcase pressure, or too much capacity to make it. If this happens even with good-sealing  rings, then.....

A big block V-8 or V-6 seems to get along with just a single PCV hose. is this because it does not spin up in daily driving to 5-6000 as often as a mini does? Is that the issue - rpms? If that were true, then plenty of today's < 2 liter engines working hard would have the problem.

Lack of windage tray?

oil squirting from an engine bearings onto a thrashing gearbox?

too small an oil passageway between rocker area draining down into the crankcase so that spewing blowby picks up even more oil mist?

what else?? all of the above??? Need a camera inside the beast?

seems all  mini engines have to much crankcase  pressure  in racing we would spit out a lot of all oil,   so i fixed both,   made a small can  with top vent to a oil catch can, two side vents from   valvecover  and lifter cover  to the can ,  a fiber section inside the can   and a bottom vent back to the clutch cover breather,  so any oil leaving the engine  from the vents  drains back into the engine  .  worked great   you can see the red hose lines  and the can  on the left of the engine bay 

By doing the head first it will may only make the ring problem worse you may end up opening it up sooner than later. But with the leakdown test you did it and the results you found it would seem the rings are not that bad though. A puff of oil smoke on start up is either bad valve guide seals and/or worn guides.

CupCake (or anyone for that matter) - do you know if those A+ vented timing covers will work on an A-series metro engine?  I didn't think that was an option so I never researched it.

I did remove it and ran simple green through it for probably 3-4 hours in a parts washer last year. I hope it's still clean....Maybe I will check again.

When I posted earlier, I was not intending to hijack a thread about my problem. The intent was to show that there are other signs that could lead you in different directions. JeffM5150 you posted about this a post later. 

Brief response to Dyelooper:

Have you checked that your vents are actually working? From what I've read the vent cannisters have metal mesh or something in them and  can get grungy (old oil and collected dirt etc.) and not breathe properly.

Have a look at David Vizard's tome.. it desribes a "negative" crankcase pressure system ..and the advantages to be derived there from.

A series engines always seem to have positive CCP.... Maybe its a consequence of the engines huge growth from its original design???

In any case, the problem (as I see it) is basically one of ring sealing (blowby won't go up through exhaust valve stem clearances as the easist path is out through the exhaust).  If you have bad postitive pressure there's not much you can do beyond a rebuild... And if you glaze the bores after a rebuild (not hard) then you just start again..

Even with good sealing you need breathers - especially if the engine is working hard.. 2 or 3 x 1 inch diameter outlets is not unreasonable.  The interior of the engine is basically a single box so the laws of physics tell you it doesn't really matter where the outlets are - as long as they're not being flooded with flung oil - so higher is better. The usual choices are the rocker cover, drop gear case and the tappet covers (if you have a small bore or S engine).  Vented cam drive covers are also an option (but there's not much room).

I wouldn't bother with leak down tests etc.  If you have high pressure then just get rid of it through adequate venting. 

Cheers, Ian

One of these might fit.

//www.minimania.com/part/LJR103470/Timing-Cover-Assembly-For-Twin-Point-Injected-Mini-Cooper