Mk3 primary chaincase breather ??

I have a set of "special" Mk3 cases that have the BSA timing port cast on and drilled through the case chest as well as the standard Mk3 that's on the side.

And, they have a hole drilled through drive side case into the primary side to allow the engine to breath into the primary drive case and engine oil to flow in and out thus maintaining the primary oil level. Perhaps that is why there is a breather pipe coming out of the primary cases?

Both 650 and 750 engines can suffer from poor engine breathing. The tunnels and tubing to assist the departing crankcase gases being fine for the 500cc  and 600C engines but not for the longer stroked / bigger versions. The later Commando motors had variations of devices to assist in controlling the issue and to some extent these worked but a fundamental problem remained. This being the crankshaft oil seal which, like tyres, appears to have a limited life span.

Once worn, this seal generally lets oil through it on the engine downstroke while the upstroke creates a partial vacuum that can suck in oil from the primary case and even the gearbox.

I suspect that Stan's Commando has a modification to allow his bike's primary case to help overcome pressurising issues.

Possibly related.......I have fitted belt-drive conversions to a number of 650 and 750 Dominators and on all of them found that the crankshaft oil seals wore out quite quickly. 

I wonder if it was an attempt to emulate the crankcase breathing arrangement on the late C range Triumphs?  The crankcase breathes through the drive side mainbearing in to the chain case, then through a fitting on the chain case to the oil tank/atmosphere.  There is a row of small holes between crank and chain cases to allow excess oil to drain from the chaincase back in to the crankcase.  Worked very well indeed, I'm copy it on my lightweight.

I think it was the C range that got it first, then the B range?