Hitchcocks Forum

Hello chaps. I've just started stripping down an old 350 Mag/dyno bullet engine and noticed there wasn't any crankcase breather outlet. I looked into the "problem" and discovered that this early type of case vents into the primary chaincase via some sort of gap in the drive shaft which aligns on the downstroke. I'd never seen this set up before. Does anyone have an opinion on its effectiveness. Why did they stop doing it. Did it not work properly, or was it just cheaper to send the pressure straight out through a pipe? Sorry for yet another crankcase breather question. Spech.

According to Steve Wilson ( Royal Enfield Motorcycles Since 1950) the oil mist blows straight into the clutchcase through a special sprocket nut. These fumes can cause the clutch plates to stick and also don't sufficiently relive crankcase pressure.

Sent too soon! This didn't help general oil leaks so the development was the banjo bolt breather just beneath the cylinder and onto the rear chain. I have no personal experience, just read it in the book, hope this helps

Spech, the vent actually terminates in a special breather cast into the outer primary chaincase. This vent points downwards and looks quite vulnerable, especially if you're doing any off roading! I rebuilt one of these engines years ago and got it together as a bitsa in a late frame. I didn't have it long but I can say it used to p**s oil out of the hole in the case! I'm not sure if there should have been a pipe to lead the oil somewhere but from experience I'd say find another way to vent it (maybe our Host's oil tank neck conversion?) and block the hole in the crank as the later engines did.

REgards, Mark

Thanks very much for the advice chris and Mark. I think that's pretty conclusive; I feel a "mod" coming on.

I think that this type of crank breather was used up to 1952, when it changed to the large breather at the rear of the drive side crankcase.
There should be a banjo bolt with a breather pipe leading from the bottom of the outer cover. There is a cork seal on the nut at the end of the crankshaft to prevent the crankcase venting into the primary case, I'm not sure how effective it is though.

The breather at the end of the crank was in hind site the best breather of the lot, and was reverted to for all the latter twins. Not only was it effective but mostly because it carried next to no oil with the fume.


The critical crankshaft sealing feature of your early engine is the cork 'oil seal' inside the outer end of the crankcase. Its fit and finish is crucial otherwise 'some' (too much) oil WILL migrate into the primary chaincase. Excess oil needs to be checked and drained on a regular basis otherwise it DOES effect the clutch operation, and will eventually end up being blown out of the primary chain-case breather vent.


This situation was worse back in the day with the use of cork inserts in the clutch plates.


If you want to optimse engine breathing and ease the work done by the 'main' breather, fit a breather off the Exhaust rocker cover. Inconspicuous.


As the ex valve guide wears, blow-by between the ex valve stem and guide is a major source of internal pollution and fume.