AJS & Matchless Owners Club

Dear all

No doubt discussed before but I am interested in members views about engine breathing.

In my younger days of BSA and Triumph twins one of the first things I always carried out was a modification to the rocker box/tappet covers to allow a breather to be attached and taken out via a long tube to beyond the rear number plate. The wisdom of the day was that by reducing engine/crankcase pressure there was less chance of oil leaks particular at the top end of the motor.

I was a little surprised therefore to read in the AJS/Matchless literature that crankcase pressure is in fact desirable hence the crank engine breather which maintains a modicum of crankcase pressure. What is the advantage of this against having a breather system which elevates it more completely?

Interesting. Obviously (?) a c/case needs to be vented to release the pressure from piston blow-by and that from the descending piston.
I found this ont' Web.
To paraphrase.........

Crankcase pressure

Positive crankcase pressure regulates the amount of air passing between the oil control rings and the cylinder liner into the crankcase. A slight pressure in the crankcase is also desirable to prevent the entrance of dust through the c/case breather. A loss of engine lubricating oil through the c/case breather, crankcase ventilator or dipstick hole in the cylinder block is indicative of excessive crankcase pressure.

I only have single cyl engines on which by design the c/case breather pipe exits at road dust level, although some folk extend the breather pipe to a catch pot or to the rear chain or beyond.
Since our old singles (and Twins?) don't have any 'soft' crankshaft oil seals the likelihood of excessive c/case pressure is negligible unless some part of the engine's technically knackered.

Many thanks, what might cause excessive crankcase pressure, and how does the pressure in the crankcase manage to find its way into the crank in order to vent out the end?

Should the crank breather - if it is operating correctly - help to reduce any oil finding its way into the chaincase?

I'm sure all this will have been discussed before but simply trying to understand how these things operate on our make of machines.

There are lots in the Tech Articles, here in our discussions and in Cristian's Archives. Doing a search will bring up tons of bedtime reading! On the twins the drive side shaft has a hole all the way through to the chaincase (except early models) . The one way flap valve in the release valve controlls the pressure. On the heavyweight singles a hole on the crankcase, near the drive side main bearings, leads to the release valve with it's flap valve.
Dave

chris goldson wrote: ↑Sun Sep 19, 2021 1:25 pmMany thanks, what might cause excessive crankcase pressure,

In the main it's due to piston blow-by from the combustion chamber as a result of a worn cylinder bore and/or piston rings, or a broken piston ring.

and how does the pressure in the crankcase manage to find its way into the crank in order to vent out the end?

I don't know as Twin engine are not my thing and the venting on singles isn't via the crank.

Should the crank breather - if it is operating correctly - help to reduce any oil finding its way into the chaincase?

Yes, because the pressure will seek an easy exit and this is usually via the unsealed crank axle bearings and into the chaincase. Fairly frequent questions on here are from owners discovering oil 'filling up' the chaincase.

I'm sure all this will have been discussed before but simply trying to understand how these things operate on our make of machines.

You're welcome. My comments are just opinion based on a bit of formal learning, previous posts on here and a few years' experience.

Surely putting a pipe off the breather to the rear chain, the exit will be above the level of the standard metal pipe exit.
This would allow oil to pool and block the breather. So the engine will no longer have controlled breathing. Excess pressure will cause air / oil to seek anywhere it can exit.