AJS & Matchless Owners Club

Greetings. I'm trying to fix the breather to my crankcase and I do see a small plate. I am now wondering how this operates or how do I even know if this is working right?
The cases are apart and is there anything that I need to check before I start with the restoration

Regards

Hi

The breather is a simple floating disc type which allows the free passage of air in one direction and restricts it in the other. As long as the disc is free to move in the housing and not damaged or distorted the breather should work.

Clean and inspect the release valve, (breather), body STD-836 and release valve diaphragm STD-835, if all OK re-assemble.

You can confirm it's operation by blowing air from either direction - it should pass air into the crank case with little or no restriction - you should notice a definite restriction to air passing out of the crank case.

Regards Mick

Mick D wrote: You can confirm it's operation by blowing air from either direction - it should pass air into the crank case with little or no restriction - you should notice a definite restriction to air passing out of the crank case.

Regards Mick

I'd have thought that it would have been the other way round.... Easy flow out of the crankcase and restricted flow back in.

I agree, the breather is there to allow air to escape rapidly from below the piston during the firing stroke so it should allow air out and restrict going into the crankcase. The little disc can get damaged so its worth checking the function.

Hi

I'm going to have to check with the engine running - I thought the out flow was through the drive side bearing and the inner chain case.

I don't believe either port in the later alternator engine is of sufficient size to allow the venting of the mass of air displaced by the piston when the engine is running.

Regards Mick

clive wrote:I agree, the breather is there to allow air to escape rapidly from below the piston during the firing stroke so it should allow air out and restrict going into the crankcase.

That's my understanding too.
I don't think the designer would have specified the short piece of downward pointing breather pipe if it was to allow air into the c/case as this would have 'sucked up' dusty air from the road (into the main bearings) rather than dribbling oil mist onto the ground.
I've seen bikes with the breather extended into a 'catchpot' that collects the exiting oil.
I see Mick's point though that it's a rather small diameter outlet to cope with the volume of displaced air............although some air must also exit through the main bearings into the chaincase.

It probably works a bit like a pump. Once the engine is running, air is forced out through the valve quicker than it can be sucked back in so there will be a depression in the crankcase. The valve doesn't have to allow the full volume to be displaced on every down-stroke so the small bore tube is probably big enough.

Hi

The crank case must breath to allow gas which passes the piston rings to vent, I don't think it's got much to do with the piston displacement, (why would twins need one if that was the case?)

Regards Mick

Mick D wrote:Hi

The crank case must breath to allow gas which passes the piston rings to vent, I don't think it's got much to do with the piston displacement, (why would twins need one if that was the case?)

Regards Mick

And here's me thinking that both pistons rise and fall at the same time on the twins, i have even timed the wrong piston at tdc. So thats double the crankcase pressure! My experience of blow by from the rings on the single is that the breather then vents an oily smoke on tickover.

However which is the cause blow by from the rings or piston movement, the point of the vent is to allow air OUT of the crankcase not IN.

I'll get my coat

Regards Mick