Crankcase vent valves - value added?

I fitted a Mike's XS breather valve to my MKIIA 850 as part of the restoration, and it's been oil-tight since I built it 4 years ago.
I recently swapped the oil tank due to a broken rear bracket, and in doing so I inadvertently disconnected the pipe to the PCV.
Huge embarrassment at the Leyland VMCC show, where my bike was doing a fair impression of the Torrey Canyon - oil everywhere

I got home, found the problem and fixed it. Took it out for a ride and there's still oil everywhere - the head gasket it seems. I haven't stripped it yet though.

So it looks like as soon as the PCV was disconnected, crankcase pressure did the rest.

B+Bogus said:

I fitted a Mike's XS breather valve to my MKIIA 850 as part of the restoration, and it's been oil-tight since I built it 4 years ago.
I recently swapped the oil tank due to a broken rear bracket, and in doing so I inadvertently disconnected the pipe to the PCV.
Huge embarrassment at the Leyland VMCC show, where my bike was doing a fair impression of the Torrey Canyon - oil everywhere

I got home, found the problem and fixed it. Took it out for a ride and there's still oil everywhere - the head gasket it seems. I haven't stripped it yet though.

So it looks like as soon as the PCV was disconnected, crankcase pressure did the rest.

Click to expand...

Unless you actually blocked the breather, nothing should nave been damaged.

Nortiboy said:

Unless you actually blocked the breather, nothing should nave been damaged.

Click to expand...

No, but it reverted to 'factory breathing'. For a moment I was worried that the head may have been damaged, and RH10 Heads seem to go for a premium, but my guess is the sealant around the pushrod tunnel has let go.
I'll have a look this weekend.

Are you sure its not a rocker shaft end plate gasket peeing through the channels and showing up at lower front of head ??? Maybe..

Hello,

if you discover the studs at the third fin as cause, I would try it with silicone under the washers at first. Perhaps could this avoid removing the head.

Ralf

Oil lines to the head are always breaking right at the fitting. It's hard to tell till you wind it up.

As a Mk2 850 owner, I really doubt the disconnect from the XS device would cause a sudden large scale oil leak as you have described. You would have the same crankcase positive pressure, but lost the negative which would not cause a sudden massive leak, that could only happen if the positive pressure was blocked. On the Mk2, the standard breather hose from the back of the timing cover is adequate to expel the positive cycle.

Just as an observation - the standard outlet hose goes into the top of the oil tank and then vents out by the smaller bore breather that runs down to the air filter back plate. The idea here is that the crankcase volume displaced (850cc) is pushed up in a column of gas to the tank and pressure drops as it enters that space and exits via the smaller outlet. On the negative cycle (pistons going up) the gas column has to reverse, but the flow has to now come back via the smaller outlet. That functions as a restriction reducing the back pressure. I think the problem in the standard design is the volume of the hose to the tank could be increased. I doubled mine, added a second small breather to the inlet rocker cap and gave away a brand new XS device as the motor is perfectly oil tight.

My only reservation about the XS device, is that I think it should be cleaned after long term use so as to prevent the reeds from gumming up.