lm breather box valve 'fix'

To hold open the so-called ‘flap valve’ of the LM style crankcase breather / collector box, this looks like it should work:

Search eBay for “star lock washers 3mm”

Push one onto the rod in breather box (ideally with something like a small diameter tube) to fix the valve ‘washer’ and its spring permanently in the raised (fully open) position:

No need to try bend / break anything or cut holes in then have to repair afterward or generally hack anything about and all done in just minutes. Seemples!

NB: you must use the crankcase breather ball valve with this mod.

NB (2): Disabling the ‘flap valve’ reduces back pressure on the crankcase breather which may typically cause oil loss via the external vent tube (what points down at the road) at high rpm due to oil being pushed up the sump drain tube instead and overflowing the breather box (which is instead supposed to trap any blown oil and return it to the sump. But to do that air must be allowed to flow as freely as possible into the box from the large diameter crankcase breather pipe, but still close in the opposite direction, hence the need for the ball valve).


That’s a nifty fix, Mike.


can’t see why it shouldn’t be robust enough, in my experience these star locks are pretty tenacious ~ aka, xxxxxxxxxx to remove… you could always put two on to make sure.

Sorry to resurrect to old issue, but having built cafe racer from LMII one outstanding issue is oil being pumped into the atmosphere.
The engine has been bored to 950cc with enlarged sump and old oil breather box removed and currently using self-made large oil breather
box, but oil just pumps through. Seems to me the standard LMII breather box is not up to coping and ball valve an issue with it anyway.
Anybody found alternative, eg Ducati breather box or other suggestions ?

Do you know if the oil is coming out of the large crankcase breather pipe or the smaller return pipe that sits alongside it?
The system is designed to work so pressure in the crank cases escapes out of the large breather. Any oil mist condenses in the catch tank and is returned back to the sump via the smaller return pipe. The catch tank includes a one way flap so pressure only goes out the way. If this gets blocked, pressure builds up inside the crank cases and forces oil back up the return pipe which then fills up the catch tank and overflows out of the overflow pipe.
The standard box also includes a connection for the breathers off the rocker boxes.
The standard system usually works fine so long as the flap works correctly. I recall Le-Mans don’t have a ball valve, but use the flap within the box to prevent suction back into the crank cases.

Don, thanks for that, clear explanation of how it works is very helpful. I must say I am not sure which pipe the oil is coming from, but on current set up there is no valve, the pipes run into a header tank so no reason for there to be back pressure. However it could be that due to poor positioning oil is not returning down the return pipe and so building up in header tank.
I will try and see if I can work a way to find out where oil is coming from.
Additional question: the return pipe does it just go straight into the main sump, or does it go into a ‘protected cavity’. I presume from your response it just sits in the sump and if pressure is enough oil is forced up into header tank ?

The return pipe returns any oil that condenses in the catch tank is the smaller pipe in the top of the bell housing alongside the larger one. It drops into the clutch bell housing and is the pipe running around the outer edge in the picture below, The bolt at the bottom is a banjo bolt and directs the oil back into the sump.
If the larger pipe gets blocked and pressure builds up inside the crank cases then oil can be pushed back up this pipe and will appear in the catch tank.
Have a look at the design of your tank and try and see what happens to any oil mist that condenses in the tank. Does it have a return pipe? Is it nice and low down so any oil is returned back to the sump.

P3210221 by Don West, on Flickr

They say every day is a school day. I don’t know about that but I have certainly learnt something today. Thanks for that picture Don. Worth a thousand words.
I often see a small drip of oil coming from the hole at the bottom of the bell housing ( especially after a long run ). I had wondered about the origin but, looking at your picture, I suspect it’s from that banjo bolt.

Mine dribbles from the bottom of the bell housing. I don’t know why I didn’t seal that banjo bolt whilst I was in there!
It is one of those jobs I might get round to one day when I have nothing better to do!

How great to have picture, worth a thousand words, I will check pipes for any blockages. I was also wondering if the fact that I have removed the ‘flap’ valve resutls in air being sucked back into the crankcase and building up pressure and hence pushing out the oil. If pipes all look clear I am tempted to put on a valve on large breather pipe (purchased from HMB Guzzi - orginally for LM3) and see if that helps.

The OEM catcher tank / box is quite convoluted inside, to try and prevent what you describe, one of the things it has is a wire mesh screen which strives to prevent oil crossing over into the air venting part of the box, and stay in the bottom to drain back back to the sump. Can still be overwhelmed though.

The purpose of the valve is to encourage incoming air to flow into the rocker boxes and thus help drain the oil down from there, via the pushrod tunnels and drain holes in the cylinders. I’m pretty sure I always used a ball valve.

Hi all. From my experience, oil dripping from the bottom of the bell housing is due to a failed crankshaft oil seal. This is usually accompanied by a sticky clutch especially upon a cold day start up. If you are digging that deep into the motor then a new seal, clutch plates and attention to detail re-assembly are the order of the day. The main bearing housing must be orientated correctly and the oil seal set flush.