Oil loss through Breather Box - 1976 LeMans 1

Thanks Phil. I’ll try that if the new pistons don’t reduce the crankcase pressure and the problem still persists.

Piston rings gaps measured as 32 thou., (0.032") should be about 17 thou., (0.017") so not ‘running fine’. Rather, implies, totally clapped out. :smiley:

Guessing bores are plated, (Nigasil?) if so can’t be rebored oversize, so I’d be looking at new barrel & piston kits too. Will also include new piston rings & gudgeon pin clips.


On the LM breather box you can easily disable the so-called ‘flap valve’ (basically, a washer on a rod with a coil spring behind it, it closes off the end of the crankcase breather tube and is pushed off by crankcase pressure), by slipping a suitable sized star spring washer (off the top of my head, sort of 4 - 5mm hole size), over the rod and pushing back the washer, the star washer then keeps it open. You then fit a ball valve in the metal pipe coming out of the bell housing. I have done this.

A tip I read recently, the end of the small diameter return tube (metal) into the sump should be below the surface of the oil, else blow back and overflowing the catch box can occur. If you have a sump extension fitted, you may need to extend this tube with a bit of hose over the end, that will reach down to dip below the oil surface. Hope that makes sense.

Another trick I did that worked quite well, is extend the hose that comes out into open air, I think normally it points out to the right side below the gearbox and is secured by a clamp. I extended mine by adding a longer piece of hose to run along the left edge of the rear mudguard and end just in front of the number plate. Never lost any oil that way. However I did start to lose oil when, after some years of the former set-up, I decided to rebuild it all with new catch box (+ disabled flap valve), new hoses and new ball valve. The vent hose hung below the gearbox, as per standard. Almost. Then I could lose oil level if the bike was ridden hard. It hardly happened using the extension along the mudguard, best as I can remember.

(Edited due to typos.)

That doesn’t sound right. The oil is being forced back up this return pipe by the positive pressure in the sump. If the pipe is clear of the oil in the sump, it can’t get pushed back up.

There should also be 2 small breather pipes off the rocker boxes that go to the breather box and 2 oil feeds to the cylinder head. I assume you haven’t managed to get these two mixed up somehow?

I got my breather box chromed (I know show pony!) and that process disabled the sprung flap, works for me. :roll_eyes:

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Thinking about it I would agree with you, but that’s what the guy wrote.

If you’re asking me, no. :smiley: