Oil loss through Breather Box - 1976 LeMans 1

Quick Precis: 76 Lemans bought in '88 as a non runner and rebuilt by me. Been running well until last year when I took it off the road and sent the heads to be rebuilt by Len Patterson at the Cylinderheadshop. Since the heads have been refitted the crankcase has been pressurised causing massive oil loss through the breather - about half a litre per mile!!.
I have run the engine both with the ball valve (NRV) fitted and removed. I have tried 2 different Breather boxes and in desparation at the weekend fitted a new agostini breather box with a brand new ball valve. The problem persisted. I reasoned that the crankcase pressure must be caused by blow-by over leaking piston rings. So yesterday I removed the barrels hoping to find an obvious cause for the oil consumption. Alas they look like new! No scoring, no discolouration.
So what now…any suggestions welcome.

What else did you take off apart from the heads?
I can’t remember exactly what the head gaskets look like or whether there are variations, but is it possible they or the wrong parts or have been fitted incorrectly so are preventing oil returning from the top end, or leaking from combustion chambers into the pushrod tunnels etc.

Thanks. That’s a good point but on dismantling I did check that the head gaskets had been correctly fitted and they were. The combustion chambers were dry and healthy looking.
Someone said I should remove both check valves - the one in the standard breather box as well as the ball valve. But I’m not sure.

Weird thata a lot of oil. I would check if the crancase pressure by the oil filler. Should initially blow but then the nrv will create a vacuum. That will show if its pumping. Be careful if you use your thumb for this!
I would also check the oil return out of the breather box, is it mist or more a flow.
I think ranton_rambler is on right track, if oil isnt draining from heads it will come out of the head breather pipes.

Yep. It’s a lot of oil. just checked again and the Barrel gaskets were correctly fitted so the oil from the heads could drain back to the sump by gravity with no restriction. The ball check valve is also working correctly allowing air/mist to escape from the crankcase but not allowing it to flow back down. All the pipes are free and unblocked.
Compression was a bit low for a Lemans - only 10.0 bar on the left and 10.4 bar on the right.

  1. if was running well, why heads off and rebuilt?
  2. So bore etc. looks good, but did you meaure the piston ring gaps? Should not be much more than about 0.017 in. (But why would you, if it was running OK before.)
  3. Something else must have changed. What else did you do?
  4. It is possible to put the base gaskets on ‘upside down’ which will block the drain holes, but you checked that. (If gasket replaced, I used to drop a long screwdriver down the barrel drain hole to double-check it’s lined up correctly before putting the head on!)
1 Like

Good point. I’ll measure the ring gap. Should have mentioned that it’s running on 88mm Gilardino barrels and pistons. But is 150psi combustion chamber compression low enough to cause all this crankcase compression?

You should only have one valve to the breather box, either the ball one or the one in the breather box :+1:


This makes sense.

Unlike this website which is nanny personified.

1 Like

@simon H. Measured the ring gap at 0.8 mm. 32thou. So maybe that’s the problem.
Will reassemble with new pistons & barrels.

If you are running 88mm barrels, is it possible there is a mismatch with heads? From memory the pushrods were moved out compared to 83mm in order to clear the bigger bore but I don’t know what else changed.
Ian P

Thanks Ian. Since the ring gap indicates wear, I have decided to fit new pistons and barrels. The leakey bores must be the source of the crankcase compression.

1 Like

You say the barrels and pistons looked perfect. I would check the clearance between the bore and pistons and hopefully you should get away with just fitting new rings to the existing pistons.

Still like to know what was done to the heads. Way it reads implies problem only occurred after refitting. (?)

That’s what threw me. The problem did only occur after refitting the heads.
The heads were rebuilt with some new valves, re ground / replaced valve seats and new/ re shimmed valve springs.
Hope to fit new barrels and pistons this coming week.

1 Like

Still bizarre - only thing I can think of, head gaskets leaked or valves leaked, so that after the rebuild and refitting the compression went up. But I would have thought leaking gaskets or valves would be noticeable. :astonished:

Extra compression was my theory too. The compression was only 10bar which is a bit low for a lemans
We will see…

I don’t know what is the cause of your problem, but if it was running fine before you sent the heads off, then what is the point of changing the pistons and barrels? Nothing has changed there, and they were working fine before the rebuild, so how can they possibly be the cause of the problem now? The only thing that’s changed you say is that the heads have been off, rebuilt, and put back together, so logic says that the problem must lie there….
The breathers should carry only air, with some oil mist, so if they are pumping liquid oil out, then oil must be gathering where the breathers exit, and it shouldn’t be there. Or so it seems to me.

If the crankcase pressure rises enough, pure oil is pumped back up the small pipe into the breather. That’s what happened in this case. And I do have a new spare pair of 88m pistons and barrels handy.

I am running a T4 with pod air filters. Had to renew the breather box because the old one rusted internally. Found new replacement breather boxes (I tried 2) had too much spring pressure on the NRV so crankcase pressure was forcing oil up the return hose from the crankcase into the breather box and this was then running out the vent to atmosphere hose and coating the rear wheel. Followed advice in Guzziology, ditched the breather box, looped the breathers high up behind the steering head and then routed them down along the swingarm to the rear wheel spindle. Works for me!