There are only 3 ways that oil can get into the combustion chamber:
- up, past the piston,
- horizontally, through a break in the head gasket or
- down, from the valves.
So I’ve removed the cylinder head from the MV to find out which it is.
UP: The piston crown is clean and shows no signs of oil burn at the edges so oil is probably not getting drawn up past the rings. There’s very little movement of the piston in the bore which confirms it’s recently had a new piston and I can’t see any scoring lines on the bore which would indicate a broken ring. However, I will remove the barrel later to be certain.
HORIZONTAL: There is a problem here, as the seal around the oil passage from the barrel to the head is poor, due to the hole in the head gasket being in the wrong place making it eccentric to the O ring.
Part of the O ring is under the gasket and part of it isn’t. This significantly increases the chance of oil leakage and explains why the front of engine was a little wet with oil. However, the seal to the bore doesn’t look like it’s been leaking, so this isn’t the cause of oil getting into the combustion chamber.
DOWN: To remove the valves I had to make a hairpin spring compressor and I then found the inlet valve was wet with oil – not good.
Next I inspected the exhaust valve and port and they were wet with oil – not good, not good at all!
The port is wet with oil around the valve guide and it hasn’t came through the combustion chamber or it would be dry.
THE CAUSE: Is therefore oil getting sucked down both the inlet and exhaust valve guides. Now most people think oil won’t get sucked down the exhaust valve guide due to pressure in the exhaust but this is not the case. It’s true there are pressure waves but Bernoulli’s principle states that when a fluid increases its speed, the pressure reduces. This is why carburettors suck fuel up the jet tube into the intake air, it’s also why aeroplanes fly and why you get some suction in areas of exhaust ports. There’s one final sign and that’s the small black patch of burnt oil directly below the inlet valve – see photo.
This indicates a small amount of oil is dripping through the inlet valve when the engine is stopped then burning when it starts. BUT WHY?
There is some wear on the valve guides but nothing excessive. The problem, I think, is that both valve stems were fitted with an O ring just above the valve guide. Now I guess the previous engine builder expected the none-standard O rings would remain stationary in the alloy spring carrier plate but unfortunately they are going up and down with the valves. Not only does this make them totally useless as an oil seal, it actually makes them act like a reciprocating pump – one that pumps oil down the valve guide! When the valve is up, the recess below the O ring fills with oil and when the valve goes down the O ring compresses the oil and pushes it down the guide – not good.
THE SOLUTION: I could just remove the O rings and hope that this solves the problem. But it would be better if I could improve the valve stem sealing, as insurance and that’s what I’m going to do. So it’s off the my local engine repair machine shop, where the owner “lends” me a large sweet jar filled with surplus valve stem seals with the instruction, “see if you can get any of those to fit” – happy days!
And it looks like one of these 2 types should fit, with a little trimming.
and then the finished job will look like this.
All I need to do is trim the underside of the valve stem seal, so it will seal effectively against the top of the valve guide. There’s a nice recess on the underside of the alloy spring holder that will hold the seal in place and prevent it from moving up and down, like the O rings – hopefully….
I’ve now removed the barrel and everything looks good. Virtually no wear and the piston and rings are near new. The piston is the 4 ring type (2 x oil control rings and 2 x compression rings) so the likelihood of oil being sucked up is very remote.
The piston ring gaps, measured 25mm down from the top of the barrel are:
- Top compression ring = 0.65mm = 0.025″
- Lower compression ring = 0.52mm = 0.020″
- Top oil control ring = 1.21mm = 0.047″
- Lower oil control ring = 0.42mm = 0.016″
The top 3 are a little bigger than I would like. However, current thinking is that ring gap is not as critical as previously thought and many modern engines run quite big gaps from new.
But to be safe, I will drop the piston manufacturer, “Asso” an email to get their opinion.
So, it’s new gaskets and back together. I just need to decide whether to make a copper head gasket or buy a composite one, as some say copper is much more reliable.