Oil on top of piston

[gs.davies]

Number one piston has a noticeable puddle of oil present on the bowl after a good thirty mile motorway speed run. Engine been sat for twenty minutes while I checked oil levels and plug colours etc.

Would a valve guide problem cause this, or is more likely a ring problem?

[timmy201]

To me it sounds surprising that the spark plug would still be firing if there’s that much oil

There shouldn’t be that much oil on the top end to leak through the valve stems (although I remember reading you thought your cam bearing was in backwards which might explain it)

If you can get access to a leakdown tester it should confirm where the oil it coming from

[gs.davies]

The plug is clean, no deposits on it since swapping out for a BP5 (hotter than BP6). This has solved the run on that I was getting.

The leak down test was pretty damning - 40% loss on number one - rings suspected, however, a leaky head gasket into the oil way could also give a similar symptom.

The engine will get replaced eventually with a 1293c but that build is a way off yet.

Consumption of oil is around a pint every two hundred miles. No significant leaks to speak of.

The postion the engine stopped in was with the piston towards the bottom of the bore and was after a long fast run, so could have had valves open and a rocker box with excess oil in it.

[gs.davies]

This is number one plug, as pulled earlier. I’ve often pulled this after leaving it a few days and found it to be wet with oil, suggesting it’s leaked onto the plug from above. I can compute how a ring issue would leave a puddle on the top of the piston but allow a plug to run clean

IMG_7596.jpeg

[timmy201]

If you have 40% leak on one cylinder I’d whip off the head and have a further look

[EMOKID]

Head gasket worth investigating. Could drain down from oil way through gasket when stopped which would still mean engine can run ok but probably a little smokey.
Are you losing / using any coolant?

[Oneball]

Have you tried pressuring the cylinder and seeing if you can hear if it’s leaking into the block or the manifold?