XS650 Guru
Reaction score
Anglezarke, Lancashire, UK
Fellers, third rebuild, still oil ingress killing my left cylinder. Engine now stripped for fourth time - but check out the video.

Any help greatly appreciated.


Valve seals? Valve Guide? How is the compression? Put it a tdc, remove tappet covers and put some air in it by the plug hole and check if air comes out from the exhaust, carb or in the *head*(bad seals)...

If everything is ok, maybe check your carb, how is your plugs? white, brown or black? Sync your carb.
If I thought the engine was wore out I would recommend

the Dorman/Spark Plug Non-Fouler, those things will stop the oil from fouling the plugs
Fellers, I think I have found the source of the oil ingress - I just rotated the crank looking at the exposed piston tops, as the cylinders went down the right cylinder scraped the lining of the cylinder bone dry - the left cylinder lining had a significant coat of oil. I repeated the test several times, same result. Ring, rings, rings! New rings, too.

Ok, I will take off the cylinders and measure the ring gap (the rings are in the right way and have the correct orientation). The only things it could be are either faulty rings or the precision Heiden bore didn't match the pistons and rings they sent me. Here goes...

Since it seems to occur despite completely different cylinder/piston/ring sets one would expect the issue to be either above or below that. You mention you have new valve seals but no mention of what the guides/valve stems are like. If a guide or stem is worn enough(or too far out of spec new), oil will get past even new stem seals. However, stem seal leaks usually smoke more on decel vs accel.

If the left cyl is fouling, we all know it is, the combustion chamber is likely saturated with oil. The right cyl has been dry from the get go, so the oil in the left is probably coming from above the piston. Don't throw those cyls under the bus just yet! Check them stems and guides.....only after another cup or four o' tea :thumbsup:
Last edited:
I am just on to checking the valve stems now.

Piston ring end gap on left cylinder is 0.33 mm bottom, middle, and top - isn't the cylinder supposed to be narrower at the top?

I measured with a feeler gauge (not an exact science poking down a cylinder I don't want to scratch), but the gap is within the 0.3. to 0.6mm (bottom) (0.012 to 0.024 inches), 0.2 to 0.4 mm (middle and top range) (0.008 to 0.016 inches).

Last edited:
... isn't the cylinder supposed to be narrower at the top?...

Only on large-displacement, large-bore air cooled aircraft engines.

This is far too much oil intrusion for fresh rings, guide seals, head gasket.

I'm thinking warped head, microscratch on mating surface (don't drag precision machined surfaces across pebble surfaced patio floors), or the rare casting porosity problem, possibly pinhole in intake port. Porosity/pinhole(s) can be found by doing the valve leak test, on a very dry/clean head, with the solvent poured into the intake port(s), and looking for solvent wetness in cam area.

We would check head surfaces by using a very flat sheet of plate glass, smear on a thin layer of fine valve lapping compound, then quick lap (moving head in figure-8 pattern), clean, inspect. Look for untouched surfaces and micro scratches. If significant, have head resurfaced, else, finish lapping.

Whew, feel like I'm full of tea, switched to coffee...
Thanks, TwoMany - new ring, on new pistons in matched cylinder should not spew forth so much oil unless it was a poor re-bore job, and there is nothing to indicate that.

I just checked the left side valves for any play in the guides - nothing I wouldn't expect. The play matched the play in the spare head.

Yes, I will do the figure-of-eight test. I need to find a sheet of flat glass (if there is porosity, would that work both ways, and oil be forced out, too, e.g. out along a micro-scratch so oil would be visible on the outside?)


I have done the leak test several times already, and I will do it again.
Working both ways usually is confined to the combustion chamber area, where compression/combustion pressures can force reverse flow, except if it's a leak from a hi pressure oil delivery zone, like maybe a microscratch or gouge between an oil delivery stud and the combustion chamber. The intake port is usually at lower-than-atmospheric pressure, so it will always try to suck oil in thru a pinhole. Casting flaw pinholes are easy to fix, but not find.

Unbelieveable, endured 11 net crashes before this post took...
One thing I noticed just now taking off the pistons - when I pressed the compression and scraper rings together (while still on the piston) on the left piston a load of oil spewed out from the grooves they sit in, this didn't happen at all with the right piston.

Yeah, it's loading-up with oil, better than being bone-dry. Wherever it's coming from, everything being oily on that side doesn't help the diagnostics. Was the backside of the intake valve oily?
looks like the excess oil is being pumped into the bore from the crankcase via the oil feed holes behind the piston rings

Why would there be excessive crankcase pressure on one piston and not the other ?
That would explain the amount of oil in the left cylinder if it were being forced out from behind the rings.

The oil in the left cylinder builds up slowly from start up exactly in sync with your oil pressure buildup from the oil pump.

Might be worth removing the rings and checking the oil holes in the ring grooves to make sure they match on the pistons and there arn't any casting flaws .

Try removing your crankcase breather assembly and see if that reduces the crankcase pressure !

Anlaf much oil are you putting in the crankcase ? just a thought you might be putting in too much oil which would cause excessive crankcase pressure.
Last edited:
Might be worth removing the rings and checking the oil holes in the ring grooves to make sure they match on the pistons and there arn't any casting flaws .

Is there any way that you could get more crankcase pressure in one cylinder than the other ?

Try removing your crankcase breather assembly and see if that reduces the crankcase pressure !

Yep, check the piston oiling holes, they simultaneously supply and drainback oil.

No way to have imbalanced crankcase pressure, they share the same crankcase inner volume, unless isolation walls were installed.

Not sure what shape your headgasket is in, but if it was carefully removed and not wiped-off, you could inspect it for an oiliness track leading into the combustion chamber.
I had a problem like this on my first XS rebuild. Turned out to be the head gasket leaking oil into the cylinder from the cam chain area. To solve the problem, I used XSJohn's method of preparing a head gasket. You take your new gasket and smear a thin coat of red high temp silicone around the cam chain area on both sides of the head gasket. Let it dry over night and install as normal. Makes a much better seal.
TwoMany, the stem and rear of the intake valve was damp with what smelt like fuel but I wondered why that didn't evaporate. Exhaust valve stem and rear of stem definitely oily. Both valve do not wiggle in the valve guides - but I am taking the head for a second opinion today.

Plane_ben, I am doing a test today for the flatness of the head gasket, and your suggestion that oil might be coming from the cam chain side is reasonable (Peanut picked up on that, too). I would expect that to happen on both sides, though - so if it is that area it will be a warping of the head that is the root cause. Even if I find it is warped and have to skim the head I will still seal as you recommend. Thanks, my friend.