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I have a leak between the horizontal cylinder and cylinder head. I think the problem is located to the large o-ring in the oil return. When I renew this ring I can drive 1000 tight kilometers before leaking starts again. I have tried various solutions: 3.0 mm ring instead 2.5 mm, applied Würth DP 300, green ring instead of black. But nothing helps. There is a space at 0.2 mm between cylinder and head flangings when properly tightened. My engine oil is 20W-50. Can anybody help?

I would take a careful look at the cylinder studs.If it is showing more threads above the top of the nut than the others or if it feels "soft" when you torque it down it may be pulling out of the case.I had a similar problem years ago and had to Helicoil the case.If that's what it is luckily it's the horiz. cyl. so the engine can stay in the frame.

Thankyou for a good and quick proposel. I have just checked the studs. All 4 have exact the same amount of threads above the nuts, and torqued to 4.5 kgm no one becomes “soft”. Could it be the Würth DP 300 which dissolves the o-ring?

the DP300 is HYLOMAR and it very good stuff to use, I stock it in my shop for sale. I highly doubt it is causing you the problem.

Perhaps there is a nick/burr in the case or barrell causing things to not sit flat?

A further thought,see if you have that gap all around the barrel/cyl. head face.Use and repeated removal/installation can result in the head warping down at the corners where the studs pass through and expansion can cause it to lift and leak.Spot facing the area on a milling machine will give some clearance.