850 engine: oil sprays out from the tank cap

Hi Piero, it is a simple test. Making up the spark plug adapter might be a hassle if you haven't got the gear. Being a mechanic, i have most of my test equipment from the days when I ran an automotive workshop . If you have a good engine then do the test on that one first, so you know what that sounds like.
With mine it always puzzled me because in my experience with old cars that had excessive blow by because of worn rings, they used to get salad dressing clogging the tappet covers and the blowby smell emitted was obvious. In my case, I had no white salad dressing nor did the blow by gasses exhibit what I would have called the obvious aroma.
We live and learn.

Dereck

If all pipework has been checked, then put your fingers under the lip on the oil tank filler neck and check for a wear ridge, it sounds like the cap is not tightening far enough, usually there is a little wear point that the cap will stop at, dress it out, though nigh on impossible when tank fitted, or try and push down and twist the cap tighter past his point. The other solution is to buy a small square of thicker PTFE gasket of Ebad and cut out a slightly thicker cap seal.

I had a similar problem a few months ago. It appeared that oil was leaking out of the oil tank cap area but I wasn't sure from exactly where, and wondered whether it could be from a hairline crack from a weld / braze somewhere in that area etc.

To try and prove from where, I took the oil tank off the bike and pressure checked the oil tank and all welds etc and there were no leaks, even the cap didn't leak any air, and so was puzzled by this result.

It turned out that the leak was from the breather pipe where it connected onto the neck of the oil tank. Despite having a jubilee clip on the pipe, this is where the leak was from. I think the pipe was marginally too big and the jubilee clamp must have slightly nipped the pipe, and of course the oil leaked out around the neck of the filler which led me to believe that the oil leak was from there.

The cure was to fit a new correctly sized pipe with a spring clip!

This sounds ridiculously simple and makes me look a bit stupid, but it had me scratching my head for a few days, and was the first time I'd had an oil tank "leak" in 39 years of Norton ownership. So check the breather return pipe where it connects onto the oil tank.

Incidentally you could get a new filler cap gasket from here;

https://www.ebay.co.uk/itm/2-Oil-Ta...-Norton-Bsa-Etc-Free-Uk-Postage-/282696520638