R80 - hissing warm piston
 

I was out for a long run on my bike the other day and noticed that the right piston seemed to hiss when I got back to the garage and had switched off for 5mins+ or so. Is this likely to be a blown gasket? I looked for water on the piston (rain?) but couldn't see any, nor could I locate the exact source of the hissing (an identifiable gasket area). I am clearly a novice airhead. Any ideas anyone?

Noticed a similare (same?) sound from my left Sibenrock overhauled head... it seems to be the re-welded exhaust threads that is the culprit in my case... Siebenrock had failed -utterly- to weld the aluminum and re-thread it propperly (I simply did not have time to deal with it so I installed the "faulty" head anyway).

The headgasket is said to produce a twittering sound. A hissing sound is more likely to be a leakage from the exhaust system (back-pressure). Check the connection between the down-tubes and the muffler/silencer also - they do tend to leak there and that does also create a hissing sound.

When the valveguides are -worn- you may also get a hissing sound; when you look at the inside of the covers (which should thus not be pained on the inside...) it should be just a clean oily surface; if light brown it could be ok, but if darker brown or black - that sides valve guide is leaking.

What should not cause a hissing sound is a loose stub. Detach the carbs and check that the stubs are seated firmly in the heads - just incase... The stubs have a tendency to got loose, and thus causing air-leakage (too lean mixture) which cause too hot engine running, causing augmented wear of the guides and risk to loose valves (valvestem braking/separation).

This can be a normal thing. There is a small gap in the gasket on your valve cover that allows water between the head and the valve cover, but not into either of the sealed sections of the valve cover, so your oil isn't getting contaminated. There is a small pocket that can fill with water and it will continue to hiss well after the engine is shut off. Look in the center of the valve cover on the top edge.

...forgot that the covers lack a hole that drains the room inbetween the "rocker-cups"...
It can hiss quite strongly when residual water is "steamed off"...

Notice the hole above the center bolt - "drainage".
On the lower side I've just made an opening/gap between the covergasket and the cover.

Thanks guys. It seems to be the 'hole that drains the room in-between the "rocker-cups" '. A major relief as there are many pressing service issues to deal with before opening that can of worms.