Problem solved: although one mystery has been replaced by another - albeit one of seemingly trivial importance.
On many pistons I've seen over the years, the piston diameter is uniform: from the skirt bottom right up to the crown. However, the attached graphic shows two Hepolite pistons of different design from the same model of Seagull FP engine. The left piston shows a 'recessed step' (dunno the correct name for it), in-between the 2 rings: something around -5 to -10 thou deep I'm guessing, which has become dark with burnt oil residue.
However, on the right piston (the subject of this post), this 'recessed step' extends
below the lower ring as well, which was to produce the illusion of a 'half-height' rusty ring when viewed through the exhaust port. I discovered that the crankcase contained quite a lot of brown sludge (so it proved quite useful to pull the cylinder off), and what I had thought were rusty rings, was in fact this 'stepped recess' coloured by some of this brown sludge.
So - conundrum solved - the rings are in fact of equal height (my error), but it raises the question in my mind of what is the function of reducing the diameter of a piston next to the rings, and why this design variation between engines which were made only 1 year apart ? Unless of course, the left piston is a modern replacement, as it shows very little wear.
Googling for "Hepolite 2-stroke piston" variations/ design etc. has turned up nothing.
Niander - you crack me up ...
