yeah i think maybe what andy is saying is that they went to a shit piston in later years? like a lot of the manufacturers do when they loose their piston supplier or whatever, they just grab whatever fits and you get stuff with gudgeon pins being in the wrong place, timing issues, etc.
the piston that has come with the ones i've bought is two thin 1.5 mm rings and very nice finish.
i'm almost tempted to pull that tomos motor back apart and check, but when i built that thing 3-4 years ago i went over it real careful and checked everything.