Well, now that you mention it, looking more closely now at the spare piston I have (I had bought two junked RY70113 trimmers from the tip shop), one skirt is indeed about 3mm shorter than the other. I hadn't noticed that before. Happily for me, it is the side that has the location pins for the piston rings, which I had put on the intake side.
Interesting that, measuring rather crudely with my micrometer, the gudgeon pin does appear to be offset toward the intake (short skirt) side by around 1 or 2mm. So as you say the rod would tend to be more vertical on the downstroke. So the reason the skirt is longer on the exhaust side is that (despite the slight gudgeon offset) there is still some lateral pressure on the lower part of the piston against the cylinder wall on the exhaust side, i.e. there is a tendency for the piston to want to tilt over crown toward the intake and tail toward the exhaust, on the downstroke, due to the angle that still remains on the piston rod, especially at mid-stroke. Right?
Mighty interesting. It certainly helps to have a cylinder and piston in hand while thinking about this. I'm happy that I started my learning with these junk trimmers, I am amazed at what I'm learning. Thanks heaps.
Last edited by Ian333; 01/11/14 09:22 PM.
