Well it took me a while and quite some extensive searching and took me over all the continents (well, while surfing that is) but today a fresh cast iron piston arrived! It is a compressor piston and looks very good. Below a few pictures. The compression height, pin diameter etc are all very usable. There are, however, a few things I have questions about…
It is a very heavy lump weighing in at 546 gramms (with rings and pin). The original piston was not a lightweight too with 360 gramms but there still is a 186 gramm weight difference… In order to keep then original balance of the crank-piston assemby I would like to put the new piston on a diet.
Looking at the pics you can see that the new piston has 5 rings instead of two on the original one. I could of course cut of the bottom section just above the second ring. This will sure save me some weight. And this would still leaeve two compression rings and one oil scraper. But it would shorten the piston an that might increase piston slap. Or not?
The original piston has a recess machined around the middle section of the skirt (the dark part in the picture). According to the Premier manual this is to keep oil for lubrication. Should I machine such a recess in the new piston?
The new piston is completely cylindrical. So no taper and no ovalty. Most pistons are not cylindrical and have at least more play at the piston ring lands. Should I give the piston a bit more play at the lands? See the image below where I have quoted the recommended play that Radco mentiones in his fabulous book.
And will the lack of taper and ovalty have negative effects? Remember, it will be doing it’s work in a 1912 250 cc sidevalve that will do about 2.500 revs max.
Any other hints?