silly question but found some old yam pistons round my parents . why do some have cut aways in the skirts and others of the same size not . i may have known 25 years ago and have forgotten or may never have known and be about to find out why we killed so many bikes

silly question but found some old yam pistons round my parents . why do some have cut aways in the skirts and others of the same size not . i may have known 25 years ago and have forgotten or may never have known and be about to find out why we killed so many bikes

 

Its to increase the induction cycle. the cut away is normally on the inlet port side of the cylinder therefore increasing the amount of time the inlet port is open increasing the air / fuel charge in the crankcase.

The cut aways are often a raised edge on the skirt or a single or twin holes in the piston wall. 2 stroke tuners, amongst other tricks usually enlarge these cut out to increase induction. I suspect you tried this resulting in weakened piston skirts and piston skirt fracture (big bang and engine seizes, sound familiar?) :biggrin: