This longer length piston and the cylinder is made specially for the current SP4 engine and the old SP5 engine. But hey.... it is also used on the very much older SE2A engine made way back.
These things do wear out when you've given your engine a bashing or the wear of the passing years takes its toll. Here's a clue to help you decide if you need a new one; there is steam and water everywhere - the wife tells you to put it away - and it hardly turns over. Either ditch the old engine or get a new piston assembly. Simple.
