Can anyone tell me the correct spark plug to use on the very early (1957--1959?) 320-01 Pope 2 stroke engine which had the rotary valves? I've been told it is B5HS, then someone else told me a Champion plug, but the two numbers do not cross reference. I doubt if either are correct as the thread length is longer on the plug than what is on the block. The original owner was using a standard Champion CJ8 which at least has the same thread length as the block. I realize some plugs do thread in a little bit further, so maybe that is the case here? I tried NGK B5HS and at full thread it goes into the cylinder chamber but at least will not touch against the cylinder head, but does come fairly close to it. The original Pope part number is unknown. Later 500 series used spark plug 500-01-35, but what does that Pope number translate into? In any case it might not even be the same plug at all. I am wondering if I should just stick a regular CJ8 back into it?
This is the engine.https://www.outdoorking.com/forum/ubbthreads.php/topics/74882/pope-320-01-rotary-mower.html