I'd say we're basically down to the cylinder at this point. Can't help but think there would be different porting for the 85240/69412 closed skirt style piston than for the half-skirt style 85239/69020. I ran across a similar discussion elsewhere about the interchangeability of the pistons and did see a post indicating the 85239 was an acceptable service upgrade for any of the 70cc saws..., presumably including those with the closed skirt pistons. Whether or not that was fact or opinion wasn't specifically established and there was no mention of possible considerations for the various part numbers given to the different crankcase assemblies -- which of course include the cylinders without individual part numbers for the most part.
We need pics of the associated cylinders for the two different pistons as the easiest way to possibly reach a conclusion about the porting unless someone can otherwise illuminate the issue by providing definitive documentation on the matter. And as someone mentioned earlier, maybe there's been some grinding on this particular cylinder that didn't exactly yield the intended results? Seems like that would have been fairly obvious during the teardowns and rebuilds unless it was very high quality work.
And without re-reading everything again, the replacement piston is an OEM piston and not a Little Red Barn knock-off, correct? And I keep thinking the compression is too low as I believe someone else has also already mentioned. Not much need to include a compression release feature on a saw that only blows 130 with a new piston and fresh rings, tho. I also can't recall if this cylinder has a cast iron sleeve or chrome bore...
So does anyone have an early 4-10 ~ 6-10 in a box somewhere longing for a photo op? And maybe a burned up 7-10 / PM700 cousin also needing some camera time to compare innards with?
