Chainsaw Repair

OK, had time to take a closer look at the NWP C/P and extra pistons. The original piston was destroyed when it hung a ring, so the owner purchased two NWP replacement pistons. Both of these replacements hang up in the bore with or without rings before they get more than a third of the way in.

I immediately suspected the NWP 52mm BB kit cylinder was the problem, so tried a Stihl 064 piston, no problem same with a Mahle 52mm 272 XP piston. Also tried Hyway 52mm piston with no problems. Now both of these NWP pistons are the ones with the PTFE coating, so I pulled a brand new OEM Stihl 064 and HVA 272XP jugs off the shelf. Tried the NWP pistons in them and they would also hang up in the same place in both the Husqvarna and Stihl jugs.

I don't have a fixture that would allow me to spin the pistons to see if they are out of round or perhaps have a bulge in them that prevents complete entry into the cylinders. The only other thing I can think of is that the PTFE is applied unevenly on the piston. So, I am going to mark the piston at the point where it hangs up and measure either side of that to see what if any difference I can detect.

IIRC when the 'moly = PTFE' coated pistons came out for race motorcycles, one of the claims was that the pistons were a tighter fit to the cylinder than bare pistons. They were supposed to reduce blow-by and reduce piston slap but not create more friction because of the coating. Husqvarna never advocated the use of them but Suzuki and other Japanese manufacturers did. My brother was a Suzuki rider and used the moly coated pistons from Weisco but found that the early coatings would start to chip and flake off in a very short time.

Any other thoughts?

Just got off the phone with my brother. He remembered a couple of other things I forgot. The coating is supposed to wear off a bit during break in. The idea being that you got a more exact fit to the bore. However out 2-stroke cylinders are not completely cylindrical through out their height, they are slightly bell shaped in the area of the spigot to allow the piston to move a little to aid in the transition from the power stroke BDC to the compression stroke. Too much bell or too much piston slap and the skirt will promptly separate from the piston = 'death rattle'!