After careful measurement and a little parts swapping I have some findings. All measurements were done in triplicate. I may not have Mitutoyo instruments but I did use a dial indicator where applicable. Cylinder bore checked with snap gauge & vernier caliper.
I used 1mm rosin core solder for squish checks. The squish measurements with solder match measuring piston deck height & cylinder head relief measurements (measure how far above/below the deck of the piston edge is, and add/subtract from combustion chamber depth at outer most edge). This solder is very soft; I do not think there is any spring back. Solder was placed atop the piston, spanning the pistons full width, directly over the wrist pin. The head was torqued down, and piston was used to crush solder by turning it over by hand 4x.
BRP, Sno-X, and Wossners all have the same amount of crown, or height difference, from the pistons edge to the crown top. The piston edge is difficult to measure. I came up with 0.080" for that measurement, however, this is the most error prone measurement in my opinion.
BRP & Sno-X measure out the same in terms of total piston height & piston compression height.
Wossner is 0.001" shorter in total piston height, but most interesting is the 0.005" taller compression height. This is why I have 0.025" squish when using this pistons in conjunction with a 0.6mm base gasket. Kelsey mentions making his pistons slightly taller on his website, this is what he's talking about.
Stock piston mounted in my engine, with a 0.6mm gasket, gives 0.030" squish and 0.078" piston projection. Using the factory selected 0.7mm gasket that was originally in this engine, squish would open up to 0.034" and piston projection would drop to 0.074". Dead center of the BRP specified 0.072-0.076" piston projection.
What about piston to cylinder clearance? Somewhat unrelated to squish, but interesting nonetheless. BRP specs piston to cylinder clearance as 0.0053-0.0063" new; wear limit is 0.0079". With 3500 miles on my stock pistons, they were at 0.0075". 2000 miles on Sno-X pistons, 0.0080". Stockers weren't quite done, Sno-X not really up to the job in my opinion.. The fresh Wossners measure 3.2205" in a measured 3.226" bore for a 0.0055" clearance. I'm happy with that. These measurements are done on the skirts of the pistons, and bore checked just above exhaust port (as per BRP manual).
Okay so back to the piston kissing the head & gasket selection. Based on measurements, I know the Sno-X pistons were at 0.030" squish all last season. They weigh 447g with rings. Stock this motor was at 0.034" squish with a 473g piston with no piston to head contact. One difference in this comparison is crank bearing wear--and I know the bearings were getting worn because the MAG rod end was starting to turn blue (pictured). Loose cylinder clearance certainly wasn't helping things either.
Stock 0.7mm base gasket would put the lighter (415g with rings) RKTek Wossner in the same place as the heavier (447g with rings) Sno-X with a 0.6mm gasket in terms of squish & piston projection. I bet with the lighter piston, correct cylinder clearance, & fresh crank bearings, there would be no piston to head contact. I'm sure Kelsey intentionally wanted to close up the squish band. Considering my engine was dead center on BRP piston projection specifications, I don't think I have some fluke build that slipped by BRP with extra tight squish. More likely, I think, is that BRP & the aftermarket are wise to the advantages of tight squish in battling detonation. I do know it is more complicated than just squish..
I hope I'm not stepping on toes here: anyone can measure parts. The Wossners are very nice pistons. The take home for me is that you need to be careful. If the pistons hit the head any more than mine already did, the ring lands would likely be crushed on the exhaust side to the point of sticking the rings. This is not easy on rod bearings either (may explain blueing on my older crank?).
Now do I run 0.8mm or 0.7mm gasket? A phone call to the piston maker is in order I think.
Edited by GoinBoardin, 10 December 2017 - 05:01 PM.