EDITED AT DAVES REQUEST
I DONT THINK THE PISTON KNOWS IF ITS 2CYC OR 4CYC WHEN IT COMES TO HEAT EXPANSION.THIS IS A PROVEN 4CYC RACING PISTON DESIGN
SKIRT LENGTH IS 65% OF THE DIA. SO A 1.0 DIA. PISTON SHOULD ONLY BE .650 LONG
NO SKIRT BELOW THE WRISTPIN
PISTONHEAD DIA IS .005 UNDER BORE DIA
SKIRT DIA IS .0018 UNDER BORE DIA
RING END GAP .004 2ND RING & .005 TOP RING
IF YOU CAN PICTURE THAT PISTON IN YOUR MIND ITS ONLY 1/3 THE MASS WE HAVE IN THESE 25CC ENGINES. CAN ONE OF YOU 2CYC EXPERTS EXPLAIN THIS TO ME? SOMEONE TLAKED ABOUT LAPPING THE PISTON. DO YOU LAP IT IN THE BORE WITH AN ABBRASSIVE COMPOUND AND JUST MAKE IT SMALLER LIKE PUTTING IT IN A LATHE AND TURNING ITS DIA DOWN? WE CANT DO ANYTHING ABOUT THE HEATED INCOMING AIR REDUCING THE VOLUMETRIC EFFICIENCY BUT MAYBE THE SHAPE NEEDS WORK.

DAVE ASKED ME TO EDIT OUT THE AUTOMOTIVE INFO SO HERE IT IS EDITED
I DONT THINK THE PISTON KNOWS IF ITS 2CYC OR 4CYC WHEN IT COMES TO HEAT EXPANSION.THIS IS A PROVEN 4CYC RACING PISTON DESIGN
SKIRT LENGTH IS 65% OF THE DIA. SO A 1.0 DIA. PISTON SHOULD ONLY BE .650 LONG
NO SKIRT BELOW THE WRISTPIN
PISTONHEAD DIA IS .005 UNDER BORE DIA
SKIRT DIA IS .0018 UNDER BORE DIA
RING END GAP .004 2ND RING & .005 TOP RING
IF YOU CAN PICTURE THAT PISTON IN YOUR MIND ITS ONLY 1/3 THE MASS WE HAVE IN THESE 25CC ENGINES. CAN ONE OF YOU 2CYC EXPERTS EXPLAIN THIS TO ME? SOMEONE TLAKED ABOUT LAPPING THE PISTON. DO YOU LAP IT IN THE BORE WITH AN ABBRASSIVE COMPOUND AND JUST MAKE IT SMALLER LIKE PUTTING IT IN A LATHE AND TURNING ITS DIA DOWN? WE CANT DO ANYTHING ABOUT THE HEATED INCOMING AIR REDUCING THE VOLUMETRIC EFFICIENCY BUT MAYBE THE SHAPE NEEDS WORK.

Does this rpm loss effect reed valve two strokes the same as it does piston port engine or are there no more reed valve chain saw, leaf blower, etc. engines?

The Ryobi 31 type engines are still reed valve.

The quadra engines bigger than the 52 are reed valve.

Some of the 3W engines are reed valve

I don't think any 3W is piston port at this time, the last piston port was the old 60....

The piston skirt in a 2 cycle piston port engine controls the intake timing. A short piston won't work, you can't have the transfer ports higher than the exhaust....And on the other side a short piston might uncover the exhaust port at TDC...

The piston ported two strokes also need the longer skirt to act as a valve. Another phenomenon with pistons when they overheat, they can collapse and go round. Lots of rattling all of a sudden

Such a short-skirt design cannot be made to work in a two-stroke because of the reasons given by RCIGN1 and Flypaper 2.

About the lapping; you're right. The piston needs to be put in the bore and moved back and forth along its stroke until the desired clearance is achieved. The big engines (>100cc) we use all have cam ground pistons, even the 2-strokes. I am not sure about the smaller ones cause the manufacturers could just get away with using a higher clearance or a more abrupt taper.

Right , the Quadra 35-42 have round pistons. I'll have to measure the bigger ones.

OK SO WE NEED THE LONG SKIRT TO DO THE VALVING BUT DOES IT NEED TO BE UNDER THE WRISTPIN? THIS IS WHERE YOU ALL THINK THE PISTON IS CHANGING FROM THE HEAT. CAN WE JUST REDUCE THE DIA IN THAT AREA FOR MORE CLEARANCE? WHAT ABOUT THE TOP OF THE PISTON ABOVE THE RING IS IT POSIBLE THAT THIS IS SWELLING AND NOT THE SKIRT SINCE IT HAS A HIGHER TEMP UP THERE?
FARMER TED

Here's one way to eliminate the rpm drop from heat. Convert it to Diesel.

Pretty much all the piston skirt is used for porting. The intake and exhaust are on the right and left. Then there are the internal ports in the cylinder that are in the front a rear on each side of the wrist pin. The only part left is the small area immediately under the wrist pin.

Enjoy,

Jim

Quadra 42 piston is 3 thou smaller at the crown. Not for certain but if you were to lower the pin, the rod would have to be shortened to keep the same compression height. This would put quite a side load on the piston due to the accute angle of the shorter rod.

The Homelite 30cc even has windows at both ends of the wrist pin to feed the transfers.