Don't use double gasket. They leak.
Double gasket won't increase volume of cylinder. It will only increase the volume of combustion chamber.
Compression ratio is the volume of mixture displaced by piston divide by the volume of combustion chamber.
You can use United 100 piston with 78cc bikes and stock head. Two things needs to be cared for. One the piston should be tooled as the bowl of stock 70 head is 48mm wide and the piston of united is 50mm. So cut only ONLY outer 3-4mm of PISTON.NOT center.
Secondly the valves will touch piston when all heats up. So create a 3mm wide relief that is 1mm to 1.5mm deep end 1-1.5 mm wide in piston center at appropriate place where valves will touch piston.
This combination gives best compassion ratio.
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