Although this page is specifically about the Mingdun and JRL 48cc engines, I am assuming the same flaws I found with these cylinders exist in the same companies 66cc cylinders.
Mingdun 48cc cylinder + piston
The cylinder top is 1.5mm too high for good compression if you have a 38mm stroke crank. So I had the machine shop lathe it down.
There was slag inside some of the cylinder stud holes that had to be drilled out.
There's casting errors near the top of the transfer channels that left a step that needed to be ground down.
It had very small exhaust and intake ports.
The cylinder plating has begun to chip off almost from the start.
And its piston expands too much with heat. Although the piston/cylinder clearance is twice that recommended for a Pro-X piston here's a shot of the piston after just a few runs using alcohol-free gas and good oil:
JRL 48cc Cylinder
The cylinder top is 1.5mm too high for good compression if you have a 38mm stroke crank.
Exhaust port is lop-sided
transfers have big ledges (inside the ports, at the curve) that need to be ground down
One transfer port is 1.2mm higher than the other which gives them lopsided durations of 120 and 111.
Transfer roofs are close to a 45 degree angle which is too steep.
Exhaust duration is 143 and intake duration is 119 (the only thing right about this cylinder).
the next cylinder I want to check out from their site. Thankfully this one still has its cylinder wall below the transfers which helps with transfer of heat from the piston.