Minh Thanh..
Piston ring is too thick and not flexible.
The right dimension will be calculated when you are use 19 mm bore:
19 mm : 25 = 0,76 mm, round of to nearest 0,8 mm.
19 mm outer diameter and 18,83 mm inner diameter. (The piston ring must be lapped with lapping ring + fine lapping paste and check the ring is snug fitting into the cylinder before parting of the piston ring from workpiece. Snug fitting the piston ring into the cylinder is better because we can not make it better on lathe, the fit of piston ring in the cylinder will be improved by running-in the engine)
The height of piston ring 0,8 mm (Create piston ring groove in piston and measure with blade feeler gauge 0,8 mm and the height of piston ring must be 0,85-0,9 mm when the piston ring is parted off from work piece. Then grind the piston ring on the fine sandpaper #400-800 on glassplate in "8" form when you are grinding the piston ring (never in one way grinding) and check the height of the piston ring will fit into piston ring groove.)
The ring gap 0,8 mm x 4 = 3,2 mm after the piston ring is heated up to near glowing then let it cool down (do not drop in water or oil!)
Follow this link:
http://www.modelenginenews.org/feeney/pg8.html
http://www.modelenginenews.org/cirrus/pistons.html#3
