yes the .20, .30 is the overbore diameter. many companies (ross and wiseco for sure) make a standard bore piston but you have to be sure that your cylinders aren't ovaled past the factory spec.
Most people just go with the .20 overbore because it ensures that the cylinder is perfect and allows added insurance that the rings will seal.
in the least you should re-hone your cylinders to remove any glaze and to put the cross hatch pattern back on the wall.
i wouldn't stick with the 7.8:1 compression, the magic number seems to be 8.5:1. i'm personally running 9:1 and have no problems running 20+ psi on 94 octane daily. if it was me i would go with a .20 ovebore set of ross's with 8.5:1 compression.
Most people just go with the .20 overbore because it ensures that the cylinder is perfect and allows added insurance that the rings will seal.
in the least you should re-hone your cylinders to remove any glaze and to put the cross hatch pattern back on the wall.
i wouldn't stick with the 7.8:1 compression, the magic number seems to be 8.5:1. i'm personally running 9:1 and have no problems running 20+ psi on 94 octane daily. if it was me i would go with a .20 ovebore set of ross's with 8.5:1 compression.
