AJS/Matchless Mod. 18. G80 Piston STD = 82,55 mm Iron Head . NOT PERFECTslight scratches - tried in Cylinder, never fired up.Gudgeon pin diameter 7/8"Compression Ratio 7.2:1including gudgeon Pin, piston rings and circlipsPiston Clearance 0.12mm or 45thouPlease look at all pictures