Why im asking. I use a gx 390 block. 66 crank. Arc rod masjined for a 90.5mm briggs piston. 308 cam. No balance crank. Still a blance shaft but masjined. A briggs head. Welded combustion chamber. 39mm intake valve and 34 exhaust valve. Std briggs aliminium rockers. Briggs pushrods. Arc flywheel...