Filling the finger area in the 62T cases with epoxy increases the primary compression ratio and helps aid the fuel mixture up through the transfer ports in certain situations. An engine with 120 degrees of transfer port duration has only .004 seconds to deliver its fresh fuel air charge at 5,000 RPM. As these engines are modified by increasing the displacement, bore, stroke, porting, A/M reed cages, spacer plates, etc., the crankcase volume and the primary compression ratio is altered. The primary compression ratio is measured in a similar fashion to the secondary CR. All the area under the piston crown at TDC / the area under the piston crown at BDC. The primary compression is responsible for pushing the fresh fuel air mixture up through the transfer ports when they open to approximately BDC. Depending on the compression ratio, blow down time and exhaust port area, the cylinder pressure may be greater than the crankcase pressure. In these circumstances, it will cause a delay in the scavenging charge entering the cylinder. An increase in crankcase compression ratio, exhaust port area and/or blow down timing would help this. Too much crankcase compression can hurt the intake ports flow of fresh fuel - air mixture from the carb, when there is too much intake port timing and or insufficient port velocity to overcome the back flow from the crankcase.
