This port fits the original factory rubber gasket, a new gasket is included also. The photos show this custom transfer port on the right compared to a standard factory port on the left. The custom port is chamfered on the inner edge of the valve-side to smooth air-flow. The inside diameter is .150" to match the ports on modern 13XX, 22XX, Discovery and our custom Magnum Valves which put out more power with these ports than any other custom valve in the industry for small-bore .177 to .25 caliber. This .150" internal diameter is where the FPS/FPE peaks, then starts to drop as you get closer to .156", then continues to drop rapidly as you go larger from there. Ports larger than .150" only become necessary with ammo above 35 grains and/or above .25 caliber. Enlarging your barrel port to .150" is recommended to get the full benefit of using this transfer port, otherwise the gains are slightly smaller. This port will not fit older 1377/1322 models with a sliding breech cover. The current Chinese models have a .156" valve port, this custom transfer port will maximize the power potential with those valves.