The transfer port (35) is the interface between the valve and the barrel on the 22xx series guns. It can be a restrictive item in the flow of CO2 and by changing this for a larger or smaller bore version you can increase or decrease the power.
Standard transfer port sizes in the UK are 2.8mm for the 2240 pistol and 3.6mm for the 2250 and 2260 rifle series.
The standard transfer port (35) sits in a rubber gasket (34) on top of the main tube, this gasket seals between the transfer port and valve. Note there is a short and long edge to the transfer port, the short edge goes in to the seal.
A good way to upgrade the transfer port in the UK is using a design by engineer John Small which does away with the need for the gasket with his design.
The bore on John's transfer ports can be in 4.2mm or 4.5mm and typically sell for around the £5 mark.
They can be purchased on eBay seller ID Josma20, just search eBay for Crosman Transfer Port.
