One of the most popular and effective upgrades you can get, along with a three wick meths burner, the effective pressure is raised to about 20-25 psi, thus giving more power to the pistons but still within the safe working pressure of the standard boiler.
The 20-25 psi valve is available in a 1/4 x 26 tpi thread for standard loco boiler, usually fitted in place of whistle when used with Water Top-up Valve.
Also available in a 1/4 x 40 tpi thread to fit the Premium High Pressure Boilers at 40-45 psi.
If you require an alternate blow off pressure please add your request to the comments at the checkout and the valve will be set before dispatch. Currently the allowable range for custom setting is 20 to 60 psi.