This high capacity water pump without a 'bypass' was first used on the Mini in 1990. (Metro's started in the early 80's)
What makes a water pump "Competition"? These pumps have the same aluminum housing and the same deep impeller as the standard items; the only difference is that the bypass adapter has not been drilled. This means that if you are running without a bypass hose you don't need to plug it; therefore one less possible leak!
It's warmer here than in the UK. This higher capacity water pump moves more coolant than the stock one, and is better quality. GWP134 has by-pass hose take off, GWP154 is without take off. These super high quality water pumps have a much larger than stock impeller; it sticks into the block slightly more than 1/2". The impeller in these aluminum bodied pumps is long lasting cast iron, not stamped steel as in many aftermarket pumps. This water pump will fit all 1275cc blocks, and with a slight amount of work on the impeller can be made to fit a small bore block.
Use of a high capacity water pump helps to reduce cavitation in the water jacket, particularly at high RPM, as well as circulating a greater volume of coolant. Further reduction of cavitation is achieved by using a large diameter water pump pulley CAM6408 as originally fitted to the Cooper S and reintroduced on the A-Plus engines. It is necessary to check clearance between the large pulley and the head; if a lot of material has been machined from the head face it may foul the pulley. Small bore engines with 12G940 heads can't use CAM6408 and it is necessary to modify the top edge of the high capacity water pump or the 12G940 head will rest on the pump and not seal properly; modify for at least 1/16" gap.