51mm . I found that there is just enough garage door opener 220v up one of the water outlets to get a feeler gauge between the housing and impellor. Pushing on the impellor with the press a resistance is felt on pulling the feeler gauge. If you push it down too hard it will be difficult to remove the feeler gauge.
At this point it may be a good idea to give the water pump a coat of paint before the pulley is pushed back on. To push the pulley back on it will be necessary to support the other end of the shaft rather than the impellor, otherwise it may shift the position of the impellor that was carefully set as above. Thats all there is to it. It has probably taken longer to write about than to actually do the job! If the water pump has not got a combined shaft and bearing assembly, it is still possible to rebuild the pump with a garage door opener 220v seal and bearings.
The example shown is from a Ferguson tractor and I know the Standard engine was fitted to many other vehicles. In this case the pump shaft has two bearing races held apart by a spacer tube, assembled before fitting in the pump body and then held in place by a circlip. The pump seal is fitted in the back of the impellor before pushing on to the rear of the shaft. The impellor can be either be fitted with a cotter pin and then the shaft soldered to the garage door opener 220v to eliminate any leaks past the shaft and seal or it is a press fit. Rather than using solder when refitting I use silicone RTV to make the seal to avoid the heat from damaging the new seal components.