Has there ever been a complete primer on all of the various water pumps that will fit the SOHC? I never realized how many different variations there are for what seems like a straightforward item. Different housings, pumps, pulleys, heater attachments, coolant return tubes, and so much more. Basically, for the majority of US X1/9s (aside from the '74), I thought there were two housings - one for AC and one without. And one pump type for each housing. It seemed all AC pumps had a bolt on pulley and non-AC had a press on pulley. But I'm finding there is so much more than that. I'd like to get an overview of all the various combinations that were (are) available. But some specific questions are as follows. Are all housings interchangeable solely with regard to the block (not accounting for other accessories, options, etc)? What pulley diameters were used? And for which applications? Are there different offsets of pulleys (aside from one and two belt grooves)? Are there differences in impellers and/or fluid volume - output capacities? I have several ideas but need to find out more about what can be interchanged with what first. Thanks for any input.