Anyone know if the water pump is interchangeable with both series?
For instance, if there is a S4 front cover and water pump, can you use the water pump from an S4 to an S5 front cover?

 

I think it is more a water pump housing issue, than a front cover issue.

 

If you look at the Mazdatrix website, they list different numbers for the S4 & S5. The S5 pump gasket has 8 bolt holes while the S4 has only 7. However, the water pump housing to engine gasket is the same for 86/95.