I want to drill and tap my water pump housing to put the sensor for my gauge in. I have two questions, will a mechanical gauge probe fit in there, and will I be able to get the water pump housing off while leaving the gasket intact

 

If you're removing the whole housing, there's almost no way the gasket on the back side will survive. It's cheap, so just replace it. Mazdatrix, or your local dealer can get it. There are locations you can put the probe. It just depends on where you want it. I've seen people put them in the water pump itself, or you could use the temp sensor location on the thermostat cover (S4 only). An A/T or turbo car will have a sensor here already. Wherever it goes, just make sure it's on the warm side of the pump/housing. Water goes in the top of the radiator, and then returns from the bottom side.

 

the pump housing i'm using is from early fc(86-87) and already has a place for this. since ur having to drill and tap a hole, i'm guessing this is a 12a ur dealing with as u didn't mention it. since the later housings have a boss made on them with threads the probe fits no problem. in ur case the probe might hit the inside of the housing not allowing it to screw in far enough. my advise would be when using the NPT tap, run it in just enough to get decent threads in the casting. since it's a taper, if u got too far with it, u want it to start to tighten up before the probe hits the inside of the housing. or u could tap larger and then adapt down to give the extra room.

 

Actually I have a 87 s4 turbo 2. I will look for the spot on the pump housing. Is it just a flat spot, do u still have to drill and tap?

 

the back side of the housing under the t-stat. the hole was already tapped and had a plug in it. i can get a pic if u need.

 

The TII had an extra temp sender on the back of the WP housing. It turned on the aux electric fan for AC equipped cars. Perfect spot for a temp probe. If yours isn't tapped you'll find a land where it can be tapped.

 

The temperature switch found on TIIs and automatic NAs is located on the top of the thermostat cover, not the back. It points vertically. If the OP has a TII, this location isn't an option unless he wants to disable the auxiliary fan. This location will also cause improper water temp readings until the thermostat cracks open at 180F.

The only tapped location the back side of the housing is taken up by the ECU's temperature sender.

Here's a picture showing the fan temperature switch on my S4 TII engine. The area on the front of the housing near the air pump mounting brackets might also be a good place to drill/tap for a sensor. It's in the hot water flow and before the thermostat.