Pressure switch replacement plug

This little plug can be screwed in place of a pressure switch. It has a 1/8 MPT thread (external diameter: 9 mm), and is compatible with almost all the pressure switches found in North American and European spa heaters. You can screw it (and unscrew it) into place using a 3/16" Allen key.

As in all cases where a screw thread needs to be waterproof, you should use either Teflon tape (a bit fiddly, due to the small size of the part) or another thread sealant.