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I use the autometer electrical unit that actually has a pressure sensor that I mounted on the schrader valve with the pressure valve removed.
If you choose to go with a mechanical version, you need to order the transducer which you fill with antifreeze on the cabin side of the transducer. The fittings are AN-4's ....
You would need to order the meter , transducer ,and a length of AN-4 braided hose.
If I recall the electrical version is complete with the meter.