Even though it's a PITA to R&I the stock water temp gauge and believe you me I know how hard it's to get under the dash, but I'm still going to suggest you reconsider and not just because I want to sell you 1. I'm a big fan of keeping it original in this case as I've always disliked adding gauges to the dash especially if just the water temp gauge is going to be replaced. Placing gauges under the dash of a 544/P210 and a 122 can interfere with the shifter and with entering and exiting the car especially if you are on the tall side like I am. You can put the gauge elsewhere, like on top of the dash or attached to the drivers side trim bracket on the inside of the "A" pillar if you don't mind the look.

Did you know that the stock front seats on PVs can be removed by simply sliding them forward off the seat tracks? When I've needed to access the back of the dash for a PV, I remove the seat and create a bed to lay down on using my back board or you can use a creeper and use cushions to support my head and neck on top of the pedals and on the floor. It's still a bit uncomfortable, but way better than twisting your back with the seat installed. Keep in mind that the seat tracks should have some grease on them.

If you insist on installing an aftermarket gauge, you need to be aware that part that goes into the head is not the normal style. Although most aftermarket mechanical water gauges have a very similar style as the Volvo, it's not exactly the same. The issue is the length of threads on the nut. The Volvo nut is longer and the reason why this is important is the opening for the water temp gauge is sealed by the flange on the end of the probe/bulb on the end of the capillary tube for the gauge and is not sealed by the threads. So when the nut isn't long enough, it doesn't reach the flange to push the bulb down to seal. This can be over come by cutting a slot in a thick enough flat washer(s) and installing between the bulb and the nut. I don't recommend using a split lock washer as it can damage the bulb.

If you want to install an aftermarket electronic water temp gauge, AFAIK, none of the manufacturers makes a water temp sender or adapters for the sender that will properly seal the opening. There's a guy on Turbobricks that fabricates a proper adapter for 1/8 NPT senders which is the size that is commonly supplied.

FYI, Volvo started using electronic water temp gauges and properly sealing water temp sending units starting with the 140s and for all subsequent 4 cylinder rear wheel drive cars. But as per usual, Volvo used non-standard ohm ranges and AFAIK, no one makes an aftermarket gauge that will work with the stock Volvo sending units. The 1 exception is the water temp sending unit used for the 1800E/ES, but you have to use a Smiths water temperature gauge.

Just to be thorough, the water temp sensors used on the 140/164 -1972 have a different ohm range than the 1973 and later 140, 164, 240 and 700 4 cylinder cars.

FYI, your original water temp gauge should be able to be rebuilt, but I like installing the later updated version as it's calibrated to read a larger temperature range and as you know, are still available new for 544/P210 and 122 Volvos. The early water temp gauges max out at 212°F/100°C. I can't remember exactly how much higher the later gauges read, but IIRC, it's at least 248°/120°C. Although it's a bit nerve racking to see the water temp gauge go near or into the red, I've told them that as long as the radiator doesn't boil over, they are still safe, but should consider pulling over if the temp doesn't start to go down. You don't have that cushion with the old style temp gauge as once it pegs, you just don't know how much over 212°F/100°C the engine is.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com