Eric, many thanks for getting back to me! So I checked my setup and, you're correct, the Skandix main plug is flat on the bottom and not tapered like yours, which I guess would succeed in the seal where the tapered bottom touches the bottom hole in the head opening.
As much as I can tell, my sensor looks like 28 tpi, quite fine, and is tapered. I wish I could send you a picture. Do you have an email?
I sure would like to get one of your setups if it would work.

The threads are Imperial, not British and are 5/8" x 18 tpi. Since the threads aren't a pipe thread, they are not designed to seal.

I know for a fact that the underside of the flared section near the top of the sensor probe is the sealing surface and the sensors for the stock electronic Volvo water temperature gauges are flared to seal on the same surface.

The problem with aftermarket mechanical gauges is the nut isn't tall enough to push the probe firmly onto the interior sealing surface inside the port, so spacer washers need to be installed between the nut and the top of the sensor probe.

FYI, there's a person on Turbobricks.com that fabricates an adapter bushing that allows the use of the 1/8 NPT water temperature sensors used with various aftermarket electronic water temperature gauges and it has the correct length and has the correct shape on the underside to seal the port.

And in case you didn't know, the B17, B19, B21, B23 and B230 Volvo OHC engines use the same style of sensor and it is also is 5/8" x 18.

Besides the water temperature sensor port, the oil drain plug on the those OHC engines are also Imperial at 3/4" x 16 tpi.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com