Aftermarket or original temperature gauge?

To begin with, the threads don't seal the sender, the seat at the bottom of the sender has to push down on the seat inside the head to seal properly.

If it's an aftermarket sender, then you need to add some washers between the nut and the top of the sender as the aftermarket nuts aren't tall enough to push the sender down far enough to seal up against the seat inside the head.

If you're using an original water temperature gauge, the seat inside the head might have pits, so putting some sealant on the seat of the sender might get it to seal. I suggest using Permatex #2 non-hardening sealant.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com