FYI, because you have a B16, it is easy to get an aftermarket water temperature gauge to fit in the cylinder head. And that is because on the B16 heads, the nut for the probe from the temperature gauge does not screw directly into the head, but into a replaceable threaded bushing. It sound like you changed that bushing when you installed your new gauge and if you didn't you might have slight leaks because the seat inside the stock threaded bushing is deeper because the stock probe from the water temperature gauge is longer.

On the later engines, the probe is inserted directly into the cylinder head and does not need a bushing, so the hole is a little deeper than the aftermarket temperature gauge probes can seal on properly. But that can be remedied by installing some washers between the nut and the probe. You have to slot the washers so they can get past the tube and you have to use washers that are small enough to fit inside the hole, but it does allow you to use aftermarket gauges.

I have seen people trying to seal the threads, thinking that those are pipe threads that should be for sealing, but in fact the sealing part of this set up is on the flanged part near the top of the probe and the threads are there to allow the nut to push down on the probe to seal.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502