Tel-Tru are very good & very pricey, I used a 4" River Country, affordable, reliable & adjustable (if needed, but I haven't yet). I didn't use any sealant, I just sprayed oil on it when I seasoned it &, with that & the supplied washer, it has sealed up nicely.
I have Tel-Tru therms. I cut a 1" coupling in half and welded it behind a hole drilled into the door. I screwed the therms into the couplings without any sealant and have had no issues with them.
I run tel tru therms and they are great. The only issue I have was that I bought too short of stems I recommend longer stems if you can accommodate them.
Did you counter sink your gauge? if so you would only be drilling through the inside skin. If not you will need to figure out how you will install the nut an washer on the thermo.
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I'm trying to figure out the easiest approach. I have not built the door yet but I figured if trying to drill the holes through both skins while they are off the frame it's going to be hard to line them up when installing the skin.
I was going to weld a nut behind the front skin to screw the gauge in.
Put the outer skin on your door. Without having filled in the insulation yet. Drill the hole where you want the thermometer (taking into account where the stem will come out on the other side so it does not bump into racks. Mount your thermometer installing the nut and washer on the inside of the outer skin. Tack the nut and washer on the inside of the skin. Remove the thermometer. Place the inner skin on the frame with clamps, drill through the first hole and you'll have where the stem will come out. Mark witness marks on your door frame and inner skin where you want it. Take it off, put the insulation in. Replace the inner skin where you want it. Install thermometer to make sure you are all aligned. Tack the inner skin and weld.
Just take it step by step and don't overthink the build.
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