Bought a new gauge for my heating oil tank because the previous owner installed one that was too short for the tank. The gauge came with a tiered fitting that is listed as a 1-1/2" & 2" Male NPT. I did the following:

  1. Screwed the old one out
  2. Applied a moderate amount of "Megaloc Multi-Purpose Thread Sealant" around the threads of the 2" fitting, covering about 80% (heightwise) of the threads all the way around
  3. Started threading by hand a few turns with the glass off so that I could keep the float inline with the tank
  4. Finished screwing in the fitting with the wrench until only 2 threads were visible
  5. Screwed the glass back on to the gauge

I wiped off the excess sealant and it now looks like this

enter image description here

I noticed that while I was met with some resistance as I proceeded it never truely felt "tight" like when tightening a bolt or nut. I could have kept going and bottomed out the fitting but the old gauge had even more threads showing before it got too tight for me to feel comfortable continuing. It had a similar looking goop thread sealant that was white and had basically worked it's way to the top of the threads.

Other details:

  • Brand of the Gauge is King by Beckett
  • MEGALOC sealant by Hercules
  • The wrench was a pair of channel locks with 8-9 inch long handles
  • 275 gallon vertical oil tank

1 Answer 1


Pipe thread differs from the type used on nuts and bolts. The threads on bolts are the same diameter from top to bottom. Pipe fittings have a smaller diameter at the bottom than at the top. Because of this the threads don't bottom out they just get tight. Based on the number of exposed threads I would say the fitting is tight enough. Oil tanks are vented to the atmosphere so there no issue of pressure buildup causing a leak.

  • Yeah, thread compound is designed to hold substantial pressure by itself. It's thick enough that it's not phased by even moderate pressure. You should have none here (though the gauge in my first home's tank leaked because the oil service chronically overfilled it up the filler pipe).
    – isherwood
    Nov 27, 2018 at 3:02

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