How many engaged threads for NPT fitting?
 

How many turns should a 1/4" NPT fitting be engaged before getting tight?

This AN867-2 from Spruce (modified) only gives me about two revolutions before things get pretty snug.



Thanks,
David

Threads
 

I would chase the threads on the alum. bung , should thread in a few turns before you need a wrench . I noticed this on the alum.bungs on the fuel tank quick drains .
Tom

You want to use a good quality thread sealant. I used Leak Lock by Highside Chemicals on everything. Available at many HVAC or Refrigeration supply houses. I tighten by feel. Your seal is made at the thread root with the sealant, so overtightening will not fix a leak. If I did not have at least 4 threads engaged on 1/8" or 1/4" NPT fittings, I would remove, clean, use thread cutting oil and re-tap the fitting. Upon tightening properly, I would say 4-6 threads engaged.

Wayne has a good point. Since most 1/4 NPT fittings have about .565 inches of total threads, 5-6 threads with a good sealer will give a really tight seal. Thats about 1/2 the total threads. Be careful of aluminum fittings and bosses.
Tom

Re-tapping female NPT threads for threading adjustment is routine.

On the parts that I make similiar to those bungs, I tap them deep enough that there are only 2-3 thread left showing on the blue male fitting you have there. That gives it plenty of mechanical strength and more than enough thread left for torquing for final assembly.

Definitely need to tap that bung deeper. It will probably be pretty difficult unless you have a good way to hold it securely.

I would wait until the holes are drilled but before riveting to your tank skin. Take a piece of scrap aluminum sheet and attach it with wing nut clecos, then tap away.

You will find our brake master cylinders are not tapped deep enough too. You don't want metal shavings in there.

hand tight engagement of a 1/4 - 18 npt is 4 turns. Also referenced as L1 dimension.

I will get an NPT tap and go at it. Thanks for the help!