Tip: NPT Thread Sealant
 

This is how thread sealant is used:



The listed sealants are suggestions with long service histories. You'll also find the brands suggested in the Vans builders manual. The actual choice is the builder's responsibility.

They are non-hardening sealants, meaning they don't tend to form solid chunks. A good sealant is easy to clean from female threads, which must be sealant-free each time a male fitting is inserted. Failure to clean the female threads can allow the male to push cured or uncured sealant further into the fitting, which may form a blockage.

The point is to avoid something like you see below, in which a poor sealant choice was sheared loose into the fitting, resulted in loss of fuel flow.

I do use teflon tape on most fittings but I make sure there is no tape on the first few threads. After I wrap the threads I go over the tape with my fingers and tighten the wrap. I only wrap the tape in the opposite direction the fitting screws in so it stays tight while threading it in.
It's hard to believe someone would use RTV to seal and use so much as to fill the inside threads like this.

Dan, thanks for posting such a clear and graphic how-to on the topic.

Dave

Thanks for continuing this discussion.

For the AN fittings, is it normal practice to skip the thread sealant because the flare is there to create a "mechanical" seal and there is a concern that any sealant could cause those two flared/beveled surfaces from completely sealing?

Thread sealant is used on the pipe threads only; not the flare fitting.

Look at Dan's drawing. Note where he shows "no sealant" on the flare side.

Yes I saw that, thus my question.

Sorry, I guess I don't understand what the question is. Sealant has never been recommended on a flare fitting. So what are you "skipping"?

As a new owner (not a builder) are there any specific tips for checking these critical fuel fittings to ensure they were done in accordance with best practices?

My plane was completed in 2007 and has 570 TT. This upcoming annual I was considering replacing all my under the cowl fuel lines, but not before ensuring it's advised at this juncture.

This tutorial by DH is awesome.

Sorry for drifting.

Cory - it is indeed standard practice to not apply any form of sealant to AN flared fittings. The mechanical design of the flared fitting ensures tight metal-to-metal contact between the female flare of the pipe and the mating beveled surface of the male fitting.

The only "sealant" which may be used on flared fittings is a thin, soft aluminum flared fitting washer, referred to in the Spruce catalog as "DEL 37 degree fitting seal". This thin aluminum material is essentially compressed between the flared tube and the AN fitting. In doing so, it allows a good seal to be made in the event the male or female portion of the flare is marred by some minor imperfection such as a scratch.

Again, just to be clear, thread sealant of any sort (liquid, paste, tape) is not applied to AN 37 degree flared fittings.

Correct, it is normal practice. There is no sealant required or recommended for the flared fittings. The tubing used most often by Vans, AL 3003, is relatively soft and deforms slightly to seal properly.

You might take a look at Chapter 7 on this link. I have not studied it, but it may help.