Brass NPT fittings in fuel pressure applications

[billmac]

I'm installing a mechanical fuel pressure gauge in my diesel pickup. As part of the installation, I will be using some brass fittings, i.e. needle valve, elbow, hose barbs, etc. My question is, should I lubricate the male threads of the fittings before tightening them. I was thinking of using an oil like 3in1.
My reasoning is that it would allow me to tighten the fitting more securely by hand to make a better seal. I do not plan on using a sealer such as Locktite PST or Permatex #2 because the fuel line connection will be post filter.

 

[johnd]

NPT fittings don't seal as well as AN oil _might_ seal an AN. I personally use Teflon tape-be advised that you can overtighten pipe with Teflon tape because of the reduced friction. I've had good luck, too, with sealants like Ultra Copper. I don't think 3-1 oil is going to do anything one way or the other, esp. as a sealer on NPT fittings.

 

[Bret Chase]

Brass is self lubricating. just use a little pipe dope (or tape) and you'll be fine. we use brass fittings all the time on our 1500psi washer lines. are you trying to find out lift pump pressure or are you going for injector pressure?

 

[porcupine73]

I would use a sealant anyway even if you don't think you need it otherwise there will be a potential leak path. I've read that with NPT fittings it's not how tight you can get them and over tightening can lead to leaks as well and the fittings should be tightened the proper tpft turns past finger tight though I'm not sure where to get the tpft specs.

I've used brass NPT fittings on my fuel injected gasoline vehicles for putting in an aftermarket fuel filter and even with the Teflon tape sealant I can see the exposed tape on the fittings has leached gasoline. I used the same fittings and tape on engine oil pressure connections with zero leaks.

I'm going to try some NPTF (Dryseal) brass fittings from McMaster this time on the fuel stuff to see if the tape doesn't get wet.
-Louis

 

[labman]

NPT fittings are an old design with simplicity being more important than sealing. They originally were used with water a little leakage was no big deal. Often the leaks would lime shut. They still can work well with with other fluids with well cut threads, a sealant, and properly tightened. The various flare or compression fittings work better. For reliable leak freed joints you can't beat the O-ring fittings. Using silicone or Viton O-rings reduces the need to match the elastomer to the fluid.

 

[billmac]

Thanks for the replies. I realize that compression type fittings and o-ring fittings are probably better in this application but I'm going with what I have. The brass threads on the fittings that I'm using are cut vs rolled and are therefore rough. I would prefer to use a thread lubricant/sealant to avoid the possiblity of metal flakes contaminating the fuel system and to prevent galling. I also need something that is high heat and resistant to diesel fuel. I'm probably going to use Permatex #2 and apply it after the first two threads on each fitting. I realize that Permatex #2 is not a lubricant per se but it should keep any metal flakes in check and not leak.