Re: Fuel tank issues

Posted by TheOldHokie on June 29, 2013 at 07:38:12 from (74.110.72.9):

In Reply to: Fuel tank issues posted by Hobo,NC on June 29, 2013 at 04:37:24:

Hobo,NC said: (quoted from post at 07:37:24 06/29/13) Whut he said his head is loaded for bear when it comes to numbers :D In the pix it does look like a bolt has been installed in the hole whats up with that I dunno,,, the seller has had issues by his wright up and its misleading... Get a pipe plug lube it with pipe dope and use it to clean up the threads before you mess up your original.. Chines pipe threads are a issue but I have had luck with pipe dope used on the threads to lube and straight'n them up... What I have ran into on these threads... have found good old pipe dope to work the best on those ell fitting fittings... If it does not feel good going together its not gonna seal,,, if it squeaks and squats its not gonna seal,,, If it appears to not engage enough its not gonna seal,,, I have found it best to pre-assemble till it feels right with good old pipe dope... The treads are so ell fitting they need lubrication that's my take on it... BTW use a brass are a non coated test plug...

Well I just read his write-up which is what I should have done BEFORE I posted my first response. His write-up is perfectly clear:

"This tank has straight threads in the sediment bowl hole. If your old sediment bowl has diferent threads we recomenned ebay item number 221081754982 as a replacement bowl. Our replacement bowl has tapered threads made to seal in the straight cut threads on this tank."

What he doesn't say is exactly what thread form is used in the tank fitting. Based on the eBay description they sound like National Pipe Straight Fuel (NPSF) or National Standard Straight Coupling (NPSC) which are designed to form a pressure tight seal when mated with NPT or NPTF male fittings. If combining an NPSC with NPT(F) type male fitting a sutable thread sealant must be used to obtain a pressure tight seal. In all straight/taper combinations the tapered male fitting is typically made from a soft /ductile material (e.g. brass) that will compress and conform to the mating straight threads. A 5/8-18 UNF male fitting will not make a reliable mechanical or pressure tight joint when joined with a NPSF/NPSC coupling - it will be loose and leak.

But wait - there's more!!!

All told there are at least 10 different American National Standard pipe thread forms (ANSI/ASME B1.20.1) and a thread pitch gage will only get you a partial ID. The most common form is probably the National Taper Pipe thread (NPT) found in the plumbing section of your hardware store which requires a thread sealant to effect a pressure tight joint. It is often confused/interchanged with the National Standard Dryseal Pipe thread which does not require a sealant and comes in 4 different variants some of which are straight thread and some of which are taper thread (NPTF, NPSF, PTF, NPSI). The many different pipe thread variants are designed for different applications, have subtle differences in their geometries, substantially different mechanical and pressure sealing requirements, and don't all play well together. You really need to know exactly what you have and what it's designed to mate with when making up new connections.

TOH

This post was edited by TheOldHokie at 09:13:38 06/29/13 4 times.

Replies:

• Re: Fuel tank issues Hobo,NC 10:05:02 06/29/13 (0)

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