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Discussion Starter · #1 ·
I cleaned the inside of the fuel tank and sealed it with POR-15. (It left the inside very nice)
But of course some of it covered the shutoff valve thread and I couldn't screw it back in.
No problem, stop by ACE, try to find the right thread size, nothing fits, until I ask one of the associate and he said it is probably a pipe thread. So off to plumbing and we find that it is indeed a 27tpi thread.

I got a tap and thought my problems were over. But ignorant me doesn't know the difference between a bolt thread and a pipe thread. Apparently pipe threads are tapered? Well I didn't know that and just threaded the tap too far.
So now the shutoff valve will screw in past the thread because nothing is stopping it.
How do I fix that. Would a thick gasket at the base be enough?
 

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Why not use the POR-15 as a thread sealer.

Or Purchase a 1/4 NPT tap, drill bit and a 1/4"x1/8" bushing. Drill, tap, insert bushing and then screw in fuel valve.

Michael
As someone who does plumbing for a living, I 2nd this. So long as the welded in boss can accomidate a 1/4"
 

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It looks like you would use a 7/16" drill for that 1/4"-NPT tap.
 

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Correct on the 7/16" drill. The existing tapped hole should have been around 11/32", but since he drove the tap in too far, I'm thinking now more like 13/32".

MJ, If you have a 13/32 drill, run this through the tank first. Then follow with the 7/16" drill. Here's the part where you MUST be careful!!! Look at the tap, count down 5 threads from the large end of the tap, and make a mark with marker, paint, anything! You can also cover the first 5 threads with tape! It's best to apply tapping compound/oil to hole, but regular oil will work better than nothing. Start tapping...go 1 turn, reverse tap 1/2 turn. Proceed until your get to the mark/tape on tap. Remove tap and install bushing hand tight only, NO wrench. Put a mark on your tank and a mark on the bushing. Start unscrewing the bushing and count the number of turns when the bushing comes out. I'm thing maybe only 1-1/2 turns! Replace tap and tap 1 TURN. Try bushing in. Typically a tap will have 3 threads showing when tapping to the correct depth. You've never done this (correctly!) and that's why I said 5 threads. When you can install bushing hand tight and then remove it with 2-1/2 to 3 turns, the threads are proper. Clean threads & tank, apply sealant to bushing, install and tighten. Install valve with sealant, and best of luck. Bob
 
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Nice write-up on the tapping process Bob. The first time I tapped NPT threads I screwed them up also. It is a learning process. NPT tapping is more difficult than standard threads because the tap is removing more material the deeper you go because of the taper.
 

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Discussion Starter · #8 ·
Thanks Bob for the detailed write up!
Of course, I see it after I did the work! So I probably went "a bit" too far.
But it looks like it is going to work, it is in there solid!
The other thing I did is that since that bushing goes in further than the 1/8, I drilled a few small holes perpendicular to it toward the top so that I don't end up with gas that can never leave the tank.

267162
 

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I'm glad it worked for you. Anything pertaining to plumbing can get confusing and easily messed up!

Just as a matter of informing people, I'll add what I call "useless information". Aside from tapping correctly, there's also 3 taps available! NPT, National Pipe Taper which is the "normal" thread used, NPTF, National Pipe Taper Fuel. a thread developed for the aircraft industry requiring no sealant, and NPS, National Pipe Straight, a straight thread primarily used in electrical work. If your doing work for let's say NASA, MORE steps are involved! After drilling, the hole is taper reamed. It's then tapped and the correct depth is determined by pipe thread gauge. So for about $150 your threads will be good for NASA! For the homeowner/DIYer, 2 1/2 to 3 turns to remove a hand tight fitting is fine.

Any other questions, just ask. We're all here to help. Bob
 

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FWIW:

Europe and or everyone else in the world (far as I know) does not follow our "NPT" specification or more specifically pipe threads do NOT have taper. So pipe thread taps herein are not the same as those utilized in USA!

Michael
 

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Michael, Yes they do! They to have 2 thread forms, BSPP, British Standard Pipe Parallel (straight thread) and BSPT, British Standard Pipe Tapered (tapered thread). I'm not 100% sure of this, but I believe metric threads are made to BSPX standards. Bob
 
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