Replacing Stuck Fuel Petcock

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Replacing Stuck Fuel Petcock

Postby bcr751 » Wed Sep 19, 2018 11:41 am

New to the forum and looking for advice. The fuel petcock on my 1948 Model 125 is "frozen" in the tank. When I was restoring the bike, I tried to remove it but it wouldn't budge. Not wanting to screw anything up, I left it in place. Lately, the petcock has started to leak fuel so it now has to be replaced. I'm afraid if I try to unscrew it from the tank, I could cause damage to the tank that would likely be unrepairable. Here is my idea:

Cut the petcock off at the point where it enters the tank. Using a small hack-saw-type blade, carefully cut from the inside of the petcock stem toward the threads in two places 180 degrees apart. When the cuts are almost at the threads, try prying the halves apart to free them from the tank threads. This way, there shouldn't be any damage to the existing tank threads. Then, using a tap of the correct size, go over the tank threads to clean them up and insert a new petcock.

Does this method sound plausible?

Doug
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Re: Replacing Stuck Fuel Petcock

Postby Mutt » Wed Sep 19, 2018 11:47 am

that sounds good. The thread is 3/8 NPT
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Re: Replacing Stuck Fuel Petcock

Postby bcr751 » Fri Oct 05, 2018 7:09 pm

Even though I think this method will work, I'm still a bit skeptical. I don't want to ruin the tank threads trying to remove the stuck petcock. Does anyone have any alternate suggestions as to how I may remove this petcock without screwing up the tank threads?

Doug
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Re: Replacing Stuck Fuel Petcock

Postby peteg59 » Fri Oct 05, 2018 8:24 pm

Empty all fuel, flush with water 3 times to be safe, & heat the bung on tank as well as the body of the petcock & it'll come out.
Not sure propane torch will get it hot enough but most homeowners have one, vs oxy/acetylene torch...
Not much that a little carefully placed heat can't loosen.
Careful being the key word.
Good luck
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Re: Replacing Stuck Fuel Petcock

Postby bcr751 » Fri Oct 05, 2018 8:45 pm

Thanks for that suggestion except the heat will wreck the paint on the tank.

Doug
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Re: Replacing Stuck Fuel Petcock

Postby Mutt » Fri Oct 05, 2018 9:14 pm

I'm afraid if I try to unscrew it from the tank, I could cause damage to the tank that would likely be unrepairable.

I honestly don't think that using a pipe wrench on the petcock, with the tank still on the bike, will hurt the threads in the tank. The petcock is no good, so a big pipe wrench won't matter. Just make sure to put the pipe wrench towards the rear of the bike and turn the pipe wrench towards the front of the bike
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Re: Replacing Stuck Fuel Petcock

Postby hennesse » Sat Oct 06, 2018 8:45 am

I agree with Mutt. The petcock is some kind of pot metal (zinc, aluminum, and whatever), so it's a little softer than steel, but not that much. The best thing is that it doesn't rust, so it's not rusted to the tank threads. It's just been in there for 60 years, and it's reluctant to leave.

Your only real choices are heat, penetrating oil, and the method you suggested. Your method sometimes works easily, but sometimes the male threads are firmly stuck to the female threads, and the "prying out" ends up causing damage to the female threads - which is what you're trying to avoid.

Penetrating oil might help, but I suspect it is not worth the effort. You'd have to drain the gas and replace with pen oil. Tap, tap, tap the petcock many, many times in hopes that a little will find its way between the threads. But since gasoline doesn't, why would oil? Pipe threads are designed to seal better than straight threads - that's why they use them on pipes! Remove the tank, flip it over, and repeat from the outside. After tapping hundreds of times, you most likely will have accomplished little or nothing.

Penetrating oil and heat combined are often the best method for parts that are really stuck together. Heat the joint where the parts meet, and spray with penetrating oil. Continue spraying as the parts cool. As the internal and external parts expand and cool at slightly different rates, a tiny bit of space may open up momentarily between them - enough to allow a little pen oil to seep in. I remember trying to get the end caps off some rusty Knucklehead handlebars without hurting the bars or the caps. Had to repeat the heat/spray process about 30 times, but finally they came apart without any damage to either.

I've taken the petcocks out of maybe a half-dozen Harley gas tanks in my life - just brute force effort with a big wrench. They always came out without damage to the tank. Sometimes I damaged the wrench flats on the petcocks, sometimes not. But since you don't care about the petcock, just grab a wrench!
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Re: Replacing Stuck Fuel Petcock

Postby bcr751 » Sat Oct 06, 2018 12:20 pm

When I was restoring this bike, I thought of exactly what you're suggesting. I put a pipe wrench on the petcock and tried to back it off but it wouldn't budge. Thinking the metal around the petcock socket was pretty thin, I was afraid to really reef on it for fear of ripping the socket right off the tank bottom. However, if you have done a number of these removals with no damage to the tank, I might give it another try, if I can get up the nerve to do it. Seeing as tanks in good shape are hard to find, I don't want to screw this one up but the wrench method sounds like the only plausible one.

Doug
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Re: Replacing Stuck Fuel Petcock

Postby Mutt » Sat Oct 06, 2018 12:43 pm

don't try to go only in the "remove" direction. If ya feel it move when ya start to take it out, stop, tighten it up a bit, then reverse. Go back and forth with it and eventually , it will come all the way out. It's tapered thread, so all ya need to do is break the bond
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Re: Replacing Stuck Fuel Petcock

Postby hennesse » Sat Oct 06, 2018 1:10 pm

Mutt wrote:don't try to go only in the "remove" direction. If ya feel it move when ya start to take it out, stop, tighten it up a bit, then reverse. Go back and forth with it


Good point, Mutt. And once you feel it move, that's a good time to hit it with B'laster or other penetrating oil. The movement has opened up tiny little spaces between the parts that the pen oil can get into, and make the rest of your job easier. Take your time - "Slow and steady wins the race".
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