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Richard Phillips
Richard Phillips
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Dodgy Petcock threads in alloy tank
#757298 11/27/18 3:07 pm
Joined: Feb 2007
Posts: 847
edunham Offline OP
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I have a '71 Triumph 250 I have been working on. I have a pretty nice alloy tank off a similar vintage B25, I would like to use. Problem is that one of the petcock holes has dodgy threads. While I am able to tighten the petcock up fairly well, if I tighten it further it starts slipping. Once I put gas in the tank, it starts dripping down the outside. I am using a petcock washer and teflon tape to seal it. Additional turns of teflon tape slows the drip down but doesn't eliminate it. As I see it, my options are:
1) forget about using the tank;
2) weld up the hole;
3) weld up and re-tap;
4) drill out and use an insert;
5) drill out and use a reducer bushing (like BSA used on some of their bikes);
6) use epoxy on the petcock threads.
I don't like option 1, because I would like to use the tank, but it is viable. Option 1 and 2 require finding a decent welder as well as the expense. I do have the right tap however I am also concerned because after drilling and tapping, I will only have new metal on the tips of the threads. That seems to be asking a lot.
I may not have room for options 4 and 5, and I would be concerned about fuel leaking around the insert, although less so around the bushing. Option 5 is a total bodge, that even if initially successful, will no doubt cause problems down the road.
Anyone have any suggestions?

Ed from NJ

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Re: Dodgy Petcock threads in alloy tank
edunham #757305 11/27/18 4:02 pm
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Well'ard Rocker
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If you or someone you know has the skills, "weld up and re-tap" is the solid, no-bodge, permanent way to go.

Lannis


Do dogs see police dogs and think "Oh no it's the cops!"?
Re: Dodgy Petcock threads in alloy tank
edunham #757315 11/27/18 5:05 pm
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A decent alloy welder is worth finding anyway, car alloy wheel repair shops are a good start, for a side job a few cans of the best local brew work too.

Re: Dodgy Petcock threads in alloy tank
edunham #757317 11/27/18 5:13 pm
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Trying to hold a tank and drill is difficult. You could use a weld-in bung such as these:
https://www.summitracing.com/search/part-type/weld-in-bungs-and-fittings/bung-material/aluminum

Re: Dodgy Petcock threads in alloy tank
edunham #757418 11/28/18 3:04 pm
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edunham Offline OP
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Dave,
I think those bungs might be the answer! My understanding is that the petcocks are 1/4" BSPF, which are straight pipe threads. The bungs are various NPT sizes, which are tapered threads. I understand that the diameter of the tapered threads is larger than the straight threads. So I assume the way to go would be to get the 1/4' NPT bung, have the hole welded up solid, drill and tap it for 1/4" BSPF, then weld that bung onto the existing bung on the tank. Do you agree?

Ed from NJ

Re: Dodgy Petcock threads in alloy tank
edunham #757423 11/28/18 3:56 pm
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This requires thinking outside the box... Tap it out to common 3/8 NPT if there enough material to do this...screw in a common 1/4 to 3/8 NPT bushing using PTFE paste..The screw in the 1/4 BPT petcock using the same paste ..I have done this on my race bike several years ago..., it will not leak if done properly and will save the hassle and cost of getting it welded and then taping it to BST


79 T140D, 96 900M Ducati ....On a bike you can out run the demons..
Re: Dodgy Petcock threads in alloy tank
edunham #757429 11/28/18 4:37 pm
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That sounds like the long way around. No point in buying a bung if you are just going to weld it closed. Any machine shop can make one with the 1/4 BSP thread. Alternatively, if you can wait this place in the UK has 1/4 BSP bungs:
https://www.viperperformance.co.uk/aluminium-pipes_10188_aluminium-female-weld-on-bosses.html

Re: Dodgy Petcock threads in alloy tank
edunham #757474 11/28/18 10:51 pm
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The threaded bungs on the gas tank is either aluminum brazed or soldered, it is not welded. That's good news. Because the bungs are attached on the outside, it should be relatively easy to change using a MAP torch an appropriate rod with flux. Just make sure that the fuel tank is well purged before applying heat. There are a number of places that sell aluminum soldering/brazing rods and flux also there are youTube video's showing the procedure.

The reason I know it was soldered/brazed is that I used my MAP torch to heat the bung on one of my old BSA aluminum tanks. Sure enough, the filler material melted at a fairly low temperature.

Peter Joe

Re: Dodgy Petcock threads in alloy tank
edunham #758006 12/03/18 9:12 pm
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edunham Offline OP
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Hillbilly,
I was going to try your idea. I got the bushing from my local ACE hardware store. The threads are not the same as the petcock, but after I ran a 1/4"BSP tap through it a couple of times, it was perfect. But the bung in the tank is not big enough. If I drill out the stock bung and tap it for the bushing, I have almost no wall thickness left.

So that idea was out. I ordered the bungs from England that Dave Madigan suggested with the right threads. On Saturday night I went to a Christmas party that a pal whose a retired vintage racer holds every year. It's a pretty cool party that a lot of racing legend types come to (Dave Roper comes every year, Ianucci was there last year).
I was introduced to Albert Bold, who some may remember of MV Agusta fame (Google him). Albert grew up in Philadelphia machine shops and started messing with MV's in the seventies. He built an MV for a collector in Austria that Giacomo Augustini rode at a track day and pronounced as the fastest and nicest MV he had ever ridden. At one point Classic Bike did an article on Albert that mentioned him machining scrap cast iron manhole covers into disc brakes! Anyway, Albert is going to take care of my tank for me, which I think is pretty cool!

Ed from NJ


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