SOTP Vintage Series
Main Motorcycle: The one between my legs
Join Date: Feb 2009
Location: Upstate NY
Other Motorcycle: A few
Extra Motorcycle: Yes
Gas Tank Leak Testing
In a thread a few weeks ago, somebody asked how to check a gas tank for leaks. I provided a test procedure that I had assumed would work, but had not yet tried. I needed to test a '66 Bonnie tank for leaks prior to painting. So I set up and tested per that procedure and it works like a champ. I talked to Don Hutchinson about paint today and in that conversation, he made me realize that I had not really checked the filler neck crimp joint because I used a plug instead of a cap on the filler neck. So, I made up a cap and retested and my neck is fine. I did however find two leaks in one of the factory welds, which I suspected were there due to obvious paint damage.
So here's the improved and tested procedure for those of you that are getting ready to repaint a tank and want to make sure it has NO LEAKS prior to painting.
Before going any further, I want to be very clear that under no circumstances should you used a air compressor or tank of compressed air to do this test. If you do, you risk injury and/or damage to the tank.
1 each 1/4 NPT pipe plug
1 each 1/4 NPT barbed fitting (sized accordingly, see text below)
1 filler neck pressure cap (see fabrication sketch attached)
1 hose clamp to affix air hose to barbed fitting
1 bicycle pump
1 small container with soapy water
1 syringe or sponge or method to apply soapy water
First, you need to make a pressure cap so you can check the crimp joint where the filler neck is attached to the tank. All the pro-painters that do Triumph tanks I've spoken to will not paint your tank unless they silver solder the crimp joint. If you choose to solder this joint yourself or have it done locally, you should test it afterwards to make sure it does not leak.
The pressure cap is simply a large 1//4" thick washer you can probably get in a good hardware or farm supply store. Mine was 3" in diameter and happened to be something I had in the shop. If you don't have a washer, a small piece of steel plate will work. You could probably even use a piece of wood if you had to. It does not have to seal perfectly. Just be able to hold some pressure in the tank.
My cap is a washer with a rubber gasket that presses against the mouth of the filler neck. I used a 1/2"-13 bolt becaue the hole in my washer was a half inch. I have a hard fiber washer to seal the head of the bolt to the washer. The backing plate is a piece of 1/4" x 3/4" barstock, drilled and tapped 1/2-13 for my bolt. I put 1/8 rubber pads on the plate to avoid damaging the tank inside. The backing plate dimension given in the sketch should allow you to fish the plate into the tank and then tighten the bolt. YOU DON'T NEED A WRENCH!! On a 1/2 inch bolt, finger tight is sufficient. Small leaks are not a problem. You just want to hold a positive pressure. You can crush the neck with this rig so fingers only, please.
Screw a pipe plug into one of the petcock bungs and the barbed fitting in the other. No Teflon tape is required. It just shreds and falls into your tank anyway and like I said, small leaks are not a problem. The barbed fitting end should be sized to fit the airhose on your bicycle pump snugly. The hose clamp is to seal the hose and the barbed fitting. Although I found that if the hose is snug, the clamp is no required. If you blow the hose off, you have way too much pressure in the tank.
Fill a plastic bowl with water and add a generous amount of liquid dish detergent. Mix it, but don't suds it all up. If you have a syringe, suck some soapy water into the syringe. Now, give the bicycle pump several strokes. I probably put about 5 or 6 strokes into the tank. Watch the sides of the tank in the flat knee area. They will start to bulge out with too much pressure. If you see them bulging, STOP PUMPING. You can now start to squirt the soapy water around pontential leak areas. If you don't have a syringe, use a paint brush. Don't squirt it like a squirt gun or that will create bubbles. Just drizzle it on the tank. When you find a leak, there will be no question. It will start to blow bubbles like crazy. Run some around the filler neck joint. This is why your pressure cap top washer cannot be too big. You need to get into that crimp area. Put water in the tank mounts on pre-OIF tanks to make sure they don't leak. On OIF tanks drown the center bolt recess on both sides (top and bottom). Do all the welded areas, including the seam down the center of the tank. Keep putting a stroke or two into the pump because your plugs are probably leaking and you are losing pressure. Just don't over-pressure the tank. And don't forget the emblem screw holes. When you are all done, do it again to be sure. When you are thru, unscrew the filler neck cap to relieve the pressure and then remove the rest of the plugs.