Fixing a Leaking Motorcycle Petrol Tank

When I got my BSA B25 SS Gold Star, there was Aspen Fuel in the petrol tank…although I must admit there seemed to be less in it when it arrived than when I went to look at it. Aspen has very little smell and leaves virtually no residue as it evaporates. On my first ride out I put a gallon of premium E5 in the tank. When I left the bike in the garage on my return, I noticed a smell of petrol and the odd drop on the floor a few days later. Examination revealed a sticky residue on the left underside of the petrol tank. The previous owner had obviously tried to stop the leak using plumber’s mate tape on one side and rubber o-rings on the other. I reckon two thirds of the gallon of petrol I’d put in, had leaked out and evaporated!

I drained the tank and removed the agricultural water valves he had used. I cleaned the bottom of the tank and installed the BAP style petrol taps, one for main and one for reserve. I also used rubber and copper ‘crush washers’ from Feked. Once installed, I thought the problem may have been solved, but there was still a small leak from the left hand side, reserve tap area.

Investigation revealed that the leak was actually from where the boss that the petrol tap screws into, was let into the tank itself. These tanks are around £200 and come from India…returning it wasn’t really an option. Having it welded and machined would have been expensive, so I looked for a suitable compound I could use to seal the tank.

I came across JB Weld, specifically their Marine Weld, which tolerates petroleum spirit and other solvents once set. I bought a pair of tubes from local supplier AV Taylor in Hexham. Having drained the tank again, cleaned the area with isopropyl alcohol (IPA), roughened the area to be sealed with sandpaper and cleaned it again, I mixed enough Marine Weld to do both sides.

Remember to leave the petrol tank cap open or pressure will build in the tank and blow small bubbles through your JB Weld…there’s a reason I know that! 😉

I made sure the epoxy sealed the join and smoothed it with a finger to get a neat finish. The unset epoxy can be wiped off using a tissue soaked in IPA, both from your finger and from the tank if you slip. After waiting twenty four hours, I put petrol in the tank, there were no leaks so now if is back on the bike.

The front brake, fork dust covers and odd drops of oil from the cylinder studs are still to sort out.

You can use JB Marine Weld on other sections of the tank, such as the seams, if there are small leaks…but remember to prepare the surfaces first; they need to be roughened but free of dust and cleaned with IPA.