Pressure testing a tank

If you have an old tyre tube, cut the valve out.

Insert the valve into a piece of fuel hose. Secure with wire or a very small hose clamp.

Attach to your fuel tap.

Plug filler with big plastic lid if your petrol cap does not seal.

Using a tyre pump, give it a few pumps, you should be able to see the tank expand. (Don't use compressed air!)

Spray the out side of the tank with Windex or soapy water  and look for bubbles.

Leak finder or soapy water. Don't use compressed air.

3 psi tops, no compressed air just use a hand pump and soapy water on the outside. 

I was talking to a bloke who was old enough to race scrambles in the 60s and 70s and he told me they used compressed air to pop the dents out of their tanks after a race.

One time they used an oxygen cylinder!!!!

After the smoke cleared he said the tank was blown completely flat in the shape of a 4 leaf clover!

Just be careful. It' worth putting a strap or two round the tank to avoid overstressing it. And yes, the tank will open out like a pair of wings if unstrapped and over pressurised.