I have used a heat gun to squeeze a 10oz tank into a GP PT-40. Taking what I say with a pinch of salt since I have yet to fly my plane (or any other for that matter), this is my story.

Tank was slightly too big for the compartment when all the control rods ar running through it. GPlanes recommends a 6 or 8 oz, but LHS "expert" said 10oz would fit, and be better. I took 10oz, and sure it fitted, but only if it "bent" the control rods for the nose gear, and throttle. I decided to make the tank longer and thinner using heat gun and re-forming. Only needed to make the top 1/3rd of the tank narrower by 1/4 inch, and needed to make the back end slightly more pointy so that I could force the tank into the compartment without cutting away too much of the fuel tank compartment floor (I had already cur away some, but needed just a little more).

So, out came heat gun, and blasted it until the plastic goes transparent. Made me a little nervouse, but it only goes mouldable at that point. Then, reshaped tank as desired.

The final result looked pretty amateurish, whih was to be expected, but the tank did fit, and I could squeeze enough foam around it to avoid impeding the control arms.

Problem was when I re-installed the clunk in the tank. Re-forming of tank means that the clunk can not reach all the corners. If I had the plane level, then I had to shorten the clunk flex so that the clunk rested on the floor of the tank. but then, it was about 1/2 inch from the back. Lengthening the clunk to reach the back meant that the clunk would never fall quite to the floor.

My concern that I would suddenly suck air instead of fuel made me decide to can the 10oz tank, and go with the smapper 8oz. My reasoning was that I probably had only 8oz of reliably reachable fuel anyway.

So, reforming the tank is easy, but reforming it so that you have access to your fuel, and you can feel confident in it is something else.

gus

P.S. I would need a huge suction to deflate the tank. I could not guess what would be needed to do it. I can not picture doing what Perdo suggests, although maybe with bigger tanks it would work (10oz is quite rigid and tough).