It has been said that the fuel tank on a motorcycle is the single defining design element. If that is true, then this project is in trouble.
The design brief was to keep it looking as stock as possible in terms of the tank so that it would be visually identifiable as an old Suzuki GT750 and yet at the same time it clearly would be something else. It had to look complete and not like a pile of mismatched parts.
Closer examination of the tank revealed a few fatal flaws....
There are several rust bubbles on the right side and at the rear where the seat rested against the tank there were rust spots that looked like they may be a problem. There's also that nasty dent in the rear and a smaller crease on the right side.
And that was just the starting point. Obviously the tank required a little minor cosmetic surgery. And since it was about to be cut, why not make a few small changes at the same time. The first change was a bit of a no-brainer. That old leaky filler cap had to go. It could either be replaced with a nice shiny new cap from Aircraft Spruce or I could just cut the filler section out of a GSX-R tank and weld that into the gaping hole I had just cut.
Fortunately, I wanted to replace the filler cap with a modern flush aircraft type. Now it just so happened that I had scored a crash damaged 2007 GSXR tank and fuel pump off ebay really cheaply. I got it for the pump and the flange the pump bolts to, but the filler part was OK.
2007 GSXR tank. What you can't see in this picture is the top which is severely dented.
Using only the finest surgical tools - in this case a 4 1/2 inch angle grinder, the fuel pump flange and filler cap were skillfully removed.
Next, the old filler cap was cut out of the GT tank with the same surgical precision. Of course I had to smooth it out somewhat before sticking my hand in past the razor sharp edges. At this point things started to get weird. The tank started to rattle when shaken, it smelled awful and there was a coating of gooey slime on the bottom of the tank.
The rattle turned out to be a fine collection of Lego bricks, crayons and one leg from a child's toy. The slime was a mix of oil and fuel residue which came out with a couple of washings with degreaser.
After the wash, the damaged areas were cleaned off with a sanding disk on that "ankle grinder", as my kids used to call it. Of course I had completely forgotten that a GT750 tank is very shallow at the front where it clears the radiator filler cap. That oversight led top a slight reshaping of the tunnel which in turn tore a hole in the steel. Ah well, just another item to weld up later.
The rust was quite deep in places as can be seen on this patch in the middle of the right side. At the rear corners I could see straight through the pin holes..
After a lot of hammering in a very restricted space, it was starting to look much better. The large dent at the rear is starting to pop out but has a long way to go. It looks ugly, but a little filler and no one will know! I'm not happy with this at all, but metalworking isn't my thing, so it takes time. i may attack it again later.
After dinner I tried again and now it's starting to look much better. I need to buy some metalworking tools to make this sort of job easier. You can also see the holes that I have started to fill with bronze weld.
The shot above is the rear corner of the tank shot through the filler cap opening with the camera part way into the tank and a Maglight to give the camera something to focus on. See how many holes have opened up and the bronze has filled them.
So much for the damage, the next step was to fit the new filler arrangement from that GSXR750 donor and then make it all look presentable.