Spring 2005 modifications

Building a new fuel tank


The old petrol tank
The first fuel tank that I fitted was a 12 litre plastic boat-style tank from K&M. This was OK, but because it was fitted underneath the seat, it meant that the seat ended up being too high and too wide. I had always planned on fitting a custom made tank, and a modified seat, so Spring 2005 was the time to make the changes - in time for the race season.

Asking around, I found that the TZR powered craft used around 6 litres or less per race, so I reckoned I needed a tank of around 7 litres capacity. I found a filler neck and cap machined from aluminium from Randall Motorsport, which I reckoned was a bargain at £10, which meant I could maybe make a long cylindrical tank to lie underneath the seat. In order to kep the seat low, I needed to keep the diameter down, so I looked at a range of dimensions.

The best compromise seemed to be 140mm diameter by 450mm long with a capacity of  7 litres, which would weigh around 1.3kg when empty. Allowing 40mm for the filler cap, this meant that the top of the seat would be around 180mm above the floor. I ordered a 2m x 1m sheet of 2mm thick aluminium (in 5251 alloy - a harder grade than I had used before) which I reckoned would be enough for 7 tanks. At £35 a sheet that's just £5 per tank. I rolled the sheet into a cylinder using a mangle roller and cut out a couple of cylinder-end blanks, and while I was at it I rolled a spare to practise on.

I was not sure that I could MIG weld aluminium as thin as 2mm, so I gave it a try with some flat sheet. With a backing piece, the weld itself was fine, but the sheet distorted very badly. I needed another approach. Talking it over with the guys in work we came to the conclusion that I needed to lap the edges to be welded, clamp the sheet firmly to a strong backing piece to prevent it lifting off, tack weld it at  2" intervals, and let it cool between weld runs. A cylinder would also probably warp less as it is inherently stronger than a flat sheet.

What about baffles? Under the massive(!!!) acceleration that I am expecting, would the fuel end up sloshing to the back of the tank away from the pickup pipe? Maybe I should add some internal baffles? A couple of discs of 1mm sheet held in place by a lengthways strip would probably do it.


Cylinder with stainless steel backing tube

Backing piece screwed to the tank
I used a length of 30mm square section stainless steel tubing as the backing piece, clamping it at the ends, and screwing it down in the middle. This all worked a treat, and by doing a quarter of  the seam at a time, and allowing it to cool between runs, it all went well.

Once the seam weld was finished, the backing piece could then be removed and the screw holes could be welded up at the end.


The backing piece around the end of the cylinder The end discs were recessed into the cylinder by around 10mm. To provide a backing piece for the end discs I wrapped a strip of thin stainless steel sheet around the end of the cylinder and held it all in place with a couple of jubilee clips in series.

Welding into a V is always easier if you can adjust the work so that the bottom of the V is at the lowest point, so I angled the practise cylinder to around 45 degrees to the horizontal. This proved to be too steep as the end disc melted through very quickly. Adjusting it to a lower angle, was far more successful.


Having finisished welding both ends of the cylinder I then thought about baffles again... ah.  Fortunately there are other ways of preventing fuel movement - such as filling the tank with foam cubes (Merlin Motorsport) or even stainless steel wool apparently.

I tested the welding around the ends by pouring some soapy water into the recess and blowing into the screw hole. I couldn't see any leaks in either end so things were looking good. Although the welding had gone well, I wasn't confident enough to tackle adding on the filler cap and the pickup pipe, so I asked Nigel in work if he could finish things off using the TIG welder.

Nigel welding This is TIG expert Nigel Jarvis. Nigel TIG welded the filler cap and the fuel pickup pipe.

Once Nigel had welded the filler on he let me have a go on a practise piece of aluminium. It was very difficult at first - especially as he "forgot" to turn the gas on! Oh how we laughed...

With the machine set up properly (that's half the battle) I appreciated that TIG is much more precise than MIG as it allows you to vary the heat and filler metal separately, whereas with MIG, the filler metal (the wire) carries the electric current (the heat) so it's largely either on or off.

With the filler and pickup pipe welded in place I drilled out the hole in the tank, inside the filler neck.
Nigel looks pleased

Once the tank was finished I gave it a quick grit blast to clean it up, then sprayed it with etch primer and green gloss. Total cost of parts and material used was around £20.