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How I enlarged my Hawk tank and how to fix leaks

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  • How I enlarged my Hawk tank and how to fix leaks

    I didn't take many pictures several years ago when I did this, so there won't be all the steps involved here. Also, it's been years since I did it so I'm sure there are steps I've forgotten.

    Using a hand pump MightyVac connected to a hose about 3 feet long allows for getting the last of the gas out of the tank. It'll probably be a good idea to then run some water through the tank to get the combustibles out of there, because there's gonna be sparks.

    I knew I eventually wanted a rolled edge (like a Harley!(?)) instead of the seam on the Hawk tank. I used a thin abrasive cutting wheel and cut the outside of the tank off above the seam and was careful not to cut into the inside pan. After doing that, there's still the overflow and other tube going through the tank that I used my Oxy-Acetylene setup to get hot enough at the joint to be able to separate the top and bottom while hot.

    You can see the extra metal from the removed tank top on the seam here:

    This is the mess I found on the top side, and there was a rubberized liner more than 1-1/2" thick inside:

    I wanted to get more capacity so I bought a VFR800 (or a VFR750?) tank that had a huge dent in it. I separated the top and bottom the same way, easily pounded out the dent, and test fit it on the bike. Also note I made sure the handlebars don't hit it, but also that it extends farther towards the headstock than normal, so much that the top of the tank extends over the front mount hole and you cannot even see the hole when the tank is in place. Mine is held in place by only the rear mount and it's mounted to solidly that I don't even need the bumpers on the side to prevent it from rocking left and right with knee pressure on it in turns:

    Slightly worrying was the interface with the stock seat, but I figured I needed to re-cover the seat and wanted to recontour it so it wouldn't always feel like my crotch is being jammed against it, so I ended up modifying the seat to match the tank after it was done:

    I don't have pictures of the difficult process of trying to 3D hold pieces in the air and making metal pieces to span the gaps between the pan and the VFR tank. Especially with the heat of Oxy-Acetylene the metal wanted to dance all around. Making sure everything is absolutely true and correct was a ton of pushing and pulling and hammering. I should have taken MANY pics between the mockup above and welding it all up. You can see the "flat" surface is all added metal to span between the pan and the top:

    This was the tough part, because Oxy welding causes lots of pin-holes that are difficult to see. I thought I was done and filled the tank with water and saw lots of leaks. I Sharpie circled them and welded again. Filled again, leaks. Same again. Same again. Finally, no leaks!
    I had been pretty careful to do small sections, so it didn't look terrible even before paint:

    I had dumb luck when I decided to put it on the the bike and operate the bike before I sent it off for paint. Thank goodness I did. No leaks with water, but with gas there were several new pinhole leaks. Grrrr.
    So I dump out the gas, use the vac to suck out all the rest and left it in the sun for a day before I'd weld again. I should have run water through it. But fortunately it didn't try to explode when I went to fix the pinholes.
    And then it leaked again. Repeated the process at least another time until no gas leaks. Yay!

    Underlayer of gold:

    Candy layers of red:

    And done:

    It is my understanding that MAPP gas and brazing will work. Fuels tanks have been brazed up from the beginning of fuel tanks existing. I believe they're FAR less prone to pinholes and they can span gaps better and the lower heat of the gas causes less warping. Seriously, I'm not even sure why I welded mine--I created several times the amount of work than was necessary.
    I'm not sure of what brazing rod is best--perhaps a conversation with your friendly AirGas or other welding supply place will tell you exactly what to do. But considering the price of entry for MAPP would be may $60 for the click-start electric torch top and then use the $4 (?) canisters--this would definitely be the way to go.

    So, to fix leaks, split them apart and braze the weak areas and braze it back together. You can even patch in metal without needing to weld.

    The above was my second tank, on my first I cut the tank where the Hawk sticker should be and added an inch of height there. The results were not good:

    Adding 3 pounds-ish of Bondo =

    It's possible your first won't be this terrible, but it was a good experience to know what to do better.
    Hawk with many differently shaped fuel tanks.

  • #2
    Maybe it is just on my end but if you have photos there I am not seeing them at the moment, just as an FYI
    88 Blue Hawk GT - Under construction but rideable (guest approved)
    89 BlackHawk 2.0 - On the lift and being assembled
    90 Hawk GT (color as to yet be determined) - Still on the shelf in crates


    • #3
      No pics for me either.
      1988 Honda Hawk
      2009 Yamaha TMAX
      North Georgia


      • #4
        Working on it.
        Don't spend money and buy, spend time and learn.