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I have a 20 gallon aluminum tank that has 3 holes in i:
1. For filling gas ~ 1.5 inches in diameter
2. Had a vent line hooked up to it ~.5 inch opening
3. A threaded fitting 90 degrees with what I think is called NPT fitting, think its 1/4 inch, and thats where my fuel like went.
I want to cap it, and close the 2nd hole I described. The caps have vents on them, so there is no need to have a separate vent, similar to the mercury plastic fuel tank I have for the 2001 13 Sport. Just a cap with a small screw on top for venting.
So it could be threaded with a special tool for cutting threads in it, and then I would need a custom cap.
I installed an aluminium Seabuilt tank access plate for my fuel tank. I tapped and threaded it for a fitting. Your ability to thread is dependent upon the thickness of the aluminum. The access plate is 3/16th" thick, threaded easily, and the fitting secured well. I do not know if the same can be said for thinner walled material. Might try a fine threaded plug/tap for thinner material. Probably get some "bite" with 1/8" thickness if careful.
Garris
If you have a "nipple" coming up from the vent hole, a short piece of hose with a plug would work
Gasoline is such a dangerous substance, especially on a boat, that I would recommend only using a purpose built tank for the purpose for which it was designed.
Twenty gallons is a huge amount of fuel to have permanently or semi-permanently mounted above deck. Aluminum is a soft easily penetrated material relative to steel.
One or several Moeller plastic tanks can be used above deck in relative safety. They are not prohibitively expensive.
So, Kings Bay? Things in the fire fighting business looks to have changed since 1974. When we were dealing with AFFF we had large tanks supplying hydro powered proportioning pumps to mix the AFFF with water in the proper ratios when discharging with the hose or spray heads.
Unfortunately, I seriously doubt much time for fire fighting would be available if a serious leak developed in a 20 gallon fuel tank and it ignited. With some luck the pax could abandon before being seriously burned. No time to don fire suits before the boat becomes engulfed. One of the few downsides to GRP boats.
