What alloy is an aluminum Peterbilt diesel tank?

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Anyone know what alloy these tanks need to be welded with? Upcoming project for my 1949 Ford F-1 rat rod: need to shorten an aluminum Peterbilt diesel tank about 2" so it will fit in bed of my truck. Tank has been off and airing out for about 15 years, and will be cleaned out again before cutting the head off one end, shortening the shell 2", then reattaching the head. Joint will be a single-v with 1/8" root opening, 1G position. Tig welding will be performed with the big Airco in my signature line. I weld steel & stainless open root pipe in an asme shop for a living, but never did aluminum pipe... looking forward to a new challenge. This tank is about 22" diameter.IAMAW Local 330Airco 300 AC/DC HeliwelderAirco Dip-Pak 200 with Profax spool gunPowr-Kraft AC225Everlast PowerArc 200stBuffalo Forge No.21 drill pressSpeedglas 9100xxAirco, Oxweld, Purox, Victor torchesLincoln Ranger 8
Reply:Hello wnywelder, very possible that the tank is made from 5052, whether or not this is accurate, if you weld it with 5356 filler you should be good to go. When you cut it apart to shorten it consider using a worm drive skill saw with a high tooth count carbide blade, almost no risk of explosions etc. as you have minimal heating and no sparks (providing you don't hit anything with the blade that isn't aluminum. As to the cleaning and such: consider that aluminum is porous and even after such a length of time it can weep undesirable residuals when heated and cause a dangerous situation even now. So take special care to clean it again and even consider purging with a non-flammable/non-oxidizing gas. Good luck and best regards, AllanLast edited by aevald; 07-22-2015 at 06:37 PM.aevald
Reply:I've used 5356 on patching fuel tanks in the past and it works well but if you are going for a shiny finished look, I'd polish out your section removed, then weld a short bead upon this section. A 600 grit scratch pattern will buff off pretty easily using red rouge after knocking the bead down with coarser abrasive. The polished weld bead will give you an accurate representation of the finished product as far as appearance. If close enough, then weld'r up, if you need a different filler, it's a lot easier to get on track rather than when the tank is back together.I'll be looking you up in about a month or so.Thanks,SlobPurveyor of intimate unparalleled knowledge of nothing about everything.Oh yeah, also an unabashed internet "Troll" too.....
Reply:I have shortnend and repaired tanks before, usually mig but also tig. I have used 4043 and 5356, both worked fine. An 1/8" root spacing? Are you talking gap or bevel width. Most tanks are about .090- .100 thick. On the tanks I worked on the end was formed like a tray that fit inside the rolled part of the tank. If you get an aluminum grinding wheel you can thin down the weld, and by cutting about 6" pieces out of the scrap rolled part you can break them away and keep the end intact. Then you are doing a lap weld. Pressure test a 2-3 psi max.
Reply:Had no idea the cap/ head was designed for a lap weld, but it makes sense. I am used to everything having an 1/8" gap, so I assumed this tank would be similar. I guess I will see when I cut it apart. As far as polishing goes, this project is rat rod in the truest sense, so there will be minimal to no polishing. Thanks for the input guys!IAMAW Local 330Airco 300 AC/DC HeliwelderAirco Dip-Pak 200 with Profax spool gunPowr-Kraft AC225Everlast PowerArc 200stBuffalo Forge No.21 drill pressSpeedglas 9100xxAirco, Oxweld, Purox, Victor torchesLincoln Ranger 8
Reply:Forgot to mention. On the joint where the end cap goes inside the rolled section there will be somewhat of a gap. If you use a body hammer you can close the gap by tacking and tapping the rolled section down.
Reply:MJD is right on the money with the ends. If you don't realize it the first time you go to shorten the tank is 'WTF' by then it's too late. Easiest way I've found is to cut it down to the length you want leaving the excess on the end and then removing it from the end once it's off. That way you can peel it off as you remove the weld...Mike
Reply:Originally Posted by mrmikeyMJD is right on the money with the ends. If you don't realize it the first time you go to shorten the tank is 'WTF' by then it's too late. Easiest way I've found is to cut it down to the length you want leaving the excess on the end and then removing it from the end once it's off. That way you can peel it off as you remove the weld...Mike
Reply:When you put the end cap back on make sure you space tacks all the way around it before starting to weld then jump around.