OK I have over my many decades of working on cars never had to drill a hole in a used fuel tank. So I am asking for ideas. Here's the layout. I have to drill a .550" hole in the TOP of a 10.5 gallon Willys jeep fuel tank to insert a Bung for a Fuel return line. The tank for various reasons can not be removed from the vehicle (to do so would most likely screw up the outlet bung to the point the tank would have to be replaced) So far I am thinking I fill the tank with water drill a small pilot hole (7/32) then using a Step drill drill out the hole. The Outlet is actually the very bottom of the tank so removing the water is not a problem (a PITA but not a problem) I can use an old school carpenters Brace to power the bits. So no sparks from a power drill. SO any tried and true methods to not blow myself up?
I would do the water fill, and pilot hole... Let it drain repeat the water fill and drain. Then go for it. Remember gas floats. FWIW I watched a guy drain a tank with a hatchett ( For the gas )
I had to drill a 5" hole in the top of my tank for a fuel pump. I was worried! I had the tank out of the car, filled it to the top with water, got the hole saw out and drilled away! I was nervous, but all went well. Took a bit to get all the chips out but I got it. Been driving the car for two years now and nothing in the filters.
You could also use exhaust pumped into the tank as an inert atmosphere. Maybe pour a bit of alcohol in the tank and toss a match in. WHOOSH! and instant inert atmosphere! (j/k!) An inert welding gas works great.
The sending unit is a SW electronic sending unit with no moving parts the body is a sealed maybe 3/4" thick round block. So no way to drill through it. I could I guess swap it out to an old school sending unit But with only a 1.5" D hole in the top of the tank not sure if there is room for the Bung. I have a sending unit that might work with the gauge I can check that out. Though I hate to give up the expensive sending unit.
The bung is an AN fitting that has a locking nut and two nylon washers I can just reach a suitable location for it and get a wrench on the locking nut working through the sending unit hole.
No junkyard is back purging fuel tanks before drilling. Just drill it. It will be fine as long as you don't do something actively stupid.
Think out side box , Cut up a wrench , box or open , Piece of round stock weld , Aviation and Aerospace makes some neat little tools, If me , if tank not in side coated , I would Probably solder ,brase ,weld
Compressed exhaust is explosive. I mean it really is. CO (carbon monoxide) is explosive under pressure. A fuel tank company ( serviced/installed) underground gas tanks in Florence Al. Instead of a compressor they got the idea of using their 1965 Chevy truck exhaust to pressure test the tank. Well somebody let it run too long and over pressurized the in ground tank. BOOM!!!!!! Nearly all the windows in Florence were busted. It was a pretty big deal. I’d just drill the hole. If you wanted to be really safe you could use a brace and bit.
I have a CJ3A and there is room for the return line in the sending unit. Then you can leave the tank alone.
My sending unit is nothing like yours. It looks similar to this but is evidently discontinued as its made by Intellitrox and was sold by SW https://static.summitracing.com/global/images/prod/xlarge/hly-19-228_mm_xl.jpg
I'm going to run down to the grocery store pickup 5lbs of dry Ice break it up shove it in the filler neck with the bottom of the tank closed off and the sending unit hole on top open when it looks like the gas has filled the tank I will put a loose cover over the sending unit hole. Take a HD ICE pick and poke a pilot hole through the tank where I need the hole and the using a Harbor Freight step drill and my hand brace drill out the hole. If I don't blow up I'll come back here and show a photo.
Have used the exhaust method many times. I usually rock out fuel,, fill the tank up with water and rinse it out.. Then you put the exhaust from a running car in the tank.. You make sure the tank gets warm from the exhaust about 15 minutes. It is now ready to drill cut or what ever you want to it. Learned this trick from Ford dealership I worked for in the early 1970's. I usually attach a match on a long stick over the neck to make sure tank has an inert atmosphere. Just a safety precaution. Cannot be too careful. Good luck. And yes blew an old gas tank in an abandoned dump truck in the woods as a kid. Scared the hell out of me.. Dumb move but lived to tell the tale!
I'm using the drain plug hole as my outlet. I have an older aftermarket repop tank the outlet has less then a 5/32 opening through the bung. I did not think that was enough for something that was calling for 6AN hose. I have been using the Drain as the outlet the whole time I have been running the Willys a little less then 10K miles now. You guys have been very helpful. I just finished drilling and cleaning up the hole I used my "ICE pick" to punch a pilot hole after triple checking the tank was full of very cold CO2 and that I had a goood surface on the inside of the tank to get the nylon washer and Nut to clear the doubler around the Sender opening. Then about 2 min with the carpenters Brace and the well used Harbor Freight Step drill and it came out just a hair to tight so I used my deburring tool to dress up the hole and make it a few thousands larger and the BUNG fits really nice. I'm waiting for the CO2 to dissipate so I can make sure and pickup any chips that might have gotten away from the magnet in the first place. Then I will anchor the Bung and assemble all the plumbing to get the return through the floor and over the tank.
If you can reach through the sending unit hole you can use a punch instead of a drill. https://www.ebay.co.uk/itm/134780889609
Don’t try to suck anything out with a vacuum cleaner , it tends to blow the cover off the vacuum outlet in a big hurry !!!
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