Sunday, June 24, 2007

AV - Found in a Drawer

The smartest thing about a riveted fuel tank is that they usually weigh less than a welded or soldered tank. Welded tanks usually use some fairly thick stock; .032 is about as low as you can go without blowing holes with your tigger. Lotsa weldors won’t even consider the job unless the tank is .050. Then too, all of your welded fittings have a pretty good flange that you melt into a nice-looking row-of-dimes when you marry it to the matching flanged hole in the tank.

Riveted tank, you can use damn near anything at all. Most guys stick to .020 because that’s what they’re using for wing skins or whatever, but I’ve used .016 and even house siding. Strictly home-built of course but it hasn’t leaked yet and that was back when Jeeter was a pup.

If there’s any trick to making a riveted fuel tank it’s in using a suitable sealant. Back in the Day they used a special neoprene tape and two rows of close-pitched rivets. But by the start of WWII they’d developed seam sealants that really worked, reducing the chore to a single row of rivets. Nowadays we’ve got polysulfide sealants that form a bond strong enough to be used as an adhesive - - you could drill-out the rivets and the thing would still hold together and not leak. (I’ve written about this before...

A riveted tank calls for riveted fittings and that’s where some folks come to grief. To support your filler-neck and outlet plumbing you need a fairly substantial flange and the easy way to make them is on a lathe. A little lathe will do but not everyone has one. That’s where your local EAA chapter can come in handy. Or your buds out at the airport. Ask around, someone will eventually point you toward a lathe you can play with, mebbe even one that comes with a machinist attached.

Personally, I hate that sorta thing because I’m poor and most homebuilders aren’t. They've got enough money to hire someone to do most of their chores whereas I don't even have enough time to do my own. But I know some guys even poorer than me and when they need something turned, I try to help them out.

(What’s ‘poor’? I’m glad you asked. According to the Bureau of Labor Statistics there’s about two hundred and thirty-three million Americans working for wages as of 2006. Line them up according to how much each of them makes and the guy standing in the middle of the line is getting about $28,000 per year. Which means half our work-force - - 116,000,000 people are earning less than $28k per annum. Like me. And a couple other guys I know are earning a lot less.)

Saturday one of these lo-buck builders dials the Secret Number then comes by the shop with his new, riveted fuel tank, complete except for the flanges. I was impressed. He has cast a flange to accept a filler-neck he was planning to glue in place with sealant when I snatched it out of his hand and threaded the thing. (Not a perfect casting but usable.) Whist doing so he told me that besides Show & Tell he’d really come by to see if I could make him a flange for the outlet, something to support the finger-strainer. He didn't want to trust his melted-down beer-cans for the outlet fitting.

Major case of Deja Vu.

Every fuel tank has some sort of strainer. For many years the standard was a finger-strainer made of bronze screen soldered to a brass bushing threaded 3/8-NPT on the OD and 1/4-NPT on the ID. The 3/8-NPT was the standard thread for the fuel tanks on light airplanes, allowing the finger-strainer to be screwed into place. The quarter-inch pipe thread on the ID would accept a small brass shut-off valve or a fitting for hose or tubing.

Finger strainers have largely been replaced by a lighter, all-aluminum set-up in which the strainer is a section of quarter-inch aluminum tubing, mashed flat on one end, slit several times with a hobby-saw, flared on the other end and installed with regular AN tube fittings. This is not only a lighter arrangement than the old finger strainer, it performs better since the kerf of a hobby-saw is typically about .028" wide whereas the bronze mesh would allow particles as large as a sixteenth of an inch.

Still, finger strainers are a standard item, they don't go bad on the shelf, and so inexpensive (a couple of bucks each) that most mechanics keep a few on hand.

A couple of years ago I needed a finger stainer, reached in the drawer and came up dry. I thought I had a couple somewhere but they’d gotten away from me.

Not a problem. I called Flo Irwin at ASS Co. and asked her to send me half a dozen. A pregnant silence oozed out of the phone then Flo asked, did I know what they cost nowadays. No, I didn’t but it couldn’t be more than a coupla bucks, right?


After she told me what they were going for - - and after I got my heart beating again, she said they were only being made by one supplier, hence the horrendous jump in price. So I ordered just one.

Next time I needed a finger-strainer I made my own. In fact, I made about a dozen of the things. Tossed them in a drawer. Forgot about them. And of course, economics being what it is, once the price of finger-strainers started being listed on the Big Board a buncha people started making them and the price came back down to a reasonable level. (I think it’s now about $5. [June 2007] )

Back when I’d made the finger-strainers I’d also made up a batch of fuel tank flanges threaded 3/8-NPT for outlets and smaller jobbies threaded 1/4-NPT for vents and the like. So after threading the filler-neck I dug around in the drawers, picked out a nice flange and an ugly finger-strainer and asked if he wanted fries with that.

Man really can fly, with a bit of help from his friends.