The Basics of Custom Hard-Line Bending

Penultimate. It means next to last, and this is it. Our twin-turbo F-Bomb Camaro has been lingering around these pages for six issues-this month we're finally done showing you how to build it, and in the September issue we'll be able to gloriously reveal what it's like to drive the thing. For a lot of the players, that moment will culminate with one big sigh of relief. That's certainly true of Mark Bohlen, who's the last in a long line of F-Bomb craftsmen we've introduced you to.

Along with his wife, Michelle, and Twig, the ubiquitous Italian greyhound shop dog, Bohlen operates Bent Custom & Performance out of a supertidy shop that's in the same industrial complex as Nelson Supercars, the instigator of the Bomb. While Bent is a full-service fabrication shop-regularly handling minitub jobs, chassis modifications, and street rod setup-he's become known around here as the go-to guy for custom-bent hard lines for anything that carries automotive fluid or air. Applications include fuel systems, air conditioning, brakes, oil coolers, vacuum lines, airbag-suspension plumbing, and anything else you can think of.

The Bent shop creates all the hard lines for Nelson Racing Engines' twin-turbo engines, and that's how we got hooked up with Bohlen for the F-Bomb. In addition to the engine plumbing, he handled all the complex fluid conveyance systems on the chassis. Ninety percent would be hard line; flexible braided line would only be used in areas that need vibration damping, such as between the engine and frame, and in spots that require quick-release AN fittings.

The first step with hard line is to choose the material. While some racers prefer it for its light weight, Bohlen rarely uses aluminum tubing, telling us, "It's easy to bend by hand, but you can hardly ever get it perfectly straight since it always comes in rolls. It's so soft that it flattens out if you try and unbend it, and AN flares in aluminum gall up after a few times being connected or if you overtighten them. It can be polished, but it gets dull quickly." Another option is mild-steel tubing, which has the advantage of being easy to bend. Even so, Bent does not use it often because it has a seam that can be problematic, and because it cannot be polished. Bohlen's material of choice is aircraft stainless tubing, despite the fact that it is the hardest material and the most difficult to bend. However, Bohlen says, "Stainless is great because if you leave it unpolished it looks race, or the show guys can polish it and it stays shiny forever. Small dings can be polished out. It's hard enough that you can use single flares rather than the double flares that you have to do with soft mild-steel tubing. It might take more effort to make an actual bend in it, but it's cleaner to work with overall."Even so, Bohlen warns that some so-called aircraft-quality stainless tubing still has a seam inside it. He's found that the seam leaves a hard bump anytime a flare is added to the tubing, and that the imperfection can cause sealing problems at the junctions. Bent sells its own true seamless stainless tubing to avoid those problems. Bohlen typically uses 0.035-inch-wall tubing in sizes 11/42 inch in diameter and smaller, and 0.040-inch-wall for 51/48 inch and up. The thicker walls on the bigger tubing help prevent it from collapsing in hand benders, which do not have a mandrel inside them to support the i.d. of the tube as it is formed.

While Bent often must fabricate custom line clamps, our car used out-of-the-package pieces from Kugel Komponents. These clamps and brackets are available for most tubing sizes in both single- and double-tube designs. Note that Bohlen put a bend in the tube to clear the attachment screw; he never lets hard line rub on other objects.

Here, on a different car, is another solution to dual fuel lines: a Bent-fabricated Y-block. The company will soon offer CNC-cut, billet-aluminum blocks for this purpose.

Bohlen advises, "Never connect hard line to flex line without a fixed attachment point. If you just leave it flopping, it's ugly and the hard line will probably work-harden and crack." This shows a bracket welded to the Bomb's frame where the fuel lines merge from hard line to Earl's AN hose through bulkhead connectors.