Fabricating Tapered Tubes -Cool Cones

Some street rod-building projects are easy, some are tougher, and some are difficult enough that the best approach for the homebuilder is to have that task handled efficiently by a pro. Where rodders have difficulty sometimes is determining what category describes the job in front of them. By underestimating the situation (and overestimating our abilities) we can wind up with a jumbo can o' worms, and most hands-on rodders have been there a time or two. The other avenue is to err on the side of caution and assume you can't do it.

Making your own tapered lengths of tubing for a special need might be something you would suspect was destined for the farm-it-out section of your project's to-do list, but it's easier than you think. If you have a welder, a vise, files, sander, and a reciprocating saw, you can do it!

Harry Hoffman, a veteran Bonneville/ off-road motorcycle racer, is a very talented craftsman with decades of experience working with metal in every form. When a friend brought him a cool '20s-style speedster, Harry had the task of installing an Oldsmobile Quad 4 engine with a modern automatic, which was simple enough for a guy who can build a whole streamliner. The kicker was that everything in the drivetrain had to look as much as possible like a vintage oval track engine, like a DO Frontenac or a Miller..Harry disguised everything in such a masterful way that we don't have enough pages to describe it, but we asked if he could show us how he made it when we spotted the water manifold atop the engine.

"I had actually never done this exact job," said Harry, "but it turned out to be easier than it looks." Typical engines of this period had several water connections that needed to be routed to the radiator, and what you see here atop the "antiqued" 16-valve motor looks just like what the old racers had. "It's a fun project that could be done, from start to finish, in less than an hour," according to Harry. The technique shown here could be applied to make a tapered length of tubing for a number of purposes-an outside nostalgia header collector, water manifold for a V8-60 flat motor, an air intake, exhaust cone, or anyplace where you need to join two different sizes of hoses with metal tubing and still have it look cool.

The water manifold on the Quad 4 was made of stainless steel tubing, with short runners cut from stainless 90-degree bends and welded on, but the work is the same regardless of the material. Our example here is made with 2.25-inch-diameter steel exhaust tubing of 16-gauge thickness. Harry's opinion is that this is the right gauge for such a job because thinner material dents too easily in the hammering phase, and heavier tubing is too hard to shape with a hammer.

Start with a length of tubing that is already the diameter of what will be the "fat" end of our tapered tube. The other end of our cone will have a 1.5-inch diameter, and a short length of straight tubing will be added to each end of the cone to accept hoses of those diameters. We're going to be taking a pie-section of the whole length of the tube to make the cone. There's some math involved here, but we promise it will be relatively painless and over quickly.

We need to work with the desired circumference (around the outside of the tubing) at each end. It's more precise to use the calculator here, rather than try to measure the tubing circumference with a tape measure, although that could work also. To find a circumference from the known diameter, just remember your old high school math. So the circumference at the existing tubing size is 7.068 inches, and we want the other end to be 1.50 inches in diameter, which means that circumference should be 4.712 inches. The difference in the two end circumferences is 2.356 inches, and that's what must be removed at the widest part of the pie-cut, tapering down to nothing at the other end of the tube. This isn't the space shuttle, so we can round up or down a little to find marks on our tape measure that work out.

The tube can be marked at one end with two marks to show the edges of the cut. If you are using tubing with a seam, as shown here, use the seam as a centerline for your cut, spacing your two marks equally on either side of the seam at what will become the small end of the cone. If you're going to plate or polish your finished cone, it's better to be cutting out the seam anyway. Just on the off chance you didn't remember the high school math required in the previous paragraph, that old formula is diameter x pi (use 3.14159) = circumference. Had you for a minute, we could hear you groaning!

Mark the tubing and Sawzall the pie-section out, leaving the last quarter-inch uncut at the big end of the tube. Do a few inches at a time and then move the pipe in the vise so you are not cutting the part held in the vise, otherwise the vise pressure will close the gap and snag the saw. The trick now is to start hammering each side of the cut line down toward the centerline of the tube, working gradually along the tube. When both sides have been persuaded to take a new shape, you'll have the basic cone all done. You don't have to move the metal as far as you think-the rest is just refinement. It happens so quickly that you won't believe it was this easy. However, not having worked with metal for as long as Harry Hoffman, you and I might want to experiment with a test cone on scrap steel before using up some expensive stainless tubing.

Remember to plan for the finished length you need. Once the cone is made, you can't shorten it or the diameter will be wrong at that end. Use the straight pieces of tubing at each end to achieve the exact overall length. Follow our photo sequence to see how it's done, then try it. There may be an existing cone somewhere you can purchase for some applications, but making your own can be quite satisfying.