Piper At The Gates Of Dawn

mmmm mmmm

o. This sodding exhaust manifold.

According to the 3D model, near as damn it 5 3/8" was the amount of tube in a 90 degree bend. So I set the bender up, clamped a reference point to it, and pulled 5 3/8" of tube round the former (Fig.l).

All of which worked alarmingly well. That meant that I could mark the tube up for the bends and the straight bits and just keep feeding the tube through the bender and, as long as I got the rotations between bends right, I ought to get a result. On the other hand, you may be the sort of asshat that draws a pipe with 2" between bends and makes a bender with a 3" long clamp, that makes the closest you can get the bends together 3" and not 2". Bugger. That meant I had to cut 1" out of the pipes for No. 1&4 exhaust ports ... but up until then it had been going swimmingly. The bender did start to exhibit a tendency to squash the tube and that looked like it was because the plywood former was getting squashed while trying to restrain the tube from moving. I think the answer to that is to redesign the former a little bit.

Given that T set out to make a bender that would do a job to my satisfaction, this isn't that bender. On the other hand, a closer look at some of the cheaper commercially available exhaust systems shows that while it might not be what I wanted, it's bloody nearly as good (or even slightly better) than some. The whole process of working out the lengths and designing the manifold would apply to using bought in mandrel bends as well, so don't feel that you need to build the bender for all this to work out.

Working from the information I'd got from the 3D model I knocked out all the bends (Fig.2) and made up the four pipes I'd need for the manifold. Accurately measuring bits of bent tube isn't especially easy to do, so I left them all with an over long tall' so that, once they were attached to the head, then I would lie more easily able to mark them in the same relative place. I'd already made the flanges for the manifold out of 1/4" steel plate. Clearly, fitting the round tube into the rectangular hole presents a bit of an issue.

Adding twice the height of the rectangular ports to twice their width gives the distance around the port, and dividing that by Pi tells you the bore of the tube you need to to have the same amount of material in the circumference of the tube as you'll need to match the exhaust port. Rounding up to an available size and adding the wall thickness of the tube gives you the OD you want. In practice that all adds up to a very limited choice of tube sizes, often just one, unless you're willing to start on some radical head redesign. Luckily, most people who design engines know more or less what they're doing so, most of the time, the size of tube the port shape dictates and the mounting studs will permit usually includes something like the ideal size.

With all that worked out beforehand and the holes in the flanges filed to match the amount of material in the OD of the tube, and the flanges attached to the head, the tube can be offered up and the corners of the flange's hole marked on the tube (Fig.3).

Unlike the Ancient Greeks, squaring the circle isn't a big philosophical issue to us, it's just a matter of aligning the marks we just made on the tube so that they're equidistant from the jaws of the vice and squeezing the tube up (Fig.4). This makes the end of the tubes oval but, by placing them on a hard flat surface and hitting them with a hammer on the rounded ends of the oval, they can be flattened out too. Repeating the process three or four times produces a reasonable rectangle and, with a little persuasion from a hammer and a little encouragement with a file, that can be induced to fit into the flange.

Since the pipes for No. 1&4 ran closest to the block, I fitted those to their flanges first. With the flange securely bolted to the engine, a certain amount of finessing of the angle that the pipes run at is possible, and a little careful tweaking had the two pipes meeting each other halfway along the engine.

With those welded top and bottom to their flanges, the next thing was to mate the middle pair of pipes to their flanges. That O


With the collectors made, I welded the tubes to the flanges on the mating face to seal them (Fig.7) and then sanded the weld flush, and filed the holes back to shape. Someone in slightly less of a piss than me might have used a TIG for this and saved quite a lot of effort...

All done, it looked alarmingly like an exhaust manifold (Fig.8). But was it worth it? Bearing in mind that you can buy something for around £150, you'll have to answer that for yourself. Mine cost me maybe £40 and I can make another one for a lot less hassle than I made this one. It's 4-2-1 which ought to go some way towards easing any problems with the 17" rear wheels, as opposed to a 4-1 which tends to rob the mid range to feed the top end. On a purely economic basis, you're probably better off buying one. If you're going to enjoy doing it, then what the hell... ©

meant trimming off some of the 3" of tube that was necessary for the clamp on the bender. I could have bent them the 'other way' and just left 11/2" after the bend, but that would have put the bulge that the bender left in a different place to the other two pipes. I measured the middle two pipes to check how much they needed trimming by and it turned out to be the same as the amount I'd worked out from the model info. (Fig.5).

Once I had all the pipes attached to flanges and ending up in the right place, I welded each pair of pipes together, and then measured up from the surface everything was sitting on and cut them to length. Once I'd made the secondary pipes and trimmed those to the shortest length I was likely to want, I made the collectors for each pair of pipes. These were formed from some 2" wide strips of steel the same thickness as the tube, by forming a "U" shaped bend on the strip and then trimming and shaping it to suit (Fig.6).

This is actually the last Trike Torque that Blackjack's able to do for us and we'd like to publicly thank him for all his hard work over the last few years doing it. Thanks, Tim, the mag wouldn't have been as good without you.


soulli .toast custom H ike and bike bu

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