Friday 9 July 2021

The Exhaust System

New exhaust rings had already been made in AB2 Ni Al  Bronze but the exhaust pipes themselves needed fabricating.

Pipecraft in Lancing have previously made the exhaust pipes for both the AJcette and the AJS K7 and did a great job. However, I anticipated that the exhaust for the V-Twin would be quite a bit more complicated and decided that I would fabricated it myself using bends provided by Pipecraft.

Why should the V-Twin be more complicated? Well, there are 2 reasons for this: firstly, on the front cylinder, there is inadequate space to have a simple single curvature pipe and the pipe would need to wrap around the side of the timing case to avoid the front mudguard; secondly, on the rear cylinder, the exhaust port is angled upwards (because of the 25 degree-from-vertical angle of orientation of the cylinder) and this would require an initial bend of high curvature to bring the pipe downwards and inwards.

Before any work could start on the pipes themselves, stubs were turned on the lathe onto which the pipes would be secured at the exhaust port. The pipes to be used are 1.75” OD and 2mm wall thickness.

As with the copper oil pipes, wire patterns were made to follow the line of each pipe. However, there are 2 differences here: the first is that formers for bending the pipes are in a number of fixed sized radii – 175mm, 133mm, 114mm and 84mm and the design must be based on these specific radii; the second difference is that, because the exhaust pipe is of much larger diameter, the wire pattern needs to follow the centreline of the pipe. With small diameter copper pipe these considerations are not important because, although a small pipe bender is used, adjustments by hand are easy.

Partial circles with these fixed radii were marked out on a large sheet of paper and the wire formers were then bent by hand to the desired exhaust pipe shape and with the constraint of using only the allowable radii.

The pieces of masking tape stuck on the wire in various places indicate each bend radius.

Pipecraft then provided a selection of bends and, of course, straight sections from which I could then cut-and-shut the exhaust in situ.

The plan was to make each exhaust pipe in 2 sections – the first part containing the bends and the second the long straight section. Each end of the straight section was therefore swaged to allow it to fit over the front section.

In the above picture, the lower pipe is “as received” and the upper is after cutting the swaged section to length.

I know that some people will find this ridiculous, but the cuts that are necessary in the swaged end to allow it to close up properly on the inside pipe when clamped have been made with a 3/64” slitting saw on the milling machine rather than just cut with a hacksaw.

Why? Well because doing it this way I can guarantee that the cuts will be perfectly parallel to the axis of the pipe, they will be the same length and they will be at 900 to each other. And it only takes a few minutes to set up on the milling machine and do it properly.

The bends were cut and set up for the front cylinder first. In the 2 pictures below there are 3 sections forming the complete bend: the first section is held firmly in place as a tight fit on the exhaust stub; the last section is held by large magnetic V-blocks and the middle section is simply sandwiched between the other two.

The next stage is to very gently tack weld the sections together with the TIG welder and then remove the pipe and complete the welds.

I’m sure a trained welder would throw up their hands in horror at my welding …but then I’m not a trained welder. After cleaning up the welds the pipe is refitted to check.

Everything seems to have ended up in the right place. The pipe has not yet been welded to the exhaust stub; that will be done when the straight section is added.

After a repeat performance on the other side of the bike for the rear cylinder (also using 3 curved sections) the straight sections are cut to length and checked for position.

Adjustments were made to ensure that the pipes were parallel to the longitudinal axis of the bike, the same height at exit and the same distance from the frame before tack welding the pipes to the exhaust stubs. Brackets were then made to support the pipes at the ends of both the front and straight sections.  A couple of bespoke copper washers for the exhaust stub/port junction were laser cut by lasermaster

to complete the installation

….and enough bits left over to make a giant French Horn

Many Thanks to Andy at Pipecraft for providing all the necessary pipework.


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