Allen gets on with the exhausts.
Just what creates that gorgeous six sound? Allen reveals all...
The exhaust system for my RC374 had been on my mind for a while and I was looking forward to getting started. I have made a few expansion chambers for my two-strokes and I have also made small megaphones for my SS100 V-twins so I’ve had a bit of practice, but these Honda exhausts were going to be harder to make. Firstly because they are so small in size and secondly because the genuine RC174 pipes are made from two pressed pieces that are seam-welded together along the top and bottom, which was a common way of making exhausts at the time. I wanted to replicate this look with my exhausts and had been thinking about different ways I could make them and researching photos online. Due to my slightly wider engine and the shape of the sump I would not be able to replicate the exact look but I was hoping to get them close. I had already made the header pipes for the test run of the engine and they were okay to use after lengthening the straight portions to fit alongside the frame rails and with a few minor adjustments to the curves. I decided to make a start by cutting out cardboard cone templates to dimensions that I had worked out from my book of pictures. When I was happy with the shape I took the templates to my friend Bernie at Project Metal for them to be cut out of 1.2mm steel sheet on a guillotine. I also asked for several 6mm wide strips from the same sheet (I will explain what these are for later). Once back home I started to hand-form the cut out shapes into cones. I do this by using a rubber and hide mallet and a few bits of steel tube. The first stage is to gently beat the two sides over my bench, which happened to have a perfect sized rolled edge, with a rubber hammer to start the curve on each side. The next stage is to bend the sheet gradually over a length of scaffold pole until the cone is about ¾ bent around then the last bit is done around a short length of 20mm steel bar. The most important thing at this stage is to ensure the two edges come together with a minimum gap. I use a file to clean up the edges and adjust the fit if needed. Then I use hose clips to pull the joint together ready for TIG welding; I usually tack weld at each end and one in the
middle of the joint to prevent the edges pulling apart or pulling together causing overlap during the welding process. Once the welding is complete it’s time to true up the cones, making them round in section. To do this I use my hide mallet to reshape the cone over the 20mm steel bar until it is round. This can take some time to get right, but after a while the cone looks good. There are portable bench rollers available that would speed up this process but I have limited space and like to make parts by hand where possible so have never felt the need to buy a set. Looking at the cones, I wondered if they would fit in my lathe so that I could machine the end surfaces ready for the end cone part to be welded on. To do this I first turned up a couple of aluminium bungs that pushed snugly into the cone, one at each end, the larger bung was pre-centre drilled for the tailstock centre. The cones just fitted and when supported with the tailstock centre and the other end gripped in the three-jaw chuck I could easily machine the larger end surface true. I was also able to polish the outer surface with fine emery cloth to remove any scratches etc. I then repeated this process for each of the six cones. The next job was to make six end cones; these were quite thin in section being only 12mm long, and I cut them out by hand with tin snips. I was taught how to develop cones in technical drawing at school but it’s easier to use an app on my phone these days! I first tried the cardboard cut outs onto the steel cones and adjusted the fit, then marked out the shapes on the steel sheet ready to cut out. Once cut out it was a simple job to form them around my scaffold pole by hand with a few gentle taps with the hide mallet. These were then welded up, trued and welded to the back of the six main cones.
I mentioned earlier that I had some 6mm strips cut from the steel sheet. I used these to replicate the seams, as seen on the original RC174 exhausts. I placed a pair of strips side by side and clamped them together then TIG welded them along the whole length. After cooling I positioned the welded pairs of strip along the top and bottom of the cone and held them in place with hose clips while I welded them to the cones. After a bit of fettling with a file and emery cloth they looked quite good and really improved the appearance of the pipes. I was now ready to start fitting them to the bike. Starting with the lower pair I located them onto the header pipes and packed them up with wood to the correct height, then the middle pair, and finally, the top pair. I spent some time making them fit close to the bike and in a similar position to that of the original bike. Once I was happy with the fit I made cardboard templates for the hangers, following the shape and design of the originals then fabricated them in 2mm steel and bolted them to the bike ready to tack weld to the individual exhausts. At this stage the headers were also tack welded to the rear sections of exhaust. After a final check I removed them from the bike to complete the welding on the bench. I then refitted them to the bike to check the fit and was pleased to see that they fitted great and had not moved during the welding process. The last job to do was to make a pair of support tubes to bolt to the upper frame and support the three exhausts each side. These were made from 13mm cold rolled steel tube flattened at each end and drilled to clear an M6 bolt at each end. The RC374 was taking shape nicely and was starting to look more complete every day but I still had a lot of parts to make; more on this next month!