Allen gets on with the ex­hausts.

Just what cre­ates that gor­geous six sound? Allen re­veals all...

Classic Motorcycle Mechanics - - CONTENTS - cmm

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!

Sheet steel sec­tions which make up the ex­hausts.

Half way curved, so still some way to go.

Curv­ing the edges of the cones...

Hose clips to hold to­gether for weld­ing.

The fin­ished ex­hausts on the bike.

Trimmed and pol­ished cones.

Weld­ing on the seams.

Trim­ming the end in my lathe.

Cut­ting sheet steel for end cones.

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