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The original exhaust was scrap - well not quite. It consisted of "Dunstall Decibel Reverse Cone" silencers, a strange choice of words given that these were commonly fitted to bikes in the sixties and seventies to make them very loud, clamped to the original exhaust pipes which in turn were fixed to the cylinder head by gigantic cast steel finned clamps. Take a look back at the opening page for a picture.
Clearly these hed to go but some parts were re-used. Firstly, I removed the detachable alloy cones and pulled out the perforated steel tubes which served as silencing (a relative term, I can assure you). This left just an open megaphone which I thought could be re-used. having looked at pictures of the works Hondas, I considered that if I were to slightly taper the cones inwards, the originals from the bike would look a bit like them. I set to with a propane torch and hammer over a wooden dolly and came up with an acceptable shape by beating out.
You can just see the small taper in the picture above. I needed to make some attempt at a silencer in order to comply with the 105db noise limit so I found a heavy duty paint tin with a flanged OD the same as the ID of the reversed cone I had beaten out and cut a hole in the end wiith a diameter of the same OD as the perforated steel tube I had removed. Then, with a bit of metal bashing and heating, brazed the end of the perforated tube into the hole in the end of the tin and removed all byt the last inch of the paint tin. This made a top hat shape insert which fitted snugly into the end of my modified silencer.
I packed the space between the perforated tube and the silencer body with pukka exhaust packing and pop riveted the insert to it. This is exactly how modern day silencers are made. I then repeated the exercise for the other side. These should provide just enough back pressure whilst keeping the noise down to a level acceptable to the sound inspector. I re-used the original exhausts, but machined off the finning on the original clamps to reduce weight and to make the whole system more pleasing to the eye. I also welded 20 degree bends between the silencers and the exhausts to give the neccessary upswept profile.
The silencers had suitable brackets welded on to secure them to the footreat hangers, and the whole assumbly bolted to the cylinder head using stainless steel cap head screws. |
