W/NG Silencer

One day, I was walking up the road to my workshop and found a piece of rusty perforated steel in the road, squashed quite flat. I thought nothing of it.

Weeks later , I was musing over the efficacy of the W/NG silencer and looked inside the taailpipe only to find a similar piece of metal, though this one was partly tubular. The penny dropped.

Apart from this shred of perforated steel, there as nothing else in the silencer.

Burgess silencers, as used by Ariel, are supposed to look like this:

The principle employed in this type of silencer is the absorption of soundwaves transmitted by multiple reflection from the inside walls of the pipe. It uses an expansion chamber which surrounds a portion of the straight pipe system, perforated to allow escape of the gases into and out of the chamber, which is packed with absorbent material to diffuse the pressure and deaden the sound. 

One particular advantage of the straight through pipe is that it is not liable to partial choking, through fouling of the absorbent packing; the gases will always get through without hindrance or increasing back pressure, though the effectiveness of the silencer will suffer. 

An alternative failure mode is rusting and the loss of the absorbent material, leaving you with a simple expansion chamber which I suspect would not be really large enough - expansion chambers should be something like five times the swept volume of the engine.

So, there was no option but to cut the end off and repair it:

We need to replace the perforated pipe, which is easily available on eBay:

Unfortunately that's where we run out of pictures. I wrapped the perforated pipe with fibre glass blanket, wrapped with wire to hold it in place while I slid the silencer body over it.

The last bit was to run a peripheral weld around the body to cap joint, and that was it. The simple finish of a WD machine is a real help sometimes.