ST1100 Exhaust

ST1100 P2 X04 Exhaust

After its first year on the road the downpipes looked rather tatty – the rust was coming through the high-temp silver paint. They are supposed to be stainless steel, but they aren’t original Honda ones. I had a spare set of pipes in similar condition, on which I ground off the lugs that once held heat shields (lost before I got the bike). On a friend’s recommendation I took them to Reddich Shotblasting, who cleaned them up, coated them in aluminium then baked on high-temp paint again. Much cheaper than a custom-made set of pipes!

ST1100 P3 X20 Exhaust

They look neater, and the rust has stayed at bay, but they did discolour near the top after more use, so they may need annual painting (from a spray can) to stay looking good. I might get round to removing the belly-pan brackets too.

For the new cut-down version of the bike I didn’t want the heavy and dull old twin silencers that weigh about 5kg each.

ST1100 P1 C03 After

Having had bought an ex-display Scorpion end can at a show I decided to have a go at making a link pipe to join it to the exhaust collector box. This is where the link pipe would need to go.

ST1100 P4 H04 Exhaust

It would also need a blanking plug for the collector outlet where the original left hand silencer went. The only welding I had done was a bit of practice on scrap and welding some brackets to the bike frame to hold the battery under the tail piece. In other words I was a complete novice,  so I started on the plug before tackling the more complicated link pipe.

I’d acquired some 2 inch (51mm) mild steel tube for this – stainless would have been better but apparently it’s more difficult to weld. I cut a short section of tube, took out a few mm down the length and closed it up to fit inside the 48mm collector outlet, with a round bit of plate to go on the end. The benefits of practising on scrap soon became apparent when I burnt away the edge of the plate trying to weld it to the tube. Much welding and grinding later, I had a functional if rough and ready plug.

ST1100 P4 H06 Exhaust

I’d made a cardboard mockup of the link pipe and end can to get the design right before starting on metal, as shown in the ST1100 Redesign post.

The link pipe needed two bends, one just outside the collector to turn the pipe horizontal to clear the footrest bracket and another bend to turn the pipe backwards. The first looked to be about 30 degrees, so a simple cut and shut would do, but the second was 90 degrees and needed to be quite sharp to get the end can close to where I wanted it. A set of three 30 degree angles close together seemed a reasonable compromise. I recently learned that what I was going to make is called a ‘lobster back’, where a series of triangular slices of tube are welded together to make a bend.

I practiced welding on an offcut of tube, but still burned away some of the first joint when I started on the link pipe, mainly because the cut edges were not straight and didn’t meet cleanly. I was doing reasonably well by the third joint when the electronics in my welding mask packed up. This meant having to flip up the mask to position the torch, then close it and weld with the window on its default, darkest setting, i.e. almost blind, so the fourth join was not very pretty. The final part was to make an insert to fit inside the collector. When I tried it in place, the first join was at the wrong angle, so I had to cut the pipe again and make another join. All this was made more difficult by using flux-cored welding wire in my second-hand MiG welder instead of investing in a bottle of welding gas and using plain wire. A lesson learned.

ST1100 P4 H08 Exhaust

The next step was to test it for gas tightness, by taping up one end and putting it in water – bubbles poured out of all the joins! There were so many holes I couldn’t mark them all, and just had to weld over every seam again. After a few more tests and re-welds, the bubbles finally stopped.

After each session of welding I ground off the excess – it looks better, and makes it easier to see where the pinholes are for re-welding. I was beginning to hate angle grinding, and the garage was covered with specs of burnt steel.

ST1100 P4 H10 Exhaust

This is the the point at which I decided I wasn’t going to improve it much more. Despite the terrible appearance it seems to be quite solid. High temperature paint should hide the worst of the pockmarks. It was a worthwhile exercise, but maybe one day I’ll get a professionally made stainless replacement.

Here’s a short video:


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