SWM TL350 Jumbo: Replacing the Piston

After getting the right piston, a simple (sort of) job of installing the new piston and getting the bike running

After identifying that the barrel was worn and that piston I had was too small (see article), I bit the bullet and ordered a new 84.25mm 2nd overbore piston from Peter Knight. It took a couple of weeks to come over from Austria and when it arrived, it turned out to be the same size, 84.17mm, as the piston that was already in the bike. The sizing must therefore be for the barrel, rather than for the piston, so sent it back and promptly got the 3rd overbore, 84.50mm piston back.

However, by then it was Christmas Eve, so no real opportunity to get the barrel bored out to the New Year. After a quick chat to a couple of people at the Herefordshire Classic trial on News Year Eve, took the piston and barrel to Hereford Rebore

Quick service, got the bored barrel back the following day. The barrel hadn’t work evenly, so there was an area low down on the barrel that didn’t get machined. Re-assembly took a couple of hours, mainly due to time taken to get the new gudgeon pin and circlips in the piston. The Rotax pistons have an arrow marked “Aus” (which I assumed was something like Ausgang or exit, ie the exhaust port), so after some deliberation decided that the this pointed to the front of the bike. When I stripped the engine, the piston had the arrow pointing to the rear, inlet port.

Jumbo in the workshop

Initially the bike didn’t want to fire up and the kickstart lever kept running off the shaft after a few kicks. You can’t overtighten the shaft as this then pinches in the internal gear change shaft, so as a shortcut, while I look at the lever later, took the kickstart off of the SWM TL280 I’d just acquired.

Changed the plugs a couple of times, though they were “wet” and had a good spark, but then the bike fired up. No rattle or slap from the barrel, but the bike would only run with the choke on and only then for a couple of minutes.

Seemed like possible fuel starvation, so took the carb back off the bike. It took a couple of go’s but where I’d changed the float needle before (see), the housing had become clogged with the stale petrol being washed through, so petrol was only seeping through into the bowl.


Changed the fuel pipe from the tank to the carb, as it had split and put the carb and tank back on the bike. Again a few kicks required and she fired up, again no rattle. Time for test spin round the garden, though by now it was 20h00, very dark and raining. Managed to negiotiate my way up a pitch black and very muddy lawn to ride round in lights on the drive, all sounded good. Had to adjust the clutch in further and also moved the bars further forwarded, as well as apply some cable ties to the main cables so that the routing under the tank was ok.

Well, its a running bike, with a few small items to finish off, this includes:
– the seat needs recovering
– front brake mounting bolt sheared and was fixed with araldite; a more permanent solution is needed
– cleaning the plastics, mainly the tank and side panels
– replace some of the old original cables and put the adjusters into the cable system

Hopefully can get an MOT done fairly soon, work commitments allowing.

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