How’s it done?

Making a 350 into a 410 just needs a few parts and Don Morley’s excellent book Classic British Trials Bikes goes into greater detail than there is room for here. Basically, the short stroke model’s piston is fitted into the old long stroke barrel and hey presto – a 410. Don also detailed how to build your own short stroke 350 using the last AMC Norton-based engines. He said at the time that the short stroke would be more difficult to ride but I suspect modern tyres would go a long way to combatting the bugbear of the short stroke… too much power to find grip unless you have the throttle control of a Jackson.