Originally Posted by barrier
Surely they could sleeve it a little without being too small for the valve layout.
But hey what do we know!
That was the other option of course, however, when John stripped the engine down, he realised that the difference between the size of the [450cc] bore and the shape of the original head would have such a pronounced lip, the combustion chamber would be all to cock (to use a phrase pioneered by James May)...
Of course technically
you could also machine a new head to mate up to the new cylinder bore, but that is getting way beyond what is realistic both time and expense wise - the idea is to produce a kit of parts using the least number of parts required to change - sleeving the stock barrel would be more complicated (and not a DIY job), while a new head as well would just not be cost effective...
As a bit of background to the project - John was actually on his way back home from seeing the start of the Heroes-Legend Rally in Spain last year (he was due to ride, but had broken his wrist a few weeks beforehand so had to drop out), and stopped off at a vintage bike rally in the mountains. By chance he got talking with an American guy called Drino Miller - who as it turns out is head engine tester for Pro Circuit in the US:
As is the way in Spain, they of course got yakking over a bottle (or two) of red wine about the pros and cons of the various ways to build this engine, and it was basically Drino who pointed John in the direction we are now taking...
Today's nerdy photo is of a shiny new con-rod!
As John explains above, the combination of a longer rod and adjusting the position of the crank pin will give the necessary stroke to retain the standard bore/cylinder etc. As he explains,the trick[y bit] will be to create a piston profile that creates enough compression while at the same time misses the valves when they are extended - the 690 Rally cam (which should help boost power) has a longer overlap than the stock cam, so things could be a bit tight in there - hence the prototype build and bench test before we go to production with the final specification...