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Old 01-27-2013, 04:02 PM   #41
JMo (& piglet) OP
Gone a bit Baja
JMo (& piglet)'s Avatar
Joined: Apr 2007
Location: Somewhere west of Laramie...
Oddometer: 4,445
Originally Posted by wrk2surf View Post
Yes they did make quite a few of them in 2004... basically just a smaller barrel (89mm) piston with a shorter rod on a 660 rally.. Ktm made a 400 and 620 cc LC4 motor for quite a few years actually.

Good luck Jenny!
Yes - thats right, there was a sleeved down version of the previous generation LC4, which kind of was the inspiration for this project to be honest!

However, once we got a 690 engine apart, we realised that a narrower bore just wasn't going to work with the shape of the combustion chamber head - the valves and everything are just too big to make any meaningful reduction...

However, John did a few calculations, and worked out that by having a longer rod and a shorter stroke, we could get the required capacity while retaining the original barrel and bore - all we would have to do is modify the piston profile to regain the compression that we'd lose...

As I mentioned in my initial post, there is a direct benefit here for regular 690 owners too (including the 654 and 690cc variants), in that it will be straightforward enough to also create a stock replacement together with a hi-comp piston for a regular 690 too - all from the same blanks... in fact because everything can be computer modeled beforehand, it is perfectly possible to dial in the compression ration you want, and shape the piston as required!

It is certainly John's intention to have a batch of 690 pistons available for sale as soon as possible, if only to help offset some of the engine development costs, which you might imagine are quite significant...

I spoke with John this afternoon in fact, and he is making good progress with the prototype crank - typically he loves to do things that people say can't be done - and sure enough, he has managed to split a stock 690 crank (by making his own tool), and welded it up - and will begin machining it this coming week - photos to follow of course!

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