Originally Posted by dentvet
102x84.5 is 690 specs
so 102 bore with 55 mm stroke is 450 cc
so they will lengthen the rod and drop the wrist pin to make up 30 mm in stroke. It sounds like they are doing mostly rod lengthening because they are going to use the same basic piston as a hi comp 690 piston (close to stock wrist pin location therefore)
This should keep the scrutineers scratching their scalps
To me, this is where it's getting very interesting for an armchair engineer/noob like me. It's been established that the bore will remain the same (no cylinder change). I think it also means no change to cylinder barrel length, and the piston top comes to the top of the cylinder at the top of the stroke - it sounds like the stroke is the only variable factor.
1) correct me if I'm wrong
2) explain the considerations between changing the connecting rod length and/or changing the length between the crank pin and the centerline of the crank.