This is a pic of the MC18R5 standard shock next to the MC18R6 (SP) shock with the MC21SP shock to the right
By the look of it the MC21 SP shock is just longer in the body and my shock guys says theres a good chance he can cut the MC21SP shock down to suit.
Much shorter hose to the reservoir for the MC18
Planning on a rebuild anyway so may look at the spring as well.
This is a pic of the MC18 linkage versus the MC21 (MC18 top)
I had thought there might be a chance the MC21 shock would fit into the MC18 with the MC21 linkage but no go. The link arm may be useful in raising the rideheight as the 21 arm looks to be approx 10mm shorter. Will measure the effect shortly
This I found on Yahoo Japan...looks very achieveable so will try this to compensate for the 17inch rear and give some extra tuning. Pic is a little off as the standard MC18 linkarm is 110mm.
Just had a measure and 10mm shorter (MC21) on the linkage arm lifts the rear 40mm! Almost too much for the sidestand. So this pic of the adjuster may be designed for racing an MC21.