I had often wondered the same thing about the cylinders. Recently, I had the opportunity to check the two different cylinders side by side, visually, and with measuring tools. I found no difference in port size or height between '97 670 and '98 670x cylinders. The only obvious difference was a factory machined "window" on the rear of the cylinder sleeve uncovering the rear transfer port. I had been led to believe that the X cylinders had some port work done on them to go along with the different pistons and head. Somebody please correct me if I am wrong....
This all came about because of a broken ring land on a '98 x. For a quick fix, I just installed a set of stock 670 pistons, along with a stock head, using the x cylinders. Ignition timing was already set to stock 670 specs, as I had done away with the DPM on this sled last year and went with a stock ignition. Sled runs great with this "mis-match" of parts.
