Hello Boucher. Yes, contemporary 600 or 620 (80 mm bore) cylinders can be used with the older 600-650 cases and heads. The major problems are: 1) the cylinders are the larger 96.5 mm diameter like the 750cc engines; but the walls are so thick (8+mm) that they can be turned down to fit inside the 600 spigots of the case, 2) they have switched the recesses for the three big green O-rings to the cylinders from the heads; you can fill one or the other with JB Weld, 3) they have also switched the recess for the small O-ring around the oil passage dowel to the cylinders; I used two O-rings. I do not remember locating dowels being a problem. It is also neat that the newer 620 engine pistons (80mm x 61.5mm), not the 600, have nice high-compression bumps in them that you can probably squeeze into stock Pantah heads. Here are photos of the No. 8 Pantah Rutter/Slinn IOM racer replica (2011 postings) with the setup I just explained. Only difference is that these were bored to 82mm. MikeV
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