The DS 4 bolt shell, similar to the 2 bolt cylinder shell pictured above, is for use with the Zenoah 4 bolt marine cylinders. A massive improvement in performance due largely in part to the 4 bolt cylinder design and corrected symmetrical transfer ports. There is less chance of base gasket blow out with the 4 bolt design, which allows for more porting and more cylinder pressures to be obtained. To utilize these cylinders and shells, you will need a 4 bolt case such as the DS 4 bolt billet case pictured above. There are other available cases as well, and even stock, cast cases can be modified by an experienced engine builder to use 4 bolt cylinders.