For those engines whose cylinders still attach to the crankcases, the ring compressors sold by the factories are really handy, if not mandatory. Here is a collection mostly from Honda and Yamaha. Each set of two like-size compressors in my collection I marked and color-coded, and fits a nominal range, such as 65mm~67mm. You don't know how painless installing a cylinder block can be until you try these! Too bad they are long discontinued.

Postscript 2017: I have managed to acquire a goodly collection of new old stock (NOS) Honda ring compressors for sale. Exactly as shown above except unused, unopened. Factory! The best and easiest to use!

These factory ring compressors fit snugly around the pistons and are designed to dodge the cylinder studs.	They are engineered to have plenty of room when the cylinder starts to go on, and be very esay to remove withbthe cylinder in place.	Like the Honda motorcycles they were designed to be used on, these are vintage, factory, top quality ring compressors in factory wrapping.
Nickel plated, exceptionally well made, quality pieces, made by Honda's special tool supplier in Japan.	Note the generous chamfer on the tool, which works together with the chamfer on the cylinder, so that the compressor and cylinder actually overlap when the tool is in place on the piston, giving the piston rings optimum smooth transition into the cylinder.