There are several different types of piston ring compressors. The two that have been around for the longest time are the “wind up” band type and the “pliers” band type. Both of these work well and have been in use for many, many years.
I have had the best luck using the wind up type of piston ring compressor on smaller pistons
like the ones for lawn mower engines and some of the newer imports with very small bore sizes.
The pliers type is a good all around tool that seems to work just fine on almost any engine.
One advantage of this type of ring compressor, especially if you are new to engine building, is that you can see the rings as you install the piston. That way you can tell when they are just about to go into the top of the cylinder.
The newest type of ring compressor is the one piece “funnel” type.
This type of ring compressor is probably the easiest to use but has the disadvantage of only working on one particular bore size. This means having several of these tools unless you are only working on one bore size engine. Lately, some manufacturers have been offering an adjustable version of this type of ring compressor. By cutting a groove in the tool and using a large hose clamp, it allows the tool to adjust for a slight amount of different bore sizes, usually in a .060” range. I have not tried one of these yet but it looks like they should work just as good as the fixed size ones.
I personally prefer the one piece “funnel” type as long as I have one for the bore size I am working on. I have all three types in my shop and have used them all with good success.
Whichever type of ring compressor you decide to use, make sure to keep it clean and free of scratches, burrs etc. Always give it a good coat of oil or assembly lube before installing a piston with it.
There are also some nice piston installation tools or piston “knockers” available that would be nice to have if you are building a lot of engines. I personally use the handle of a large dead blow hammer.