The usual issue is not spreading the rings, but compressing them flush with the piston so you can slide the barrels over them.
My usual technique requires no tools at all. Put on the new rings. They are flexible enough that you can wiggle them into their proper grooves. They will be loose. Your manual should tell you how to space the open part of the rings. Usually 120 or 180 degrees depending on how many rings. I then slide the barrel over the top part of the piston above the topmost ring. The barrel will stop on top of the highest ring. Keep a little pressure on the barrel and then with your finger tip or fingernails slowly push the ring deep into the groove. As you do this, the barrel will descend slightly and just enough to hold the ring in the groove. Work your way around the ring and the barrel will eventually be completely covering the top most ring on that piston. Repeat the process on the other piston. Repeat on the next highest ring until ...voila the barrel has slid over all the rings.
If the above does not work for you, you can make a very simple and cheap ring compressor. Buy a hose clamp large enough to go around the rings. Take the hose clamp and put it on a wooden block (not a hard wood, something like pine or a piece of construction 2 x4). Take a large cold chisel or something similar and hammer a sharp crease in the hose clamp in 3 or 4 places equally distanced around the hose clamp. The purpose of the creases are they will stick out a bit even after you have tightened the hose clamp around the piston and rings. Tighten up your new home made piston ring compressor enough that all of the rings are compressed within the piston. Slide your barrel over the top. It will stop on top of the protruding creases. Take a rubber mallet and womp the barrel down (being very careful not to hit the cooling fins ... that would really spoil your day). The ring compressor should slide down as the barrel descends over the rings and piston. If you try this without the creases, you will likely find that the hose clamp will wedge in between the piston and the barrel. That is not pretty and can damage your barrel. Again, this would spoil your day.
With either method, remember to use lots of oil while assembling. I usually use 10w-30 because it is a bit lighter than regular motorcycle oil and I always have some kicking around.
Peace & Grease, Dennis
