Sometimes you need to add some material to make a PVC part bigger or fill in the inside diameter. You can cut a ring from something close to fitting and change it's diameter. I needed a cylinder to be a little bigger. It needed to fit inside another pipe. I also needed some o-rings on it. Here some photos of the process.

, I cut a ring and took a section out of it.

, I placed some rubber bands around it to pull it together.

, Next put it in the microwave under water.

, 6 minutes (time to get boiling) and look, it pulled together.

, Pull out, hold it tight around the target piece, burn your hands, and run cold water over it. Oops, I cut a little too much out of it. The ends should meet.

, I patched it up with a sliver, turned it down in a lathe, and cut o-ring grooves in it.

, Finished product.