Is it bad to build a mechanical stop to keep the piston from going the full length?

If we stop a piston from fully extending, will it damage the piston in any way?

Not if you do it right. Just be mindful that the cylinder will still supply the full force it can exert on your stop. I would recommend one of these to slow the piston down as is hits your stop.

Pictures would help explain the situation, but ill take a shot at it. For pneumatics it should not hurt anything as air is compressible. We have put collars around the actuator shaft in the past to limit the return stroke of cylinders. Hope this helps

It is the opposite of bad, it is a good thing particularly if you have the stop cushioned in some manner.

Yes, it might, if you don’t design it correctly. You don’t want any sideways bending force.