I have used O rings on quite a few engines and not had problems getting the pistion to move. I don’t have Stuart drawings for an O ring piston so what depth of groove do they show and what is the actual measured cross section of the supplied ring. Really you only want to compress an O ring by about 5%. Max

For example the 24mm bore engines that I make have 2.4mm section metric O rings and the grooves are 2.3mm deep so very little compression of the ring. You also want to lap the cylinder bore as straight off the machine is likely to shred the ring quite quickly.

Lilewise with iron ring sI have never found the need to use or make a ring compressor, they can generally be eased in by hand or if narrow a poke with something thin but not metal. A small chamfer on one end of the cylinder helps and a honed bore.

Or if you just want to run slowly for display and you have  agood fitting piston then just don’t bother with any form or rings. I quit eoften leve them out of my engines although the groove is there for an O ring if I wanted to fit one.

Bit OTT for a 1″ ring that you can easily compress with finger and thumb

Also make sure you gap the rings before trying to fit them, only needs a 1 or 2 thou gap not the larger gap you might us eon Noel’s bigger rings.