(re: crossflow)

After honing piston bores, does it need new piston rings fitted to the piston?

If so, what is the theory behing that?

Surely the "roughness" in the bores will wear the rings so old ones can be used, after all they are spingy so should contact the bore.

Rings need to be gapped. If they were gapped correctly when they were first fitted, they will be no good now the bores have a bit of wear and a hone.

yeah, know about the gapping but wll the gap really open that much? never played with rings before so don't know.

02GF74, last time I built a crossflow was 25 years ago, but what I did back then was got my local engine people (sutton rebore) to do the honing, check my piston size (I had 1300gt pistons in a 1600) and supply me with the correct rings.
Don't reuse the old rings when you have gone to all the bother of stripping your engine.
Do them now and forget about them.