Replacing a zetec piston last night I managed to break the middle piston ring . All the others are fine so don't really want to buy a whole new set. I do have a spare set for a zetec which are fit the bore perfect and have the same profile...but...they are about 5 thou (inches) thinner and therefor are slightly looser in the ring groove. I suspect this will make little difference but thought I would seek the wisdom of those more experienced!

Don't risk it, buy another set or you may be able to get one from your local engine re-conditioners.

I think I would replace the whole set - I'd be really upset if I built it with this 'new' ring and then had trouble with it that meant I had to strip the engine to replace it later.

I've never bought piston rings (or replaced them myself) but they can't be that expensive or difficult to get hold of can they?

Maybe they are, maybe that's why your thinking of doing this hhmm

New set probably about £70. The issue is that the zetec bore needs to be re honed to allow the rings to bed in properly...If I just change the one I should be OK as the main and oil control rings are already run in the old bore.

Imagine you build it up and the pot with the rogue ring in is down on compression, all that work for an 'un balanced' engine. Not saying it will happen, but is it worth the risk, I'd go all out to source a new ring.
Rgds.
Pete.

You deffo. shouldn't fit a ring that's too narrow as it will chatter in the groove and wreck the piston.
As for a one-off someone must have one kicking about - just make sure it's within tolernace before fitting i.e. place it in the bore and measure the gap with feeler guages to check - you should be able to obtain that info in the appropriate Haynes manual.
Cheers, Pewe

You are all right...Found engine parts supplier who will sell me a single cylinder set, so I'll do the right thing and fit the correct one...cheers chaps.

If you are replacing 'A' piston then the bore would need honing anyway. While you are honing ONE then you might as well do all FOUR and fit a complete set of rings. At least that way you KNOW that you have done everything RIGHT and it will not need doing again.

Do it right and do it ONCE.