If I use a flex hone (ball hone) to clean up the walls of its light scratches, can I just re-ring it with the same size rings (but new of course) as it had before.
For example, it's a 4.030 bore w/piston and rings in it. I take out the internals and hone it very slightly, then can I put the pistons back in with the same size rings as it had before or will I need to get different size rings?
A flex hone will not correct any bore geometry problems, and it will not leave a good bore finish. Some machine shops use the flex hone to plateau a hone job already done with a proper honing machine, but it is incapable of creating a good bore finish by itself. It CAN work, and has before, but this isn't a tractor motor we're talking about here, and the difference between a good and mediocre bore finish could be an oil burner.
As for the bore geometry, without the right measuring tools you're rolling the dice. if the engine has a not-insignificant number of miles on it, the bores MIGHT still be straight and round enough for another set of rings to conform to, but it sure wouldn't be fun to have to pull the motor back out when the rings don't fully seat because the bores were worn out of round.
Most competent machine shops don't charge much for a simple deck-plate hone job, just have a shop do it and save yourself the worry.
FTR motor has less than 5K miles on it. i think a slight hone would be fine with some rings that came off it. are you looking to touch up the scratches to see if they come back?
i can see this is waning on your thoughts quite a bit. some people try to see things that are alot more complicated than in reality. it is simply this:
Running a fine ball hone on it will clean up the scratches. In all honesty though, will you be happy with just that? What abut if a spot looks less dull or something like that? Sometimes it is just material in the sleeve. But, proper measure of the bore will probably quiet your nerves. That does require complete disassembly, meaning retorque and all. Then do you check your bearing clearances, because the engine has been ran and it may appear that the engine was not warmed up to a good temp to keep the stroker slugs from rocking enough? It is a very very vicious cycle. It can become very costly too.
In the end, i know you want a good engine. You want it to also satisfy your piece of mind. I also know you are prob tackling this for the first time. Very honorable. Just make sure you have good tools, take your time. You are already asking questions right now that you are unsure of, which is good. IF you do not know how to do something, and it is beyond the realm of your capability due to not having all the equipment or ability, do not hesitate to call someone else, or take it to someone that does this for a living.
I, myself, hate file fitting. It is not that I cannot do it, it is just that I do not like doing it. So, if i have any rings that need to be file fitted, I take my machine shop what they need, and go from there. When i get it back tho, I do check the gaps to make sure they are what is specified.
If you're on a really tight budget or time-frame, I'd say your best bet is to run the flex hone through for a few strokes (5-7 maybe) and re-use the old rings (in their respective bores, of course). If the engine didn't have any cylinder sealing problems before, this course of action is pretty safe.
If new rings are needed for whatever reason, or you got the old ones mixed up, it really needs a proper hone.
My buddy builds motors, but he doesn't machine things. I'll probably have it honed at a shop just so I can sleep at night. I'd rather take longer to put it in than put it in sooner and have to take it back out.
I would NOT re-use a set of rings. Using a very fine ball hone is ok but PLEASE use new rings. Once a set of rings has been pulled from the bore, it has been my experience (and that of many other builders as well...) that they will almost never seat again. You can get a set of Mahle file fit rings for a very reasonable price.