I’m doing a ls swap on a 93 mustang coupe and about to purchase some pistons rings.
The engine was a 5.3 ls but is bored out to a 5.7 and is using stock ls1 pistons and rods.
What size piston rings should I get? I called one builder and he said a 3.898. Another one said 3.905. So I’m confused.

I would suggest talking to the machinist that machined your block to see what the final bore size is. Its probably about 3.898”, which is the stock bore. If you get rings for that 3.898” bore, the end gap built into the rings right out of the box might be a little looser than what you want and you wouldn’t be able to do anything about it. If you got rings for a slightly larger bore, like 3.905”, then they will be really tight out of the box, but you would be able to gap them to whatever you want.

Thanks for the replies.

Im going to talk to the machine shop hopefully Monday. I got the ls1 rods and pistons out of my old ls1 and when I took them to the machine shop I forgot to ask what size I should get since he will bore the 5.3 out. Tried to call but apparently his wife was sick so he will be out for a few days.
I was going to try and get the rings to give to him first thing Monday morning but I guess I’ll wait to be sure it’s 3.898 size rings.