The issue with ball sizing is the ball can drift a bit side-to-side depending on local hardness. So even if you measure to spec, you could have unseen "wander" in the cylinder, which is likely a no-no here.
I'm with the honing crowd, ditto those that say check with the customer as to how they plan on measuring the parts. You'll need to copy their method.
Yes I’d be concerned with ball sizing. There is a circlip groove in the bore that has to go in when the bore is rough turned. To me te possible the ball would make a different bore axis when it hits the groove.
This part is disc shaped with zero flat surfaces to grab for second op except for the bore itself and a small flat surface to reference on. Basically imagine a triangle revolves around the bore. Second side machining is tricky as is.
The single pass diamond hone is what I’m going to be looking into deeper. Nobody with a sunnen for sale at the moment.
I’m fairly confident I could do it on the lathe but I think it would take much longer and high rate of scrapped part. Material is pretty expensive to have any scrapped parts.
Off topic: Do you guys have a bs13 manual by chance?
The more you guys talk about it the more a sunnen sounds like the right machine. But like someone else said I don’t want a oil leaking mess all over and a machine I’ll be using for one job 4 times a year.