What is the optimal surface finish for cylinder bores? New rings and pistons. This is for a RB block and I don't think that makes any difference but I can't recall a magnitude number or unit? Honing stone number for the best finish too. Cylinder bore would be Ra or microinches? I seem to recall something about plateau as well??

I don't machine 'em, but I found it interesting that the guy who won the Engine Masters Challenge in 2005 said this:

"Believe it or not, the least friction and best ring seating technique was to have a fairly rough stone to hone the cylinders with and then using a brush to knock the peaks off the honing cross hatch. When we ran really fine stones we didn't improve anything as far as friction but lost all the oil control. The hassle is getting low friction with good oil control. You can have a low-friction motor, but with no oil control, and you don't want to do that. After the brushing, the cylinders have good texture and cross hatch to seal the rings, but feel as smooth as glass. If you feel the exhaust ports and they are a little wet in there, it's a lack of oil control; mine were absolutely dry."

When I spoke w/ the machinist who prepped my last 2 blocks, he said his best results have been from doing the final hone w/ a 320 stone, followed by a pass w/ a plateau brush. It sounded similar to what the late Lennart B. said above, so I just said "Works for me." and let him do it his way. I'm anxious to see how things work out w/ the freshened-up engine that had this work done to it.

Yes, that is the preferred finish of today. Called a "plateau finish". What they mean by that is it is done with a rough stone, so the grooves will be deep to hold oil, and then at the end you use a fine stone, (or a brush in his case) to knock the tops off the rough finish and make it smooth on top. So what this is giving you is a smooth RA to the touch, like they would have done in the old days, but with crosshatch grooves deep enough to hold the oil.

what grit is considered a rough stone? I always herd to use a flex hone( or dingle ball hone) then use the plateau brush after.

Most all of the ring manufacturers would advise an RA between 10 and 20 on high performance applications. You are asking about a hone stone number, but it's not like the old days where you use a certain hone number to give you the finish you want. 220 used to be the hot tip, it will give you about a 30 RA, then 280 was the going thing, then everybody wanted 400, (it will give you about 10 RA). Well, then you have no crosshatch to speak of to hold the oil because it is so smooth, so now we have plateau. Nowadays your RA is dictated by how many strokes you give it with your finishing stone (or brush). Everybody has their own technique but it is something like 280 or 220 very close to size and then 6 stokes with 400 to knock the tops off and give you a "plateau finish". This will give you an RVK (the depth of the valleys in the crosshatch) of over 30 to hold the oil, and an RA of 10-20 for a quick seating of the rings.

Ring preferences have changed over the years and ring type has a big influence on the cylinder wall prep needed. When I first started building engines back in the '60's chrome rings were popular and so were pistons requiring lots of piston to wall clearance. Also very few people were using torque plates for boring and honing either. Chrome rings were a bear to get seated and no doubt the loose pistons rocking in the bores didn't help either. Things are definitely better today. I recently built a new short block for the 'Cuda and with run of the mill standard tension moly filled rings and hypereutectic pistons ring seal was almost immediate. My machine shop has been using a plateau finish for some time now.

My comment wasn't simply about using a plateau finish.

Look at all the engine articles & books in the past (some not that long ago) where the goal was to achieve a smooth surface using 500 or 600 stones (regardless of whether a plateau finish was applied after).

I don't know how common the belief is today that the final pre-plateau surface should be a rougher 280-320 stone, but what Lennart B. stated in that 2005 EMC article did seem to contradict the other approach of going for a much smoother 500-600 finish.

Here's what Larry Widmer said about cylinder wall prep a few years ago, this quote is lifted from his website:

"The boring and honing process is one that should be accomplished by someone who does little other than cylinder wall preparation. Good boring and honing is an art form / science, and while good equipment is essential, a machinist who has a lifetime of experience (and who charges by the hour rather than having a set price for the operation) is the only way to go, as the right clearances and a good ring seal are all-important to making power. If the rings don’t seal, all the other modifications in the world won’t do you any good, as the power will go right past them.

We bore and hone the block to the same specs we provide our piston customers. All boring and honing operations are referenced from then main bearing bores of the block, so the cylinders will be absolutely perpendicular to the crankshaft. We don’t ever use the engine’s deck surface as a reference point.

The honing process must be performed using Sunnen equipment, such as a CK-10 (or newer) automatic honing machine. BTW, the “automatic” part is a joke, as it still takes a machinist with lots of experience to do it “right”. Right in this case would normally be a clearance of exactly .0028” (to the tenth), with no more than .0001” taper, or out-of-roundness. Our honing expert (who does cylinder wall preparation of winning domestic Pro Stock engines) is also giving the cylinder walls “teeth”, for an even greater effective ring seal. These “teeth” cause the bores to be smooth when you run a finger down the bore, but extremely “rough” when you pull the finger “up” the bore. This texturing is just one of the many “tricks” necessary when competing in a class where all 16 qualifiers are within .02 second on elapsed time! Lose effective ring seal on one of these 500 cubic inch normally aspirated gas-burning engines and you’ll lose a hundred horsepower!"

Interesting, to say the least.

R.