Would like to see what racers prefer in a piston's pin hole finish. Do you prefer a honed hole or do you prefer a burnished finish?
Okay, my personal views on this. I prefer the honed finish because it has oil retention and much less chance of galling. The burnished finish leaves a much smoother finish. Maybe an 8 ra or close to it. The honed finish maybe a 32ra finish. Firms who buy pistons from the manufactures are demanding a smoother like 8-9ra . Sounds foolish to me common sense tells me the smoother finish will displace oil instead of retain it. What say ye Mopar racers?

I think the clearance is more important than the finish on a piston pin. The pin does not spin in the piston, it rocks, and gets pounded on. I guess it is not really scientific, but I judge a pin fit by the feel of how it slided after putting some assembly lube on it. I have no problem with or seen any bad results from honing a pin hole in a piston or rod. I have seen bad results from having the clearance too tight.

Press or floating pin?
I like at least .0010 or more pin clearances, even on the Chevy .990 pins, on all my pins to piston and rods on floating pins
I haven't seen a super fine finish on any of the pistons I've bought and had pin fitted in a lot of years, no problems with them either, unlike the early TRW and Speed Pro press pin pistons sticking the pins

I'm asking about finish. Just don't like the industry demanding a 9 RA finish. Cost more to make. To me it would be like polishing a cylinder in a motor. No oil retention, extreme pressure would displace the oil. All my theory and just looking for opinions.

I follow the mfg recommendations unless I'm willing to spend the money for my own R&D program.

Who is demanding a smoother pin bore finish?

All the past KB Icon pistons i’ve put together in the past ( about 4 motors) and the last Diamond set that went into a RB stroker had vary fine almost polished bores. Like merpar is speaking about. But, they all needed to be final honed to fit. They were all delivered slightly tight.

I'm sure you can order pistons from either company you mention pin fitted or not pin fitted, you do have to pay extra to either company for the pin fitting though
I tell them no on pin fitting as I want my machine shop to fit the pins to the rods and pistons to my specs.

I'd rather have the piston mfg do the pin fitting. I figure they have better tooling and more experience doing it than the local engine shop. Someone like Diamond probably fits dozens of pistons a day.

Nothing to get analysis paralysis over -

Don't know your application, so I'll pass this on -

1. Some NASCAR applications are using burnished pin bores with very small DLC wrist pins, no direct feed pin bore oiling, piston/pin squirters, high pan vacuum, at .0005" clearance.

2. As long as the wrist pin is robust enough for the application, direct feed pin bore oiling, and depending on pin diameter, a .001"-.0013" clearance honed finish works well.

3. If the wrist pin is too thin for the application, up to .0025" clearance honed finish may be needed in application #2

Don't have any RA info for you, I like the pin bores on the smooth side, as long as I can see cross hatch without magnification I'm happy. Make sure to measure pin to pin bore clearance in all pin bores, one of the major custom piston manufacturers is terrible regarding their pin bore sizing, sometimes their sizing is almost .001" off, in the very same piston!

I'm with Andy on this one, Most, if not all of the piston manufacturers would have hours and hours in R&D on works best. The manufacturers that work with OEM's would have more so.

If the manufacturer could make them more cost effectively, they would be doing so. Especially these days. If they're suggesting a specific finish, they'd have testing and data behind it.

LA360, Agree with you and Andy, Manufacturers through countless hrs of R&D know exactly how to make the roundest and most accurate pin hole. Plus oil retention of a honed hole. After years and millions of pistons being manufactured using diamond coated mandrels. The buyers ( corporate america ) decided they wanted an 8-9 RA finish. Why? that is my question.

Who are these "buyers" you are referring to?

Do they have names?

Didn't think so......

Would be foolish to disclose their names. Corporate America wouldn't like that. Didn't think so! What is that supposed to mean?

I have many relationships/connections in the design departments of several reputable piston companies, not one of them said that they are finishing pin bores to a specific 8-9 RA, or are being asked to do so.

Maybe a lower tier piston company is involved in that, but I have no connections at any of those, nor would I have any interest in buying any of their products anyway.

What I stated here and earlier is the real world, based on real world results in a competitive racing environment (where the real R&D is done).

Who has the ability to burnish piston pin bores to the .000X"?
I'll bet next to nobody.
Doug

Mahle and Pankl do it every day on round and tapered pin bores

Like I said few and far between. What is the upside?
Maybe on extreme power stuff ?

Creates a compacted and more durable surface, can handle more loading in applications with minimal pin/pin bore surface contact area.