Ok cool
It occurred to me that I have a bearing clearance target I want, but the only reason is, “well that’s what everyone does”.
I’d like to know why I’m shooting for that target.
The rule of thumb is .001" of clearance per journal diameter. A 2.750" journal size gets a ballpark clearance of .00275". The bearing that's used, the coating, the application, how weak/unrigid your rotating assembly is, how far the machine work is out, etc., all "tweak" that by a little bit, where you fudge it one way or the other.
On a 302 main, which is 2.250", you would not see me run a .003" oil clearance, nor would you see me run a .002" oil clearance on a 460 Ford, which has a 3" main journal.
It's not only the rule of thumb that I use, but it's what bearings look like when engines come back for freshening. Race engines are the best ways of learning and when an engine comes back and I can see my bore mic tracks on bearings, then I know that I've done something right. That's also why I have dyno mules, so if I want to "experiment", I can do that without consequence.
Since the coating on a coated bearing is harder, you can take a little bit of that clearance away, and I do. The coating is excellent for extreme performance scenarios, or even on a daily driver where the coating would give some added protection when you first fire it in the morning. They're expensive, but it's something that I've been putting in all my engines as I see it as an added value.
