Hey guys. Let me put some "shade" on this topic.
We need some clearance between pin and connecting rod for first of all to have good lubrication (oil film has some thikness) and secondble we need to be able to put connecting rod on the pin when we assemble engine (this requirement make clearance slightly bigger then we wanted to) . On custom built engines clearance is way smaller (just good to have oil film there and have oil circulate little bit). But there is draw back-To assamble this kind of engine first we put connecting rod alone on the pin, then from the top we put piston with installed G-clip on front side (booster side) only, through the exhaust window we put wrist pin and then we put second G-clip also through the exhaust. It is pretty tricky procedure and required some skill and special tool which is not widely available. In this case usualy clearance designed to be 0.015-0.018 mm.
To make life easier we have to sacrafice clearance and for mass production engines clearance designed to be as follow:
In inital design of shaft and conrod every engineer put some clearance. I will give you sample of let's say 12 size engine. Crancksaft's pin 4.3 mm is laying in following tollerances 4.294-4.296 mm and hole in connecting rod is 4.325-4.330 mm, so clearance is in range 0.030-0.036 mm. This numbers give to us ability to lean connecting rod slightly and put it on the cranckshaft with piston install already on connecting rod.
As far as when we have to change crankshaft ( when pin conceder to be worn out) I would suggest to say if it measures 4.290-4.291 it is gone. Also needs to be measured if pin not eleptical. If make two mesurments ( 90 dereese to each other-vertical and horizontal positions) and find the difference-if it is greater then 0.008 then it is worn out and can't be use.
Visiualy we can see the following-if under the good light and magnifyed glass you find that pin has blue or dark brown spots ( regardless if it still measures right numbers)-then it is gone, if you can see that mirror on the pin doesn't look flat-then it is gone, if you can't see even mirror on the pin-it is gone.
I will try to make pics of my collection of cranksafts with wornout pins (with visible damge), but not sure if my camera can catch it. I will try anyway. If anybody has good camera and have worn out shafts-plaese let's halp each other and post your pics here. I was ale to catch with my camera only one visible damage
Yes to determinate correct number we need to have ID and OD measuring micrometers and it is pretty expencive tool. Good OD micrometer costs somewhere between $180-200 and tool to measure ID on the connecting rod is about $100.
I hope I was able to help somehow.