I am in the process of replacing the rod and main bearings in my 312,  Looking at the manual indicates red and blue measurements for the crankshaft.  Why do they have different measurements for the journals?
Larry

Larry

Have you measured the crankshaft main and rod journals to determine if they are already undersize?  Do the journals measure round, tapered, or scratched?  You must measure everything accurately to determine if it will need to be reground.  Then buy correct replacement bearings.  Replacement bearings are not color coded to be matched or mix-matched.  Joe-JDC

JDC

The normal variance for the journal diameters on the crankshafts from the factory was +/- 0.0004” for both the main and rod journals.  Doesn’t sound like much but that’s almost 0.001” from the low side to the high side.  And then there’s a variance specification for the thickness of the bearing shells.  Between the variance in the bearing wall thicknesses and the journal diameters, the oil clearance can be too tight or too loose.  That’s where the red and blue bearings come into play.  For crankshafts that were ground on the low side of spec, the blue bearings would be used and for crankshafts ground on the high side of spec, the red bearings would be used.

The wall thickness for the ‘red’ Ford Y-Block connecting rod bearings is 0.06095”-0.06145” while the wall thickness for the ‘blue’ bearings is 0.06135”-0.06185”.  You will notice that there is some overlap of the values between the red and blue bearings.  The same issue with wall bearing thicknesses occurs with any bearing you purchase, standard size or undersize.  Simply means that you do need to ‘ball’ mic the bearing wall thickness of both the main and connecting rod bearings to know for sure what you are using.  If grinding the crankshaft undersize, then it's recommended to first install the bearings that are going to be used in their respective holes, mic the I.D., and then grind the crankshaft appropriately for the desired oil clearance.  That takes potluck out of the equation for obtaining the desired oil clearance.

Lorena, Texas (South of Waco)

The crank rod and main bearings meet the specifications for a standard crank.  There does not appear to be any taper.  It has not been turned and is in excellent shape. I was able to find standard bearings for rod and main bearings.
The crank has been polished.  
Thanks for the response.  My crank meets the red standards.  Larry

Larry