MG Car Repair: A series crankshaft bearing sizes, main bearing, british engines

Question
QUESTION: I am very new to automotive repairs and hope you can enlighten me with some information. The first under-size bearings for most American and British engines is referred to as 0.010 in the repair manuals. Does this mean 0.010" will be ground off from the crankshaft journals plus a further clearance between the bearing shells and the crank pin journals? How will the machine shop operator know the amount of metal to be ground off the journals or do we have to provide him with the original dimension of the crank pin journals in the first place if the engine is new to him?

ANSWER: Depending on the amount of wear and damage, the machine operator will take off material from a crankshaft in multiples of .010".  This is known as ten thousandths of an inch, commonly abbreviated to ten thou'.  If one main bearing journal needs to be reground, then every main bearing journal is reground to the same undersize.  If one big-end is worn then every big end is ground to the same undersize.  

Main bearing sets and big-end bearing sets are available in the following oversizes to match the reground crankshaft:  (Standard, .010, .020, .030 and .040).  I have also seen .060 although I have never seen .050.

If you have an obscure engine, then it is better to find a set of bearings BEFORE having the crank reground.

---------- FOLLOW-UP ----------

QUESTION: TQ for your prompt reply. Actually my car is a British Mini 1275GT. According to the repair manual, the dimensions are as follows:

Main bearing journal is 2.0012 to 2.0017 in (50.83 to 50.84 mm),  
Big-end journal is 1.7497 to 1.7504 in (44.44 to 44.46 mm).

Assuming that the wear is minimal and it calls only for the first regrind of 0.010 in, how much further has the machine oprator to provide for the bearing clearance?

Answer
The clearances are machined into the bearing shells when they are manufactured.  

Standard main journals should be 2.0017 inches diameter, but due to manufacturing tolerances they can be up to .0005 of an inch (half a thou) less than this.  If your machine operator can polish the journals within these limits, then it is unnecessary to grind ten thou off the journals and you can still use standard shells.  Similarly for the big ends.

If you need to grind the mains, ask for 1.9917 inches, but allow a tolerance of .0005 inches, so anything between 1.9917 and 1.9912 is acceptable.

By the way, some standard crankshafts were very slightly undersize, so it is just possible that you could find some +.002 shells somewhere.