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Is it right that i cannot use a different top half crank case for an engine i have?
Reason i ask,is a mechanic friend of mine says there cut as a set and that changing the top half will not work.
Is this true for the 3HE crank cases,as i have got one thats got a piece missing out of the top half and i would like to try and rebuild it with another one.
Thanks in advance.

Yes it is true. You can not use crank halves from two different engines. The milled part he is referring to are the sections that the crankshaft sits in. If you look at the crank case, you'll find a set of numbers. Those numbers give you the tolerance sequence for each slot. You have to use the corresponding numbers on the crankshaft with those on the cases to determine the correct size crank bearings need. Same rule applies to the con rods. You don't want to mix up a con rods parts as they are ground specific.

Or you could check with your local automotive machine shops...see how much a "line bore" would be,

They will bolt the cases together and run a boring bar thru them in order to true up the bearing bosses. Then, it's a matter of mic'ing and plastigage-ing crank and bearings