Bearing oil clearance

When measuring the crankshaft journal, such as main or big end, when identify the crank size , you then select the bore gauge rod size to suite the crank size and then checked and set to zero...say for example if the crank size is 42.50mm but the size of the bore gauge rod is limited the size, then you choose 43mm bore gauge rod then set it to zero. Before you measure the inside of the bearing journal.

I'm sorry I don't quite understand your question? If you're referring to selecting the bore gauge extension then this needs to be selected so that the bore gauge is within its measurement range when zeroed.