Not trying to nitpick, but this statement deserves some comment.

Ah! a reread makes me think that you may have meant "before you take the block off to the machine shop for a "Piston Cylinder" re-bore! If so please discount the first two paragraphs!

Plasti-Gauge would as you are later seen doing, is used to ascertain the tolerance between the bearing shell and the crankshaft. This measurement will have nothing to do with the engine block's crank shaft bearing bore. Without some noticeable bearing failure the block bore would remain unaltered and would not need a "re-bore".

If there where a question regarding the block's bore the way to ascertain it's condition would be direct measurement of multiple quadrants and for square and between all the bores for alignment. Fortunately MB lower ends are exceedingly robust and absent a bearing shell being spun out and physically damaging the block bore I have never heard of a 615/6/7 block bore being tweaked!

After using Plasti-Gauge you would simply know whether the tolerance between the bearing shells and the crankshaft journals are within specs or not. If within specs, button things up, torque to spec and forget about it. If the measurement is outside of the FSM specifications the first thing would be a competent inspection by direct measurement of the crankshaft journals, once that was determined the question would be if replacement bearing shells by themselves would correct the out of tolerance issue or if the crank would require a regrind altering it's dimensions and requiring alternative repair sized bearing shells.


P. S. I almost forgot to mention it but I was wondering why you used the Plasti-Gauge in the manner you did as I’ve always used it across the bearing as described by the manufacturer and have always observed others to use it in that manner also?

http://www.plastigaugeusa.com/how.html