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The 4.000" measurement that you are getting accross the top of the cylinder means that the block has not been previously overbored. This doesn't mean that there's not cylnider wear further down the bore where the rings scrape against the cylinder wall. If you check with your finger tips, you will probably feel a ridge at the top of the cylinder wall. That's from the wear created by the piston rings against the cylinder wall.

Generally speaking, a .030" over bore will make the cylinder bore smooth and true. The machinists that I know don't recommend boring more than that on the thin wall casting of the small block Ford engine.

My dial caliper uses a regular scale for the first .100 past the inch scale. The dial portion measures additional thousanths of an inch. If I had a 4.0 on the regular scale and the dial read a 30 that would make a measurement of 4.030" which is what my original 289 block was bored when the rebuild was done.

Your machinist will evaluate the block to make sure he has something good to work with. He will check the cylinder bores for enough to make a good true cylinder. He will also check for cracks and other problems before he does all that work.

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