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Reply to "Boring 351C"

0.030" oversize is Ford's recommended limit.

Sonic testing to insure a minimum cylinder wall thickness of 0.120" on the "thrust" sides is another method. As you have been advised by others this sometimes allows a bit more overbore.

When taking the cylinder walls to the limit, I highly recommend using replacement pistons having full round skirts, i.e. endurance racing skirts. Such as those sold by Ross Pistons. This spreads piston skirt loads over a wider area of the cylinder wall, and is very effective at preventing cylinder wall cracking. This was standard practice for all forms of racing back in the engine's hey day.

The other thing a machinist can do to lessen the thrust forces on a cylinder wall is to "index" the boring machine to the crankshaft axis during the boring process. This insures the bores are perpendicular to the crankshaft's axis, rather than being canted to the side, or fore to aft.
Last edited by George P
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