Tim. 1957 ECZ 6015-C engine blocks were factory machined as either 292 CID or 312 CID depending upon the main bearing caps for the crankshaft used.
ECZ 6015-C engine blocks with ECZ main caps have a 312 crank and were factory machined as 3.800 bore for a 312 CID
ECZ 6015-C engine blocks with EBU main caps have a 292 crank and were factory machined as 3.750 bore for a 292 CID
With your main caps showing ECZ, it is most likely that John (Hoosier Hurricane) is correct and that your pistons are .060 oversize with a cylinder bore of 3.860
Unfortunately, there are some off-brand piston suppliers who supplied some crazy oversize pistons for 292 y-blocks that read .080. I've seen these.
IF they supplied .080 oversize 292 pistons, they may also have supplied .090 oversize 292 y-block pistons.
Originally, the difference in a FoMoCo 292 piston and a FoMoCo 312 piston was in the piston skirt. The skirt of the FoMoCo 292 piston would not work with the longer stroke of the 312 crank. That said, the skirt of a 312 piston would work with any appropriately oversized 292 cylinder overbore.
Some 'off-brand' (my term) replacement piston manufacturers MAY DO some 'different' things to fit as many applications as possible and reduce production costs.
Example: they could use the 312 skirt design, the common 292 / 312 piston pin compression height, and simply machine the piston diameter and piston ring diameter grooves for 292 + .020, .030, .040, .060, .080, or .090 oversize dimensions.
The result of this is that 292 + .080 piston can be used for 312 + .030 bores, and 292 + .090 pistons can be used for 312 + .040 bores.
An ECZ 6015-C engine block with ECZ main caps and oversize 312 pistons stamped .060 that measure 3.860 will displace 322 cubic inches
An ECZ 6015-C engine block with ECZ main caps may have off-brand oversize pistons stamped .090 that measure 3.840 will displace 319 cubic inches
Click the link below to see John Mummert's web page for the engine block identification info. I have cited and the second link below for JM's displacement chart for bore stroke combinations and the displacement that they yield.
http://www.ford-y-block.com/Block%20identification.htmhttp://www.ford-y-block.com/displacement.htm The off-brand piston manufacturers are the source of this confusion. Measure the bore size you have to know what the bore you have with your 312 crankshaft.
Hope this helps!
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York