The problem with extinct engines like the Ford Y-block V-8 is that information isn't just hard to come by -- more often than not, the factory specs have been outdated by decades of research from enthusiasts and builders. Combine those often less-than-ideal factory specs with the fact that original factory replacement parts are hard to come by, and even something as simple as piston-to-bore clearance on a Y-block can turn into a quagmire of guesswork.
The original Ford specs for the Y-block V-8 were 0.008 to 0.026 inches; anything between that, and Ford says you're OK. It wasn't uncommon for engines of the era to vary by as much as 0.01 inch from one to another, just because of the loose manufacturing tolerances and procedures of the time. However, any experienced engine builder would tell you that this is a pretty massive variance; enough to make the specs themselves near useless for building purposes. This is especially true given the fact that unless you're spending five times as much for New Old Stock replacements, you're going to end up using pistons made of a more modern alloy, which will require different clearance specs. Your piston manufacturer should have that information on hand.
If your engine has been bored and honed using a torque plate and you're using pistons made of a modern 2618 T-61 forged aluminum or equivalent, then consider 0.004 to 0.006 inch your target area. You could go as high as 0.008 to 0.009 for a street engine, but clearances as high as 0.012 to 0.014 should be reserved for really high-powered supercharged, turbocharged or nitromethane-fuel engines. Add 0.005 for engines that haven't been honed with a torque plate. This is a good guideline for forged and cast pistons, though high-silicon hypereutectics can theoretically get away with 0.01 inch or so less clearance. Remember, a fast race engine is light and loose; a good street engine is tight and smooth.
