ClassicCar

The first casting numbers are not definitive. They just tell you the date code when the mold was produced. Look for a string cast "A" or "C" on the back of the block (make sure it is not welded). Ford put this letter on the casting by placing a piece of string on the back of the block casting so it actually was cast into the finished block. BEWARE OF FORGERIES.

Next look for "428" cast into the center freeze plug hole, on the floor. The heavy webbing on the main supports, a third rib going toward the main journal may or may not be a 428 but it is a step in the right direction.

There is also the "drill bit test". Take a 15/64 drill bit and see if it will fit between the cylinders in the center freeze plug hole. If it fits, even with some resistance, it's a 390 bored out. If it doesn't fit, it's a good sign.

Keep in mind that some 390's can be bored to a 428 and then go 60 over but these blocks were rare truck blocks and were cast as 428's. Most FE block were cast as 390's only a few were cast as 428's. On the outside they are all the same and most parts will interchange. The difference is that the O.D. of a 428 or 428 capable block cylinder is much thicker (that’s what the "drill bit test" is all about the cylinders are all bored the same center to center).

In the end, even if the block says 428, it may only be a 390 Truck engine.

The service part number for all 1966 - 1970 428 blocks is C6AZ-6010-F.