[FFR428]

You can knock out the 6 freeze plugs on the block and see what size drill bit shank fits between the cyl walls inside. That will give you a good idea what you have until you get it to the shop for a sonic. A 13/64" drill bit shank is a snug fit in 428 wallcores. 17/64" for 360/390 wall cores. Check at the top, middle and bottom of the cyls. Clean away and crap or crud with a knife so you get a good gauge reading.

G.