I went to the junkyard today to get a strut mount retainer for my 86 z28 cause mine had snapped in half and while I was there I saw a couple blocks and wrote down the casting numbers (a habit of mine at the junk yard), and in a third gen camaro I found a 3951509 casting 400 inch small block with 4 bolt mains (1970 - 72).

I havn't had a chance to go back and mic it yet but the heads are pull and the pistons look pretty beat up so I am sure its been bored over atleast once.

The question is, what is the limit to which you can bore over a 400 small block? And also, what is a good way to get an accurate micrometer reading a on block with rusted cylinder walls?

 

400's have really thin walls especially between the center two cyliders. Like any small block you can go 60 over if you wanted to. however I wouldn't recommend going more then 30 on a 400 because they are thin to start with.