What is the largest overbore safely performed on a production 400 smallblock?  

.030 is max, but try to clean it up in .015 and save some wall for future efforts should it become necessary.

Bogie

i think it depends on the year but i have a 71 400 punched out 0.040 and it is a beast of a motor ( in a sleeper truck ) and they say grandpas truck was slow!!!!!!!