My 70 Chevelle has a 66 396 block casting number 3855961. In the Mortec listing it indicates that this is a 2 or 4 bolt block. Mine is a 2 bolt. Also under casting number 3855961 there is a listing for a 66 427 block 2 bolt. Can I assume that I can have the block bored to 4.250 for 427 cu in. or even go .060+ as I might have the same block as a 427. Any way to check????

Take a pencil and try to get it between the center two cylinders through the freeze plug hole. If it goes then it will not even bore to 427 if it doesn't then it should. I have a 961 block that will not bore to 427 and can't remember ever seeing one that will.

Ed Staffel's book "How to Build Max Performance Chevy Rat Motors" say's on page 34 the 3855961 block has been used to go from 396 to 454 with no grinding at all!!

But it is a good idea to sonic check the cylinder walls if in doubt..

pdq67

I won't argue with the man but try the pencil trick on a 427 or 454. He may mean the 962 4 bolt block which will go to 427 and beyond.

I've known people that have bored the 396 to 4.25 but i wouldn't do it myself as the performance difference isn't that great. A good sound 396 is better than a 427 which might break at any minute.