Can the 68 69 small bore block be safely bored to accept 455 pistons? Thanks

 

Not in my experience.

 

Thanks

 

Have a good shop map the walls with sonic testing. If you are lucky, you can bore to 4".

 

The stock bore of a G-block 400 is 3.870. The bore of a 455 is 4.125. You need to remove over a quarter inch from the bore (0.1275" from each wall, to be exact) in order to reach the 455 bore diameter.

Many people claim that the G-block has very thick walls and can accommodate a significant overbore. You may not get to 4.125" bore, but you can probably get quite a bit larger than stock. Custom pistons aren't that expensive for a build like this. You might get to 420-ish displacement. A 4" bore with the stock 4.250" stroke yields 427 cu in. If your intent is to preserve a "correct" block for your application, then start by having the walls sonic checked. That will tell you how far you can go. If you don't care about the original block, starting with a 455 is MUCH less expensive.

 

Many years ago an acquaintance took his newly purchased .060 over 400 pistons with his 1969 G block to the machine shop to be bored and honed. When he picked it up he was being charged more than he expected; asking why it was explained that the block had to be bored much more than .060" for the pistons he brought in. Found out that the pistons he was sold were for a 1966-67 E block and not the 1968-69 G block. He decided to go with what he had. The E-block pistons have a shorter compression distance than G-block pistons and would be too far in the hole so we found him a set of 7" 400/425 rods which put the pistons up out of the hole a small amount which he had cut off the piston top. The assembly was balanced and he ended up with a G block bored to 4.060" giving him about 440 ci and plenty high compression. He never had any cooling problems and drove the car for many miles afterward.

 

set the 400 aside & use a 425 or 455 block. You will thank yourself later.