396 L78 block
 

Hey folks-
Can a L78 4 bolt block 69 vintage accept an over bore to 4.250 for 427 displacement or will it become unstable?

asking for a friend

Pull a soft plug and see if you can stick a pencil between the cylinders. I believe this block will be too new and it will fit, but that is the easiest way to see if it is even a candidate.

I know the 854 70 L78 blocks can take a big bore job, they are actually 30 over 396's (402) from the factory

The cylinders are going to be awful thin if you do.

If you are looking for performance not sure the 427 has much over the L78 configuration to risk that bore.

A 69 big block of any C.U. Or horse is worth more with as little overbore as can be done to clean up the block, imo. A heavy overbore diminishes any 396 or 427. With so many gen.4 454 blocks available, there is no need or reason to bore a 396 out that much, if it can even safely be done.

Bore it .060 buy a set of UEM Pistons($650) and a Scat 4" stroker crank and you have a 434ci. motor. Step up to Ross pistons ($1100) and a 4.25 stroke crank you have a 461ci. from a 396 block.

As JRSully said,if the block is one of the late 1969 production & really a 402cid,you should be fine.
Easiest way to identify one is if it has a 3 letter Suffix code on the machined pad beginning with the letter "C"...example CJD or CKF,as opposed to just JD or KF.

The early 65 Fang 396 block has lots of meat also.

I have a 65 396 punched to a 427 bore. we used hard block to stabilize the cyls. been in my car for many years but its an early block