396 L78 block

[chris slawski]

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

[dustinm]

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.

[JRSully]

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

[big gear head]

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

[Salvatore]

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

[bbbentley]

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.

[wvhylander]

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.

[Mr70]

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.

[L78steve]

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

[plumL78]

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

[the427king]

Sonic test it to be certain .....cylinder wall thicknesses can be all over the board on the same block even within the same bore,so be sure to measure 4 places on each bore . 854s are decent for boring to 427 but #962s are the best.....

[big gear head]

I've got a 854 L78 block. I removed the center plug and checked the space between the cylinders with a pencil and it went through. I don't think I would bore this block to 4.250.

[EZ Nova]

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

For the cost, WHY?? you could probably buy a ready to go 4.250 or 4.280 bore block for less?

Keep the L78 for L78 stuff or people.

IF your looking to go fast, the added bore will help some, but in the end is it really going to be worth it? I myself see no reason for this unless to mislead people or a class by claiming to be a L78/396 and be stroked? Like some sort of stock-block class.

FYI I did a mild 402 to 575Hp back in the 90's with mostly GM stuff. Used OEM L78 short block, Ported sq port closed chamber heads, strip dom intake and comp .660 solid roller. Ny '69 Nova would run 10.40's with 9" slick, thru muffler and 4.56 and M21. I'm sure with what we know now, I could put that deal in the high 9's or low low 10's, WITH A OEM "163" intake.

So tell us WHY your thinking this bore?

Thanks, John

[EZ Nova]

There is a reason why stock and super stock guys use SLEEVED 454 blocks to go back to 396 also!!!!!

Your friend could very well run into a overheating issue too with that thin of a cylinder wall.

[Vortecpro]

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

NO

[Vortecpro]

There is a reason why stock and super stock guys use SLEEVED 454 blocks to go back to 396 also!!!!!

Your friend could very well run into a overheating issue too with that thin of a cylinder wall.

The reason there sleaved down 454s is to move the bore away from the intake valve. My engines are aways filled for NHRA stock.