What is the largest over bore you can go on a 402

[92646]

I have a .060 over 1970 402 4-bolt block. It will clean up at .090 but I can only find custom made pistons if I go to this size. Has anybody bored their 396 or 402 to 4.250 and used 427 pistons?
Mark Sheppard

[nuch_ss396]

Mark,

It's been done before. Is this for a street or restoration
application? Did you factor the cost of custom pistons
against sleeving all 8 cylinders? If this is a numbers
matching project.........?

Steve

[HiHorse]

Yes, I've seen the older 396 blocks go 4.25, in fact in early 1966 the 396 block # 3855961 was bored to a 427.

[nuch_ss396]

[ QUOTE ]
Yes, I've seen the older 396 blocks go 4.25, in fact in early 1966 the 396 block # 3855961 was bored to a 427.

[/ QUOTE ]

It was my understanding that some early 396 blocks had thicker
walls and as such were able to be bored to this extreme.
Since the 402 was a 396 +0.030 anyway, aren't you already
behind the 8-ball regarding cylinder wall thickness? Or,
were the 402's cast with thicker walls to compensate for
the overbore ( 402 vs. 396 ). Remember, the 3969854 block
started life as a 396 ( late '69 production ), then morphed
into the 402 in 1970. I would tend to believe that the
cylinders wall thickness was not modified once 402 production
came to be as this would have mandated a different part number.

Steve

[Unreal]

I thought the Cxx coded 396 were actually 402's in the extended Camaro production year

[92646]

The motor will go into a street driven car. It is an original warranty replacement block. I have not considered sleeving the motor but that may be choice because I would hate to toss the motor aside and not use it in something. The .090 over pistons are made locally here in Huntington beach and I was told that they are about $800.00 for a set versus a little over $400.00 for a set of off the shelf pistons. I kind of like the idea of going to the 4.250 bore (.125 over)and I have read that it has been done but I do not know of anybody that has done it for the street. I have a set of 990 open chamber steel heads. I thought I could build a L-88 style motor that has the look and casting numbers of a L-78 402 under the hood but have more power.
Mark Sheppard

[427king]

Mark, dont bore a newer 396/402 to a 427 for a street car. The 962 from 1965/66 is about the only block id do that with. If you do it on any block be sure to sonic test the block I also have new pistons in stock that are close to l78 compression for not alot of money i have 030 060 070 090 ...

[92646]

Chuck you have in stock .070 or .090 forged pistons for a 402? Can you e-mail the details please.

[HiHorse]

no doubt the 962 is a good candidate for a 4.25" bore, but the 961 block was actually used by GM as a 427. Good news is the 961 is pretty common compared to the 962.

[70-SS/RS-L78]

90 is a bit thin for street use on a 70 402 block, As Chuck mentioned I would sonic test it first. You will have overheating problems and cyl wall distortion (ring seating issues) if they get too thin.