Well there have been many discussions of this around here in the past. Lets start with the basics.
A 396 with a 0.030 overbore is a 402.
Now why GM did this is a mystery to me but from what I've seen around here a few things could be considered culprit.
1. Core Shift during the casting of the blocks leading to an overbore correction. It makes sence considering (from what I can recall) all Big Blocks were made in Tonawanda NY. All 402 began life as 396's made at Tonawanda.
2. Emmissions. Bigger discplacement engines were required to follow slightly less requiremnts than smaller engins. 400cid was a cutoff for the tougher requiremnts.
3. Numbers Game. More cubes meant greaty popularity. What strikes more fear, a 383 or a 427?. However GM homeoffice said no one could put more than a 400cid (and/or more than 10% curb weight worth of torque) in an RPO (or off the floor/line) midsize passanger car. Corvettes are sprots cars, Olds 455's had the 455s put in at the hurst plant. 427 Chevelles were COPC'd etc etc.
4. Popularity. GM wanted to make more power for more sales and found that a 402 gave a little more umph using off the shelf parts but the "402" was not the "396" so they opted to leave the name the same considering they are identical (almost).
It was corporate policy prior to 1970 that no Chevy engine would exceed 400 cubic inches in passenger cars, only Vettes which are not passenger cars. That's why hi-performance COPO's existed in the pre 70's years. Another reason why the illusive 1967 Chevelle SS427 cannot be documented because Chevy would not factory install these hi cube engines into passenger cars!! But in the 1970 model year they had to compete with Ford and Chrysler because they were doing it with the 428 Cobra Jet and 440 Magnum.
The 396 block was overbored to make it a 402 cid. They continued to call it a 396 because of name recognition fearing if the changed the name 396 to 402, sales would suffer, and no doubt they would have.
[This message has been edited by elcamino (edited 10-29-2000).]
Let me clarify, the 400 cube ban was on intermediate or mid size and smaller passanger vehicles. Vettes are sports cars, COPOs are COPOs and Impalas are full sized and were also used in fleet, taxi, police and "commercial" work like ambulances and hearses.
Let's be clear. The 402 did not show up until calendar year 1970. Many of the SS396 Chevelles of 1970 model year made during Sept.-Dec. 1969 received true 396 engines. The 402 was phased in starting in either late December 1969 or January 1970.
The 396 was bored .030 over to 402 to meet emmission standards. The slight increase in size allowed Chevrolet to use the larger motor emmission standards and allowed them to keep the same HP and torque ratings. Starting with cars manufactured in January 1970, higher emission standards were in effect. To address these new standards additional strain was placed on the motor (either via smog pump, lower compression, other emissions equipment or various combinations) so the few extra cubes were needed to make up for the additional power drain.
Since they had invested so much in the SS396 name and mystique, they simply kept the name and gave customers 6 extra cubes for their $$$$. I don't think they would have hurt anything by renaming it SS402, but that's just my opinion.
Pontiac did something similar in the mid sixties; their 326 really displaced about 331 cubes for some reason.
BB 402s and SB 400s of course are completely different motors. They do NOT share the same bore and stroke measurements.
so my "396" in my 70 is a 402? It is an oval port 396/350hp, and the block is CTW I believe. Since I'm think about an overhaul, and its already .030 over, that means I cannot bore the block? Is there any truth to the "402's use the little (peanut) spark plugs, but the 396's use regular 13/16" spark plugs"??? When I bought my Chevelle in '94 I looked, carefully, at a '69 SS396 and a '68 SS396, looks like the same motor to me (neither was the 375 hp motor)
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