Why the overbore from 396 to 402
 

Can anyone explain GMís decision to overbore the 396 032 in 1970 to make
it a 402? And did they use the same casting or did they thicken/widen the cylinder
walls to compensate for the larger bore? [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Thanks
Mark

Re: Why the overbore from 396 to 402
 

Thats a real good question , I also have wondered the real reason for the over bore. I have herard many reasons but i think most were personal opinions but one that stuck in my mind had something to do with the onset of tighter emissions. I did read in an artical onetime that when the over bore was introduced the 396 engine ID was to remain because they didn't want to change the legandary SS396 name plate because of sales. Does anyone else out there have any clad reasons for this? And didn't they change the casting from the 272 to the 854 when the over bore was introduced? I also remember the confusion at the parts counter because it was refered to a 400 mark 4 and they also introduced the 400 small block later on. [img]/ubbthreads/images/graemlins/confused.gif[/img]

Re: Why the overbore from 396 to 402
 

My opinion
GM started the 400 Chevy SB in 70 and for the 396 Chevy BB oups problem GM overbore the 396 to 402 BB good no problem [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]
Just my opinion [img]/ubbthreads/images/graemlins/smile.gif[/img]

Re: Why the overbore from 396 to 402
 

[img]/ubbthreads/images/graemlins/biggthumpup.gif[/img] [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

Re: Why the overbore from 396 to 402
 

I know some guys who have 69 SS 396 Camaro's and Elcamino's. They told me the reason for the overbore from a 396 to a 402 was to compensate for the powerloss due to the A/C conditioner option. [img]/ubbthreads/images/graemlins/headbang.gif[/img]

Re: Why the overbore from 396 to 402
 

[ QUOTE ]
They told me the reason for the overbore from a 396 to a 402 was to compensate for the powerloss due to the A/C conditioner option.

[/ QUOTE ]

Um... no.. The 396 was increased to 402 cubic inches for 1970. It's called Growth. It also had to do with GM lifting the 400 cubic inch limit on intermediate cars. And like I said, it's called growth.. There was a cubic inch war going on at that time. 427 grew to 454... 396 grew to 402...350 grew to 400... etc.. And I also have heard the story that GM stayed with the SS396 emblems because of success.. Why mess with what works? But who's going to argue with having 402 cubic inches compared to 396?? I'll take six more if I can get em. [img]/ubbthreads/images/graemlins/grin.gif[/img]

Re: Why the overbore from 396 to 402
 

Engines over 400 cu in were allowed higher emission spec. So they avoided putting some emission equip on some BB engines by making engine slightly bigger. 402 cost them less to produce then 396. They end up making a 400 SB turbo fire and a 400 BB turbo jet that was really a 402 and a 396 that was really a 402. They were crazy not stupid. [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

Re: Why the overbore from 396 to 402
 

I would agree w/Joe C. One reason: EMMISSIONS

Re: Why the overbore from 396 to 402
 

I seem to remember car insurance cost going higher if the motor was over 400 cubic inch and that the 396 was always closer to 402. [img]/ubbthreads/images/graemlins/confused.gif[/img] So GM said that 402s were 396 to keep below the 400 cubic inch insurance benchmark. But maybe what I heard was just an urban legend. [img]/ubbthreads/images/graemlins/dunno.gif[/img]

toner
p.s. is this pick on charley week? [img]/ubbthreads/images/graemlins/grin.gif[/img]

Re: Why the overbore from 396 to 402
 

I would tend to believe JoeC's explantation as most plausible thus far. When I 1st read the post, I was formulating a guess that the overbore did 2 possible things (both emissions related) but purely speculation: 1. Larger bore unshrouds the valves and contributes to more efficient combustion, thusly lower emissions? 2. Larger bore helps to create a larger combustion chamber for lower compression? (this 2nd one seems flawed though considering the year implemented...and probably a few other factors.)

Just guessing,
-Mark

Re: Why the overbore from 396 to 402
 

The "other" one I've heard, is that GM had a batch of flawed blocks, and rather than trash the blocks, they bored them .030 over [img]/ubbthreads/images/graemlins/bs.gif[/img] [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Re: Why the overbore from 396 to 402
 

Joe: The emission delete makes sense, In the engineering department one of there "mottos" was parts not installed have no warranty problems. Also weren't they having problems with the smog pump on some hipo engines over heating the exhaust systems? I could be wrong about the smog pump over heat thing it could have been an isolated thing at the dealership i worked at the time.(or maybe a loophole in the law to remove it) [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Re: Why the overbore from 396 to 402
 

Gm has done some weird things. They put 400 emblems on trucks that had the BB 402, go figure. [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Re: Why the overbore from 396 to 402
 

Another strange move for GM in 70
GM never put the new 400 SB in a Camaro for what ???
emission or warranty problems or bigger problem with this
new block for 1970 ???
GM like more overbore the 396 to 402 [img]/ubbthreads/images/graemlins/dunno.gif[/img]
Strange move [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

Re: Why the overbore from 396 to 402
 

Its funny you mention the truck emblems, My 72 Cheyenne is a 402 with the 400 badges. [img]/ubbthreads/images/graemlins/3gears.gif[/img]

Re: Why the overbore from 396 to 402
 

I've seen a few Chevelles that way too. Non-SS cars. I've seen a few 71 402 Heavy Chevy Chevelles with 400 air cleaner decals on the 402. And I've seen a 71 Malibu with a 402 BB that had 400 fender emblems and air cleaner decal.
Probably too many Chiefs going on back then. That's probably why there were so many different things going on at the same time. I'm sure everyones been in that situation. [img]/ubbthreads/images/graemlins/headbang.gif[/img]

Re: Why the overbore from 396 to 402
 

I've heard stories about the 71 Heavy Chevy Chevelles, and that depending on what transmission you ordered along with the 400 engine, you would either get a 402 big block or a 400 small block, and both were called "400" on the order sheet... I don't know if that's fact or fiction, but wouldn't it be a bummer if you and your buddy both ordered 400s and he got a big block and you got a small block? [img]/ubbthreads/images/graemlins/blush.gif[/img] That would be my luck! Anyone ever hear of this? [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Re: Why the overbore from 396 to 402
 

I can't remember ever seeing a 400sb in a Chevelle. Seen plenty of them in Monte's and Full Size but that's not to say a Chevelle never had one. [img]/ubbthreads/images/graemlins/dunno.gif[/img] The 400SB was a fairly mild engine wasn't it?

Re: Why the overbore from 396 to 402
 

400 Turbo Fire was SB 400 Turbo Jet was BB
I think the "Malibu 400" was a model name

Re: Why the overbore from 396 to 402
 

I will have to check it out later but I have a 72 super Cheyenne that has a 402 emblem on it. [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Re: Why the overbore from 396 to 402
 

I think that Jerry McNeish said in his book that some of the late 69 SS396 Camaros were actually 402's, I will have to go back and read that chapter again just to make sure, I remember him saying that the block casting #'s changed.

Re: Why the overbore from 396 to 402
 

On the same note, some head changes were made along the way in '69 as well. I found this out by accident, as I bought a set of 840s that had never had valves or seats put in them, but were made for the 1.88 exhuast valve, and tapered plug. On the shelf we had a set of 840s that originally had the 1.72 with flat seat plug...the interesting part is they were cast in the SAME WEEK (in May sometime). So, I had the last of the old style with specs of the new style, just by luck. I called MacNeish and Colvin and they said that was correct, that they did a changeover in May sometime to get ready for the upcoming year, and my 840s were kind of a mix between the old 840s and the new 291s. Some cars ended up with 291 heads on them, and it's probably safe to say some got "funky" 840s like the ones I have.

Some wierd stuff took place back in the day... [img]/ubbthreads/images/graemlins/smirk.gif[/img]

Re: Why the overbore from 396 to 402
 

some late 69 Chevelles have the 402

Re: Why the overbore from 396 to 402
 

[ QUOTE ]
I think the "Malibu 400" was a model name

[/ QUOTE ]
I don't know about that Joe. The one I saw, I'm thinking it was a 71 but it may have been a 70. It was a real light green color, like the sea-mist green that almost looks silverish. It had the malibu interior, malibu dash, I think it had a black vinyl top, but it was factory ordered with a 402bb. Standard orange valve covers.. pretty sure it was an automatic car. 400 decal on air cleaner and the body emblems had Malibu on the fender sides behind the front tire and the slanted 400 emblem on the fenders in front of the wheel opening. There was no doubt about the car, it had a build sheet and everything. Now that I think about it I'm thinking it was a 70.

Re: Why the overbore from 396 to 402
 

[ QUOTE ]
I've seen a few Chevelles that way too. Non-SS cars. I've seen a few 71 402 Heavy Chevy Chevelles with 400 air cleaner decals on the 402.

[/ QUOTE ]

Found an example.. This ones a 72
http://www.dealsonwheels.com/search/...-200308-000006

Re: Why the overbore from 396 to 402
 

JoeC got it right. It was emission rules related.

The GM literature (parts books, manuals, etc) got real confusing, calling the BB a 400 alot of the time.

I assume the new block handled the .030 overbore.
Interestingly, the late 69 blocks did not use the 70 3 digit code format, but the late 69 axles did.

Re: Why the overbore from 396 to 402
 

The three letter suffix began on the new 402 CID blocks.
Their cylinders were overbored by .032.
Example:
Code JH is 396.
Code CJH is 402.(or 400 as Kurt mentioned)
I have seen late built 1969 Chevelles with the three letter suffix block.

Kurt S
You say no 1969 Camaros have been found with the three letter suffix,even thought the 1969 Camaro model was carried over late into the 1970 model run.
I would have thought those later built Camaros recieved a three letter suffix,as the Chevelles did.
Anyone know why they didn't?

Re: Why the overbore from 396 to 402
 

Another example...
72 Malibu 400 [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

72 Malibu 400

Re: Why the overbore from 396 to 402
 

[ QUOTE ]
I would have thought those later built Camaros recieved a three letter suffix,as the Chevelles did.
Anyone know why they didn't?

[/ QUOTE ]

I'd guess cause it was a model year extension and they chose not to change the codes midyear (though it's not really changing them, just adding a digit...).
I've heard of 2 Camaro blocks with the 3 letter code, never seen them.