Official Difference btween the 396 and 402 BB Because you asked.

[70cst]

The official difference between the 396 and 402 BB according to the Feb, 1970 Chevrolet Dealers Memo

The top power option for all Chevrolets Series 1030 trucks (except the Blazer) is a big 400 V8 with 310 horsepower. It has the same power output as the 396 V8 it replaces, but it has a number of significant differences, such as:
1. Gasketless, tapered-seat spark plugs and conforming heads.
2. Throwaway-type oil filters.
3. Two-step automatic choke for better engine warm up
characteristics.

The increase in displacement was accomplished by enlarging the cylinder bore rather than lengthening the stroke. This, plus the use of a low profile inlet manifolds, keeps overall height low for more room under the hood. And remember, with this engine, you get a lot more horses under the hood than competition can offer. Here’s the way the top engine of each major make compares:
Biggest V8 & Max. Gross H.P. CHEV. 400/310 FORD 390/255 DODGE 383/258 IHC 392/236

Now you know the rest of the real story...

[Monte Carlo Man]

Actually it was marketed by year and model and not just by the numbers.

If you read in the Motor Repair Manual for 1970 - it will say 396, marketed as a 402. Not 400 - because the 400 technically was a small block Chevrolet which came out in 1970 which was listed with 4 bolt mains and had 25 more horsepower then the LT1 350

Most truck engines smaller then 1 ton had High Performance Passenger Car cast into the blocks and heads. My buddy had one in a 1974 Chevy wrecker they bought when he was a kid.

It came with 454 rocker covers and when we went to rebuild the engine, the pistons would not fit in the bore.

When you get into Square Port vs. Oval port, you will have a heated discussion as to which was better. The last set of Oval Port heads I had, I had traded for a set of double hump SBC heads and then sold for $500.00

I probably sold them too cheap, because they were already built up, but I needed the money more then I needed another set of heads on the shelf!

[JimKshortstep4x4]

70cst,

Thanks for posting that bit of interesting information!

Jim

[70cst]

Originally Posted by Monte Carlo Man

Actually it was marketed by year and model and not just by the numbers.

If you read in the Motor Repair Manual for 1970 - it will say 396, marketed as a 402. Not 400 - because the 400 technically was a small block Chevrolet which came out in 1970 which was listed with 4 bolt mains and had 25 more horsepower then the LT1 350

Most truck engines smaller then 1 ton had High Performance Passenger Car cast into the blocks and heads. My buddy had one in a 1974 Chevy wrecker they bought when he was a kid.

It came with 454 rocker covers and when we went to rebuild the engine, the pistons would not fit in the bore.

When you get into Square Port vs. Oval port, you will have a heated discussion as to which was better. The last set of Oval Port heads I had, I had traded for a set of double hump SBC heads and then sold for $500.00

I probably sold them too cheap, because they were already built up, but I needed the money more then I needed another set of heads on the shelf!

I have read this over and over and I am still not sure how this reply fits the topic at hand?

[70cst]

ttt for those who missed this.

[leddzepp]

Does it say why they went to a 402 instead of a 396? Not a lot of people know this...

[70cst]

Originally Posted by leddzepp

Does it say why they went to a 402 instead of a 396? Not a lot of people know this...

Nothing was written in the Dealership Sales Brochure. It would be great if someone had an official document/paper that would spell that out. Good point.

[leddzepp]

As told to me by someone who worked for GM (you won't find it in any book or brochure): The 396 blocks had a casting imperfection in the cylinders, so rather than scrap all of them, GM "fixed" the problem by punching them .030 over making it a 402.

[OldSkool71]

good to know info...hope the 396 I just bought is punched out enough...guy told me it sounded really healthy...keep your fingers crossed...

[JQ-72]

I've heard the one about a casting defect and even a fire in an engine plant. The one that makes the most sense to me is that in 1970, Chevy introduced the 400 small block. It would not make much sense for Chevrolet to have a 396 "big" block and a 400 "small" block!

Joshua

[jbristo67]

Yeah except that a 72 402 had 185 horse due to lower compression

[Pickupmanx2]

So when I go to NAPA, and they are looking up parts,,, it is either a 396 or a 402, but the side of the truck says 400!! And the SPI says L47 396 cu.in. V-8 engine
http://67-72chevytrucks.com/vboard/s...d.php?t=389526
So I go with 396,,,,

[D-Rat]

The old IHC 392 might only show as 236hp, but I bet it has the lowest torque curve of all the engines there.

[monte0185]

Originally Posted by leddzepp

As told to me by someone who worked for GM (you won't find it in any book or brochure): The 396 blocks had a casting imperfection in the cylinders, so rather than scrap all of them, GM "fixed" the problem by punching them .030 over making it a 402.

Close but not quite right. The reason they were over bored making the 402 was because the cylinders were cast crooked. To straighten them they had to re bore them straight making them 402 instead of 396. This came from my best friends grandfather who was one of the people fixing them at the time it happened.

[leddzepp]

Originally Posted by monte0185

Close but not quite right. The reason they were over bored making the 402 was because the cylinders were cast crooked. To straighten them they had to re bore them straight making them 402 instead of 396. This came from my best friends grandfather who was one of the people fixing them at the time it happened.

Huh? That is essentially what I said... I would call "crooked" an imperfection wouldn't you?

[junkyardjohn]

Well... I'm glad that is cleared up. john

[monte0185]

Originally Posted by leddzepp

Huh? That is essentially what I said... I would call "crooked" an imperfection wouldn't you?

There has been a story I have heard about the plant flooded causing rust issues and them needing to be bored out cause the pits (imperfections) in the cylinders. Thought thats what you were talking about. Sorry if i misforstude ya. Not a big deal now that its 40 years later anyway.

[leddzepp]

Originally Posted by monte0185

There has been a story I have heard about the plant flooded causing rust issues and them needing to be bored out cause the pits (imperfections) in the cylinders. Thought thats what you were talking about. Sorry if i misforstude ya. Not a big deal now that its 40 years later anyway.

Wow, that's a new one on me! I have not heard that before. I was referring to them being crooked as you stated. No biggie, it was 40 yrs ago. I learned something new today!

[70cst]

Good info guys...

[str82666]

Originally Posted by Pickupmanx2

So when I go to NAPA, and they are looking up parts,,, it is either a 396 or a 402, but the side of the truck says 400!! And the SPI says L47 396 cu.in. V-8 engine
http://67-72chevytrucks.com/vboard/s...d.php?t=389526
So I go with 396,,,,

So can anyone answer why the side of the truck says 400? And whether it would actually be a 396 or a 402? I am very interested in this answer.

[LONGHAIR]

I have heard the flood thing before too, but I'm not so sure that I buy either of the "damaged" block theories.
If the bores were rusty, the rest of the block would have been too, lifter bores, main saddles, etc...
If the bores were "crooked" they would have gone back to the supplier....

They couldn't possibly have been "sitting on" that many blocks. If that really had been the case, they would have either used them and just said nothing, sent them back to be "recycled", held them as warranty replacements, etc. They would not have spent the time/money to re-bore them and create new pistons to make it happen. Plus, they would have gone back the "standard" 396 configuration when the "supply" of these "bad blocks" was exhausted.

I think the 400 in small block form was the issue. The smallest "big block" needed to be bigger. The 400 came out in the same year, coincidence?

Now why they used the 400 name badge for trucks? and 402 in cars? Who knows?

[GCncsuHD]

Originally Posted by str82666

So can anyone answer why the side of the truck says 400? And whether it would actually be a 396 or a 402? I am very interested in this answer.

Originally Posted by LONGHAIR

Now why they used the 400 name badge for trucks? and 402 in cars? Who knows?

My 70 Chevelle 402 BB had "400" on the side as well. I assumed either it was close enough, or it was for the TH400 backing the 402.

[str82666]

So is the motor in the truck a 396 or 402?

[too much stuff]

Originally Posted by str82666

So is the motor in the truck a 396 or 402?

What year truck?

[70cst]

Originally Posted by str82666

So is the motor in the truck a 396 or 402?

If I have my facts correct and that's a big IF...it seems that 1970 was the year the the 396 and 402 terms were used interchangeably. My truck SPID say 396 but it is a 402 with 400 emblems ...that may have carried over into 71 and 72. It's sort of like the CST package. The least year for that package was 70 but in 71 some trucks read CST verses Cheyenne...go figure.

LONGHAIR....you make some valid points.