Greetings all;

please correct me if I'm wrong, but didn't the 396 get punched out .030 over for '70 to the 402 displacement? The 396 was no longer available for the '70 model year. Or did some slip through?
Any and all comments welcomed.

The change from 396 to 402 was made during the end of the 1969 model year. The 396s have two letter suffixs and the 402s have three letter suffixs such as your CTW. Most, or all, of the 396s should have been used up by the time the 1970 model year started.

thanks Bruce;

pretty much what I thought. Mention that to some and they go "really", like they just didn't know. Gotta know!
thanks again.

Hey mfinger 1: In late 69 cheverolet accidently overbored the 396 by .030. Well the 396 had such a great reputation they did not want to risk the name change and loose customers so chevrolet choose to stick with the 396 badges until they were all used up. Then in 71 they advertised the 402. This information is availble through Chevelle resto catalogs.

"Accidentally"....come on! Seemed to do much better on ring wear and oil consumption that the original 396. Comments?

It had more to do with GM lifting the "400ci rule" than anything else.

It had more to do with GM lifting the "400ci rule" than anything else.

That is how I understand it, also. The Mark IV was orginally designed to be a larger engine, but the 400 c.i. maximum for the A body forced them to use a 4.096 bore. When the limit was changed that allowed the 454 to be used then the 396 was modified to a more standard 4.125 bore. I think this reduced the cost of the engine.

Many people believe the reason for the .030 larger bore to 402 had to do with meeting emissions standards. The thought is that the standards were higher (numerically) for engines over 400 cu in in 1970.

Montes had the same engine but it was called a Turbo-Jet 400. It is confusing in that case since the Monte had a 400 small block and a 400 big block offered in 1970.

Here's what I heard from a former co-worker, who has built and raced chevy's for a long time (and who was old enough in '69-70 to remember)..

I heard there was a strike at GM in this timeframe, and the 396 blocks were left sitting for some period of time. They rusted. So instead of scrapping them, they went .030 over and the rest is history..

T or F???

I'd have to say that's a neat story but false. I'm sure those blocks would be kept lubed or protected in some manner. Who knows? By the way, can someone answer that? How are machined steel parts kept from rusting? Lubed, bagged ( that might cause condensation) ? ? ? Inquiring minds want to know!

BTW, I do recall in the service, new parts would be coated in some form and wrapped in paper. That was 28yrs ago though.
If blocks were coated in some fashion to prevent rusting, wouldn't this coating need removal before assembly?

OK, how about this:

Both the big block and the 400 Small block were built in Tonawanda, NY.

The small block 400 was introduced for the '70 model year, same as the BB 402.

No one at Tonawanda wanted to be bothered to reset the boring bar when the line changed from 400 sb to 396 bb, so they just punched the big block to the same bore size as the small block.

I'm not saying this is fact, I'm just pointing out that general lazyness seems to me to be as good an explanation as any other I've heard.

It's my understanding that the MK IV was originally designed to be a 427, but NASCAR was considering a displacement limit of 400 cid or thereabouts. The lower limit wasn't implemented, but Chevy had to develop a 396 version of the big block "just in case".

plausible

mfinger 1 and jtm 60: The GM strike in 1969 was a fact. The overbore was a fact as well. According to "The history of Chevelle Book" Also fact was that GM did not want to distort the good name of the 396. Now whether a GM bored these blocks out on purpose or not, well I can only relate to what I have read. Also about the blocks being rusted during the strike. Probly true. You can spray the block with what we machinist call a rust inhibitore and it will stay rust free for years. It is a thin coating of protection applied with an aresol can. Now think about this, if your company made you so mad that you were willing to strike, would you protect the parts you were working on just before you went on strike. I think not. So perhaps the .030 bore out was to save the blocks from the scrape pile. I always learn a little bit every time I pick up a book. I am in to history.

I thought the big strike was in '68, (hence the chevelles with buick interiors) was there another one in '69??? :confused:

Originally posted by gab712:
mfinger 1 and jtm 60: The GM strike in 1969 was a fact. The overbore was a fact as well. According to "The history of Chevelle Book" Also fact was that GM did not want to distort the good name of the 396. Now whether a GM bored these blocks out on purpose or not, well I can only relate to what I have read. Also about the blocks being rusted during the strike. Probly true. You can spray the block with what we machinist call a rust inhibitore and it will stay rust free for years. It is a thin coating of protection applied with an aresol can. Now think about this, if your company made you so mad that you were willing to strike, would you protect the parts you were working on just before you went on strike. I think not. So perhaps the .030 bore out was to save the blocks from the scrape pile. I always learn a little bit every time I pick up a book. I am in to history. I question the rusty block theory. Why would only the 396 blocks not be protected and rust? Why not 307's, 350's, 6 bangers, etc.

Have to agree with von here. Why were the 396 blocks the only ones to rust? Since Chevrolet already had the 454 to compete with the over 400 cid rule, why bother to bore the 396 just .030? Why not use the 427 engine? I'd be more inclined to believe it had to do with emissions since GM didn't want to mess with the SS396 mistique they'd cultivated for the previous 4 years.

I can't see how the use of Buick and Olds interiors could be the result of a GM strike. Chevelles were still being built, just not with Chevelle interiors; maybe a strike at the suppliers GM was getting 68 Chevelle interiors from (?) There was a major strike in mid Sept of 1970 for two months causing a delay in '71 models.

Did some 70 chevelles built in late 69 have actual 396 ci engines in them with 3 letter codes?

Originally posted by cortez:
Did some 70 chevelles built in late 69 have actual 396 ci engines in them with 3 letter codes? No, all the late 69 3 letter code engines were 402 engines badged as 396s

I like the boring bar theory. That sounds more resonable to me.

Maybe valve clearance? .015" isn't much, but could make a difference. Steve

How about it didn't make sense to have a BIG block 396 and a SMALL block 400?

OK... Here's what I heard. GM had a quality problem with the blocks. Seems there was some core shift when they were cast. Rather than bore blocks over size hit and miss to clean up the bores, they just flat out bored them ALL .030 over. Sounds possible but who knows? smile.gif
Steve

Originally posted by jnorth:
How about it didn't make sense to have a BIG block 396 and a SMALL block 400? Along the lines of my thoughts... with the introduction of the SB 400, making the smallest big block a 402 helps keep a clear division between SB and BB with no gray area, but with the 396 still being marketed, it didn't seem to be that big of a deal. It doesn't make much sense on one hand to say that you can't have a sb with larger displacement than a bb, and at the same time market the said bb as a 396.

Gene, I agree. I also think a company like GM that thinks nothing of crushing dozens of pre-production versions of one vehicle would have no problem at all scrapping any number of engine blocks because they had some rust or were incorrectly bored. Think of the cost of the different boring bits, setup, and all of the expense of changing the printed specs, service manuals, pistons, rings, etc., etc. I just don't buy the "save the damaged blocks by overboring" theory. There wouldn't be time to stop production and make all these changes anyway if they were a last minute deal. Anyone heard of a '70 block casting date months ahead of the vehicle build date?

i worked in a foundry 3 yrs i don't buy the core shift theory.that would have been only in a very few. the rust i doult that either when castings came back from heat treating they were rusted like crazy looked like they sat out in rain for a year.30 would be a lot to get a little surface rust off.i'm sure they didn't store them out side after being machined the bore size is always bigger than the original casting size of cylinder.
i don't know why they did it.
would that make more engine parts compatable that were made for the new 454?

My reason for doubting the rust theory is exactly that same rust.If the bores rusted so would everything else.Bolt holes,lifter bores,mains,etc.
On the assembly line they don't have the time to go through all the steps involved in removing that kind of rust.When we prep a block thats been "seasoned" we do it cause these things are getting hard to find.
My guess is the production cost theory.Probably had something to do with the introduction of the 454 as the 396 didn't become a 402 until after the 454 came out.Whew.