How can the 3855961 casing number be the same for a 396 nd a 427 ci motor. There is no way a 396 can be bored out to a 427. Confused.
I can not find anyone who can explain why GM used the same casting number for two different cubic inch blocks.
Help me solve this puzzle.
Thanks George

 

Well when Zora Duntov was asked how come a 425 HP 427 Corvette was so much quicker than a 425 HP 396 Corvette, he is quoted as saying: "Well we bored it out 30cubic inches. Just check how much 30 cubic inches of cast iron weighs".

 

Yes, there are in fact some 396 castings that can go all the way to 427 bore.

 

Not true, the early engines can handle it. Later engines would probably hit the water jacket at that level of overbore.

 

The 1965 396 "962" block (all 4 bolt mains) is a THICK wall casting, in most cases, unless there was significant core shift during the casting process, they can be bored out the additional 1/8" to 4.250" (427 spec).

The 1965-66 396 "961" block (2 bolt & 4 bolt mains) is a THIN wall casting. Regardless of core shift, boring one out to 4.250" (427) will result in either breaking through a water jacket, or dangerously thin cylinder walls that will cause the engine to run hot and be impossible to keep cool without boiling over on the street. Of course you'll always hear somone claiming how they bored a 396 "961" block out to 4.250 inches "back in the day" and raced it, but I doubt they got further than a 1/4 mile before it was puking antifreeze.

..and to answer the next question that usually follows, NO - there were no documented 1966 427 Corvettes ever produced with the 961 block ...even if GM did slip a few onto the assembly line early in Aug/Sept 1965, they would've rolled off the showrooms with 396 cid.

 

It's not the first time GM used the same block for two different engine displacements. I have first-hand knowledge of two others and there may be more:

1. In 1957, some passenger cars came with 265 engines. The 265 block has the same casting number as the 283 block and differs only in the bore.

2. In 1961, 409s were made from 348 blocks. The 348 casting number was ground off and a different "casting number" was stamped in its place.

Jim

 

The engine that was in my 62 when I first got it, was a 57-issue 265. Sold it to a drag racer 'cause he thought it had a beefier cylinder wall for better boring.

 

Well, there was at least enough cylinder wall thickness to bore it .125" to be a 283.

Jim

 

Jim,
As to the '61 409, most 348 blocks will not survive the 3/16" overbore. The 011 348 core casting was modified to provide thicker cylinder walls. There were also some changes at the main webs due to the new crank. The '61 409 was indeed a new casting rather than just a bored and stroked 348.
As you say, the new casting number was obviously overlooked on some early blocks, but corrected later.

 

Thanks for the clarification. I was mistaken about the 409 block I saw and I appreciate knowing the real story.

It's been 25 years so my memory is hazy. What I think I recall is larger numbers on the "casting number" stamping than those shown in your photo (3/8" or maybe 1/2" tall) and a slightly different font, one with (for lack of a better description) more "style" to it. I do remember the casting date was sometime in October of '60.

And apologies for hijacking this thread which was really about 396/427 engines.

Jim

 

...and here's some useful info on overboring early SBC's for anyone interested;

http://books.google.com/books?id=g46...3OLojcd705wfcg

 

One of my racing friends had one of these five 4-bolt main cap 396's out of a 65 Chevelle that we ran in a 68' Camaro. When a rod scored a cylinder wall in the block we looked to add a thin wall sleeve. Our machinist ran the block through a sonic test and came back to tell us it could be bored an 1/8th over, to a 427 bore, to clean up the cylinder. The set of 427 pistons cost a little bit, but worth the expense. One of our rivals who ran a SS legal 375hp/396 67' Camaro heard a rumor of what we did and claimed it could not be done safely, so we had our pin-striper paint "396 ? - 427 ?" on the sides of the hood scoop (just to keep him guessing). The guessing lasted about two drag race meets, before the track times improved to answer the question.

We ran this over-bored block for a year of racing with L72 pistons, with no failures. We then tore it down and swapped in L88 pistons (12.5:1 CR), and at the second or third weekend outing at the strip with the much faster configuration we blew a hole in the #5 or #7 cylinder wall. We added a sleeve and continued to run the block for the balance of the race season, and the entire next year.

This photo is from a month or so before we scored the 396 block (we suffered a drunk hitting the front end of the car the weekend before this photo, and we were forced to run with no hood).

 

Sorry for the dumb question, but is a 396 considered to be a true BB? I always thought that the 427 was the BB and a 396 was a short block.

 

396, 402, 427, 454 are all big blocks

265, 283, 302, 307, 327, 350, 400 are all small blocks

any motor with crank, pistons and no heads is a short block

 

AHHH, 400 came either way. There was the 400 small block and the 400 big block (a .030 overbore 396, actually 402 ci).

Rich

 

Forget the old greasy cars. Who is the chick with the pink top?

 

You too? Thats what I saw,

the hell with the Camaro

 

LOL!!! Me too.

 

nice rack !!!

 

NASCAR allowed the Chevy to run as a 427 in 1965 to compete with the 427 Ford and 426 Hemi.

I have not seen it in writing, but would not be surprised if this is at least one reason why the early 396 block would take the 427 bore size.

They all wanted to win in NASCAR including Chevy even though Chevy was not supposed be involved in racing.

It's funny how the 1965 396 could be run as a 427 as only 2 years before management stopped the 1963 427 NASCAR engine program.

by the way, very cool Camaro picture