I'm a metallurgist and was superintendent of the melt department at the Tonawanda metal casting plant. Jon's statement was true. We ran all 4 of our cupolas with the same chemistry base iron. We were producing about 2,500 tons of iron a day - feeding 7 molding lines, so as a practical matter, it would have been impossible to route different base irons to different lines in production. Alloying with ferro-chromium and silicon was performed as the iron was poured into the hot metal crane ladles (these cranes traveled on monorails that transported the molten iron to the individual molding lines). All lines received silicon (a late alloying agent designed to refine the grain structure of the cast iron by providing more grain nucleation sites and reducing carbide formation). Ferro-chromium was only alloyed into the lines that produced blocks (mold line 1 & 2 at Tonawanda). It was added to promote pearlite formation which increased the tensile strength of the cast iron. You may ask - then why wasn't ferro-chromium added to metal being delivered to all of the lines? the reason is that chromium can cause carbides to form in thin sections of casting. The cutting tools at the motor plants would not like this, so chromium was only added to blocks where increased tensile strength was required (class 30 gray iron). The chrome content in class 30 iron was targeted at .032 %.

Just food for thought - nickel is also a strong pearlite stabilizer and would increase the tensile strength of cast iron (similar to chrome, but without the tendency toward carbide formation). Ferro-chromium (not metallic chromium) is a relatively cheap commodity, and, therefore, a good choice for alloying in a high production environment. In smaller job shops - nickel alloying could be used to increase tensile strength in cast iron. In these low production environments - the shop may not want to risk the carbide formation tendency of ferro-chromium. So - this is just conjecture - over the years people familiar with job shop practices may have assumed that nickel was being used in high volume environments.

Very informative post. Thank you for clarifying the process Bruce.

That is an urban myth. The last 3 numbers of the casting were typically cast in both front (area under timing cover) and rear (area covered by the bellhousing) of block. 3970010 have the 010 and 020 because they are last three numbers of blocks in that casting timeframe. The 3970010 and 3970020 blocks.

You will also see this on small blocks made throughout the 60's.
1967 3892657 blocks have 657 and 944. (3892657 and 3896944)
Some 1965 3782870 blocks have 870 and 721. The 3790721 was the predecessor.
So it was a foundry thing to do this.


Its just a coincidence that the 010 and 020 blocks ended in those numbers.

I agree - front and rear housing cores were common among several part numbers. Every time a casting was modified, a new part number was supposed to be assigned, but if the change didn't involve the front or rear casting face - the same old housing core boxes would be used. These front and rear housing cores were made on equipment that was the oldest at Tonawanda. The binder used was actually a derivative of linseed oil. The dimensional tolerance capability of these "oil sand" cores was not great, so it was gradually being phased out in favor of phenolic urethane binders. the "oil sand" front and rear housing core boxes were very old & were used for multiple part number blocks over the years.

If anyone knew, it was the late great John Z. I asked him numerous questions about Chevy SB Heads and he always answered my emails.

Most 307 castings did not get hot rodded because of the small bore size, and they are all two bolt main. Although four bolt mains aren't essential, they are so prevalent and cheap, most folks would prefer to start with a 4 bolt block.

"One interesting fact regarding the Chevy 307 V8 was that it was also manufactured by GM for boat use and sold by the Outboard Marine Corporation as a high performance marine engine. The marine version of this engine produced between 235 and 245 horsepower and even used the same aluminum valve covers that were used in Corvettes and Camaro Z-28s." Quoted from this article: https://www.enginefacts.com/chevrolet307/

Don't know about "kids", but many others have use 307s as a starting point with good results.

https://www.onallcylinders.com/2021/...307-big-power/

https://www.motortrend.com/how-to/su...e-performance/

https://www.onallcylinders.com/2020/...blowing-it-up/

Ok for the sake of discussion. Bergy:


Why were the later 010’s all the same after the mid 1970’s?

2 bolt mains and 4 bolt mains?

And I just called a local guy who has overhauled engines fir a living for decades

The metal is definitely softer on the later blocks.

Explain please.

And as an additional bonus isn’t it true that a flint block could be built as a lower horse motor but not the other way around?

How many LT-1’s 302’s were cast in NY??

Phil - I don’t want to argue. You always draw your own conclusion anyway :-)