Have a 350 block, 3970010. With 0.10 S stamped on it. What is the S for

The "010" embossing on your small block chevy indicates that the block is 10 percent nickel/iron alloy. These "010" blocks can be of either "2 bolt or 4 bolt" crankshaft main cap variety. The 020 blocks are rare, but preferable to 010 blocks as they are 20 percent nickel to iron and are stronger. The higher nickel content makes for a slight add in weight, but a much stronger block. The "S" I'm not sure of yet.

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