even the 1963-66 421 HO rods can be heat treated to 32 Rockwell to improve their strength, durability. the reason there was problems with those rods is, they were 1137 steel and not heat treated, and put in 421HO engines with the longer 4" stroke, stressing the rods more. the 1958-62 rods never saw a stroke longer than the 3.75" of the 389. a rod that will last forever in a short stroke engine, may break in a week in a long stroke engine, especially if it's NOT HEAT TREATED AT ALL- like the 1958-62 and 1963-66 HO rods.
Carbon content: As a resulfurized carbon steel, 1137 has a carbon content of 0.32–0.39%. Steels with higher carbon content are more responsive to heat treatment for hardening.
Manganese content: With a manganese content of 1.35–1.65%, the hardenability of 1137 steel is improved, meaning it can be hardened more effectively than plain carbon steels with similar carbon content.
Free-machining properties: The added sulfur in 1137 steel improves its machinability but makes it unsuitable for welding and somewhat reduces its toughness compared to equivalent carbon steels.
Tempering: Achieving exactly 32 HRC will depend on the specific tempering temperature used after the initial quench. Higher tempering temperatures will result in a lower hardness, while lower temperatures will result in higher hardness
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