I find this interesting. As you can see in the picture, my bolt head has five lines with a circle and an “M” in the middle. I’ve looked for the last half hour on the internet and can’t find any place to identify the grade of this bolt . I’ve even consulted my bible, Machinery Handbook to no avail.

Anybody care to take a shot?
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Phil
Moderator, The Engine Shop, Interiors and Project Journals

1952 Chevrolet 3100, Three on the Tree, 4:11 torque tube
Updated to: ‘59 235 w/hydraulic lifters, 12v w/alternator, HEI, PCV and Power front Disc Brakes
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