High Strength Bolts - Headmarks

[Joe C.]

Here is what I found when I removed all 10 main bearing cap bolts from my L76 engine.

3 bolts: "N" inside single circle.
2 bolts: "TR" inside single circle
3 bolts: "TR" within 4 radial lines set at 0, 90, 180, 270 degrees
3 bolts: "M" with 4 radial lines set at 0, 90, 180, 270 degrees.
1 bolt: "TR" with 4 radial lines set at 0, 90, 180, 270 degrees.

I have bought 5 new bolts from GM, which are marked with "O" with 4 radial lines set at 0,90, 180, 270 degrees.
I have bought 5 new "stud/bolt fasteners", which are marked as pictured. These are to install a missing windage tray.

I have looked at numerous SAE, and ASTM publications, but cannot find any correlation to neither a single circle nor a "4 lines" headmark. I assume that these fasteners are at least Grade 8. Does anyone have any further info.

I understand that common grade 5 bolts have 3 radial lines @ 0, 120, 240*
Grade 8 have marks @ 0, 60, 120, 180, 240, 300*.

Thanks in advance.

Joe

[Joe C.]

SUFFIX

Make that 1 bolt: "M" ..........

[Duke W.]

Re: High Strength Bolts - Headmarks

SAE grades are number of radial lines plus two, so they are grade 6. Grades 3, 5, and 8 are the common commmercially available replacements.

Not sure what the circle means, but given the size of these bolts and the torque spec, grade six is perfectly adequate.

Duke