Hardware class
 

Looking at buying bags of nuts from McMaster and I'm a bit confused by there classification of the grade hardness of what we should be using for automotive use. Basically looking for nuts to use with 8.8 grade bolts and studs. I asked McMaster this and was referred to there web site for info. They are selling what is classed as grade 10 Zinc-Yellow plated steel part no. 92497A400 and grade 8 Zinc-Plated Steel part no. 90591A161 as follows.

Metric High Strength (Class 10 Alloy Steel)—Typically used with Grade 10.9 screws. Equivalent to Grade 8 steel. Minimum Rockwell hardness is C25 (unless noted).

Metric Medium Strength (Class 8 Steel)—Used with Grade 8.8 screws. Equivalent to Grade 5 steel. Minimum Rockwell hardness is B80 (unless noted).

It looks like they are telling me to use the medium strength class 8 for the 8.8 bolts but what do they mean by "equivalent to grade 5"? Would using the high strength class 10 be to hard and damage the 8.8 bolt and studs?

Had to look at their site to figure it out...

They are trying to describe the two different bolt strength classifications in relation to each other. There are strength classifications for SAE and Metric hardware and they are different. I shall attempt to translate:

Metric high strength bolts (Class 10 bolts) are the same as SAE Grade 8 bolts.

Metric Medium strength bolts (class 8) are the same as SAE Grade 5 bolts.

My engineering books agree that this is the equivalents between the two.

The metric designations can be deciphered pretty easily:

For 10.9 for instance, the "10" corresponds to an ultimate tensile strength of 1000 MPa and the ".9" means that the yield strength is 90% of the ultimate (or 900 MPa). So, a 10.9 means 155ksi ultimate with a yield of 140 ksi. You can divide MPa by .00645 to get psi. Grade 8 is 150 ksi/130 ksi. 8.8 is 800 MPa ultimate and 640 MPa yield or 124 ksi/100 ksi, etc. Metric 12.9s are the good stuff... :)

If I understand this right, the standard automotive grade for SAE would be class 5? which is the same as our 8.8 metric.

Yes. Certain fasteners are of higher grade (crankshaft puller bolt, flywheel bolts, etc on a Chevy for instance are Grade 8), but if not marked on an American car with inch fasteners it is grade 5.

I'm not sure what you mean by "not marked"? Grade 5 fasteners are marked. The only SAE fasteners not marked are the really crappy Grade 2 and weaker stuff, but I wouldn't expect to find those on a car in any sort of structural application.