What is the Difference between ASTM A194 Grades 2H and 2HM Nuts?

A194 2H nuts are a common high strength nut used with many different bolt grades, including A193 Grade B7, A325, A490, F1554 Grade 105 and others. Occasionally Portland Bolt will be asked to provide A194 Grade 2HM nuts, most often with A193 Grade B7M. In many grades an ‘M’ refers to metric, which is not the case here. Below is a list of similarities and differences between the two grades.


The main difference between 2H and 2HM nuts is in the testing. 2H nuts only require a hardness test on a small sample:

Lot Size Samples
Up to 800 1
801 to 8000 2
8001 to 22,000 3
Over 22,000 5

2HM nuts require 100% of nuts to be hardness tested. The tests for 2HM nuts must also take into consideration 3 additional things during testing:

  1. Protect legibility of markings
  2. Minimize exterior dimensional changes
  3. Maintain thread fit

Chemical Composition

Both 2H and 2HM nuts have the same chemical composition.

Hardness Requirements

Grade Completed Nuts Sample Nut After Heat Treat
Brinell Hardness Rockwell Hardness, C Scale Rockwell Hardness, B Scale Brinell Hardness, min Rockwell B Scale, Min
2H through 1½ ” 248 to 327 24 to 35 179 89
2H over 1½ ” 212 to 327 35 max 95 min 147 79
2HM 159 to 235 84 to 99 159 84

Proof Load

Grade 2H nuts have a higher proof load requirement. While not showing all of the sizes, this is a representative sample.

Nominal Size, in. Proof Load, lbf
Stress Area in.2 Grade 2HM, Heavy Hex Grade 2H, Heavy Hex
½ .1419 21,800 24, 830
5/8 .226 33,900 39, 550
¾ .334 50,100 58, 450
7/8 .462 69,300 80, 850
1 .606 90,900 106,000


Grade and Type Nuts Hot-Forged or Cold-Punched Nuts Machined from Bar Stock
2H 2H 2HB

An underline as a marking requirement has been removed from 2HM but is permitted.

If you have any questions about ASTM A194 Grades 2H or 2HM nuts, give one of our experienced team members a call, and we will be happy to help you out.


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