Understanding bolt specifications...

[Steve Plucker]

I understand just how to measure a bolt for it's length...you measure the "shaft" length only for length.

But just what are they talking about when a bolt specification is listed as:

3/8---24 NF 2 x 2-1/8 hex head bolt (with cotter hole)?

OK: 3/8 is the diameter of the shaft; it has 24 threads to the inch and is considered National Fine (NF); is a hex head bolt; and has a cotter hole on the bottom of the shaft.

BUT what is meant by "2 x 2-1/8"? Is it 2 inches long or 2-1/8 inches long or the shaft is 2-1/8 inches long with the cotter hole at the 2 inch mark?

Hmmmmmm.

Thanks.

Pluck

[Clydes 31 P/U]

Steve, What context is that used in? I write my shopping lists like that when I need 2 of 2-1/8 bolts could it be 2 is the quantity? just a thought

[Steve Plucker]

Originally Posted by Clydes 31 P/U

Steve, What context is that used in? I write my shopping lists like that when I need 2 of 2-1/8 bolts could it be 2 is the quantity? just a thought

No, that is just how it is listed in the Ford Parts Price List of Standard Parts in describing A-20968 bolt used to bolt down the radiator...two (2) are used for this purpose and I do not think the "2" in the designation is the amount used.

Pluck

[criswellnk]

The "2 x 2 1/8" dimensions that are given would be for the overall bolt length, at 2.125" and the first dimension of 2" would be from the center of the cotter-pin hole to the extreme point of the bolt/screw thread. Not the actual end of the blot/screw. At least that is the explanation from the S.A.E. Handbook pictured below.

[Roland/Pittsburgh]

Steve

I'm on vacation and don't have my handbook with me so I cant quote tolerances but the "2" is the class number. Class 1 is a looser fit in the nut (or tapped hole) Class 2 is slightly tighter and class 3 is tighter still. Class 2 is most frequently used and the fit we are all familiar with. This number does not have to do with the cotter pin hole.

Roland

[Marco Tahtaras]

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Roland is correct except that most of what you find today is class 1 at best. Basically what they are saying is "3/8-24x 2-1/8 hex head bolt (with cotter hole)". They are just including more specifics. As Roland stated class 1 is loose fitting and it goes to tighter tolerances as the numbers climb. That applies to both male and female threads so both are usually combined to control the limits. Today class three seems to be the tightest tolerance that I see special tooling for but Ford commonly went to class 4 on areas of the model A.

Go to your favorite hardware store and pick up a matching bolt and nut such as 3/8"-24 and screw the nut on half way up the bolt threads. See just how much the nut can be wobbled on the bolt. This hopefully will be a fair example of class 1. Class 3 would have little if any discernible wobble but in any case the contrast between the two would be substantial.

[Steve Plucker]

Thank you for this clarification...hopefully it helped others.

Pluck

[Peter J]

Good info. That explains why a repro. nut will start jumping threads but swapping it out with an old OE will torque down.
Thanks,
Pete

[JoeWay]

Here's some info on thread limits from my favorite supplier.
Thread Limits

As Marco and others have frequently noted, tap & die sets tend to have big taps and small dies resulting in loose fits. I buy H3 taps from MSC such as this one which would be used to chase head and manifold stud holes
7/16"-14 bottoming tap

I chase threads every day using these taps, and find that they do not cut metal. Often there is still discoloration from rust/dirt left in the threads after chasing. At most they shine up the threads without making chips as the taps from the sets would.

Joe