Identifying bolt sizes

[dlminehart]

I've measured my existing screw topped casters in order to figure out how to get replacements, and have a question. While the pitch is pretty obvious (mine is 16 per inch), does the listed diameter of a screw refer to the outer diameter of the threads, the diameter from inside of threads, or something else? My $15 HF digital caliper says that the outer diameter of mine is approx. 0.360", while inner is about 0.320". (The inside diameter of the nut is 0.323".) This screw diameter doesn't seem to match up with any of the expected fractions: 5/16 is 0.3125, 3/8 is 0.375.

Metric doesn't make much sense either: outer screw diameter is 9.15mm, inner is 8.13mm, nut is 8.22.

So maybe the sizes expressed like 8, 10, 12, etc. are what I'm looking for? But I don't see caster screws expressed in those terms.

In short, what size "standard" thread should I be looking for in finding a replacement caster with the dimensions I've listed?

[Tequila]

Here's a chart of thread dimensions for future reference. Unfortunately it doesn't seem like your measurements match up to anything on the chart. Your best bet might be to get a small assortment of nuts and see what actually fits.

http://www.engineersedge.com/screw_threads_chart.htm

[dlminehart]

So what are those "major, pitch, and minor" diameters? I found no glossary.

[Tom Miller]

The listed diameter is that of the shank, or the hole size required for the bolt.

Sure sounds to me like you have 3/8-16. You measured 0.36" which is just 1/64" shy of 3/8".

Regards,
Tom

[Tom Slick]

quote:Originally posted by dlminehart

So what are those "major, pitch, and minor" diameters? I found no glossary.

Major diameter is the diameter of the tips of the threads, minor diameter of the diameter between the threads, and pitch is the distance between the threads. Unless you are a machinist you really only ever deal with the major diameter and the pitch.
you bolt sounds like a 3/8 x 16, which is a major diameter of 3/8" and a pitch of 16 threads per inch.

the metric system uses a similar measuring system. for instance an 8mm x 1.25 is a major diameter of 8mm and a thread pitch of 1.25. they don't measure

[LCHIEN]

3/8" is the only widely used bolt size with 16 tpi.
and its close to what you measured.

[dlminehart]

Tom, I figured that "pitch" meant distance between threads, but here are the values in the table Joe posted for 3/8-16:

Major diameter (basic): 0.3750, pitch diameter (basic): 0.3344, Minor diameter (max): 0.2983.

The table also had max and min values for major and pitch, but only max for the minor diameter.

[gonesailing]

If I am not mistaken, bolts and nuts have different grades of precision as well as hardness. Some fit pretty loose, others are designed to fit tightly. Thus I would expect the outside diameter of a 3/8 inch bolt to always be somewhat less than 3/8 in.
Mike

[Tom Slick]

here are some common thread sizes for SAE ("standard" or "inch") fasteners
8-32
10-24
10-32
1/4-20
1/4-28
5/16-18
5/16-24
3/8-16
3/8-24
7/16-14
7/16-20
1/2-13
1/2-20
9/16-12
9/16-18
5/8-11
5/8-18
3/4-10
3/4-16

I really wouldn't get too concerned with major/minor diameters etc. in reality it is only going to be close in diameter and the pitch is either going to be fine or course thread. you are not going to mistake a 3/8-16 for a 5/16-18 if you are paying attention.
if the head has no markings it less than grade 5 but you have no way of knowing exactly what it is.
if there are 3 lines on the head it is a grade 5.
if it has 5 line it is a grade 8
grade 8 has a higher tensile strength than grade 5.