# Thread: Understanding machine screw sizes

1. Senior Member
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Thanks for all the information. I knew there had to be some sort of formula for figuring this out, just like they have a formula figuring out rebar sizes.

-JW:

2. Senior Member
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Originally Posted by Bented
The major diameter of a numbered machine screw in the US system is:
The size number X .013" + .060".....Even more inexplicable is the fact that someone devised a system of screw sizing using this formula 100 years ago.
You think the present system is bad?

Prior to 1908, the standard was, Nominal Major Diameter = 0.05784 + (.01316* the size number).

SO a #10 screw had a diameter of 0.18944 instead of the current 0.19000, etc.

Starting in 1908, the ASME had a committee working on changing the standard-they issued reports in 1908, 1917 & 1920, and the new system was finally adopted in 1920.

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Thanks Andy.
I always appreciate historical information like this.

-D

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It was much later than that before SAE got involved. Many of the early cars had all manner of sizes. Most produced right in the car factory to fit a specific part. This included special bolt head sizes and shapes.

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Originally Posted by Bented
The major diameter of a numbered machine screw in the US system is:
The size number X .013" + .060"

Therefore a #4 screw has a Maj. Dia. of .112" maximum.

I do not know why I have remembered this for 30 years, it is inexplicable.

....
I bet you have remembered it for the same reason I have: it was such an eye-opening revelation ...a forehead slapping moment. A piece of info that's handy to know.

6. Senior Member
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I have usually used the following for clearance holes:

Size - Fraction - Decimal - Clearance
#4 --- 1/8" ----- 0.125" -- 0.013"
#6 --- 5/32" ---- 0.156" -- 0.018"
#8 --- 3/16" ---- 0.187" -- 0.023"
#10a - 13/64" --- 0.203" -- 0.013" (close fit)
#10 -- 7/32" ---- 0.219" -- 0.029"

That made it easy to use common fractional drill sizes, and easy to remember. Also, they give between 12% and 15% clearance.

7. Member
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Jun 2019
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Originally Posted by lynnl
I bet you have remembered it for the same reason I have: it was such an eye-opening revelation ...a forehead slapping moment. A piece of info that's handy to know.
I think that I was just young and could remember useless knowledge at the time (-:

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Originally Posted by andywander
You think the present system is bad?

Prior to 1908, the standard was, Nominal Major Diameter = 0.05784 + (.01316* the size number).

SO a #10 screw had a diameter of 0.18944 instead of the current 0.19000, etc.

Starting in 1908, the ASME had a committee working on changing the standard-they issued reports in 1908, 1917 & 1920, and the new system was finally adopted in 1920.
X2 Thanks, historical information is invaluable. I'm trying to get a time frame for some of my taps and dies, thanks again.

9. Member
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Jun 2019
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Originally Posted by CCWKen
It was much later than that before SAE got involved. Many of the early cars had all manner of sizes. Most produced right in the car factory to fit a specific part. This included special bolt head sizes and shapes.
ASTM has a broader range of ASTM standard threads then the SAE.

However this does not preclude anyone from making any thread combination that they desire, you simply can not claim that they are ASTM/SAE standard dimensions. Because I am not a particularly nice person I have made mating threaded parts that have very proprietary thread dimensions simply to confuse people in the future (-:

I once made some dedicated gear pullers for a repair company, the threads were 1 1/4-15 11/16 threads per inch, they never figured that out.

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F it, I'll just use metric hardware!

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