Explain barrel thread pitch to me please, I am clueless


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grimjaw
August 8, 2006, 01:56 AM
I have a Romanian SAR3 that had a threaded barrel when I got it. I don't know if it was manufactured that way. It's a left hand thread. The barrel is 1/2" in diameter. There are approximately 21 threads (pictured below) in just under an inch. The rifle included a left to right hand thread adapter and a AK74-type brake. The threads on the brake were much wider apart than the thread on the barrel.

I've seen all these numbers: 24x1, 24x1.5, 14x1, 14x1.5, 22x1, 1/2x28. I still can't make sense of it, and I don't know if what I have even fits in that category. I've read up online trying to find a definitive source of information to no avail. I'd just like to know:

- What is the thread pitch on that barrel?
- Is it 24x1 or 24x1.5, etc?
- What the heck does that number actually mean?

Please forgive my ignorance on this topic, and thanks for any info.

jmm

http://www.grimjaw.net/mig/albums/theboomgoesthere/1807.jpg

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Sylvan-Forge
August 8, 2006, 04:27 AM
Here ya go :
http://www.boltdepot.com/fastener-information/TPIandPitch.aspx

Note the Fastener Information menu on the left.


With any threads, you are basically looking for diameter, thread and length.

Metric:
Example:
M10 x 1.5 x 20
diameter of 10mm,
1.5 thread pitch (distance between each thread in mm),
20mm long.

US:
Example:
1/4 – 20 x 6
diameter of 1/4 inch,
20 threads per inch (course thread),
length of 6 inches.

Sylvan-Forge
August 8, 2006, 05:13 AM
SWAG guesstimate on your threads shown in pic :

12.7 x 1.0 x 24

12.7 mm Diameter
1.0 mm Thread Pitch
24 mm Length

dfaugh
August 8, 2006, 09:11 AM
You can buy a "pitch guage", that you can use to determine the pitch of any threaded (Male or Female) device. My dad was a machinist, so I inherited his. I'll try to explain as best I can....Its actually a "set" (I only have the American version), of "leaves" on a central pivot, such that you can rotate one out at a time (kinda like a feeler gauge set, or swiss army knife.) Each "leaf" has a set of "teeth", w/ different spacing, you just lay it on the object, and the one that fits properly into the threads will tell you the correct pitch. I don't know if you can get them at your average hardware store or Home Depot, but any place that sells machine tools would have 'em. Shouldn't cost much I wouldn't think. I don't use it often, but when you need one its real handy to have around.

Note: this will only give you the TPI (threads-per-inch), you'll need to measure diameter w/ calipers, etc.

brickeyee
August 8, 2006, 12:56 PM
Threads per inch is the recpiprocal of thread pitch.
If you have 20 threads per inch the pitch would be 0.06
With 25.4 mm/in, 20 threads per inch is 0.787 threads per mm. A PITA to deal with, so the pitch is used and is 1.27 (probably 1.25 in reality).

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