A metric thread is easy to measure. A simple pair of dial calipers can be used to do this.
First you need to know the two basics in understanding a thread and how it is measured.
1.The diameter of the thread also called the major diameter will give you the size nominal.
2. The distance between one thread to the next equals the pitch.
The pitch = the distance in millimeters that the thread advances in one full turn.
Before we can go any farther dial calipers are typically made in English OR Metric. Some may have digital readout that will switch from one to the other at the users command.
Using the more common English dial caliper that is found on most of our hobby bench's we need to know just one more thing. That is the conversion from Metric to English to measure our Metric thread.
Conversion 1mm = .03937
Now lets measure that metric thread with your English dial calipers.
Example:
Thread diameter = .195 nominal
Pitch = .0315 nominal
.195 ÷ .03937 = 4.953 or 5mm thread
.0315 ÷ .03937 = .800 or just .8 pitch.
So now in this example we have a M5-.8 thread size.
Add the length of the thread to the type of screw for the complete designation which may be read something like .. M5-.8 X (insert thread length) Socket Cap/Set Screw/Fillister Head etc.
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Removing a broken shell in your 7.62X39 firearm.
The easiest way is to use a broken shell extractor and most of the time this works.
There is a small chance however that it will not. What now?
1. Obtain a 3/8-16 tap.
2. An extension long enough to fit the end of the tap and be able to turn it from outside of the rear receiver.
3. A rod long enough to fit into the muzzle and knock out the broken case.
Tap the inside of the case up to the neck. The tap will stop cutting. On my sample that I did on the bench the tap cut easy and was never close to exiting the wall of the stuck case marring a chamber. A modest amount of care was used to keep the tap cutting straight.
When the tap stops cutting remove the drive extension and insert a rod from the muzzle. The tap still in place acts as a stop for the rod. Knock out the stuck case.


