Tread pitch of of Polytech gas cylinder lock???
- Thread starter jimlost
- Start date
Gee, now you're making me work for it. 
I don't have my machinery handbook with me (it's at work) and the closest I can measure here and convert, it would be something like a 17 x 1 mm thread. I measured over the threads on the barrel and come up with .663" which I convert to metric by multiplying 25.4 millimeters to the inch and come up with 16.84 mm. This would be too sloppy for a 18 mm diameter and I have never seen half sizes in metric threads e.g. 17.5 mm diameter.
You would have to get a tap custom made (even up here) to chase this. I think the biggest standard tap @ 1.0 mm pitch is 10 mm diameter. I might add that 17 mm is a non-standard thread diameter.
I was expecting you would use a lathe. A 1 mm pitch would work out to 25.4 threads per inch. You will find the same problems working in metric that you do in imperial, the fine thread pitches are non-standard to the diameter of tap.
If you've messed up the threads on one side of your gas lock, maybe you can turn it around and thread it on backwards to push the threads straight. Use tapping grease when you do this so you won't just bind the threads together. If the threads on your barrel are screwed, you can get a thread file - Nicholson Thread File Type No. 8 - to straighten them or use a triangular file since the thread angle is the same between imperial and metric.
I'm sorry if I am not much help but gunsmith threads do not conform to the 'real' world and the chinese probably converted to the closest thing they could find in metric.
Ask in the exchange forum if there is anyone who would sell you their old norc. gas lock. This is probably the best way to go. I wish you good luck in your endeavours since a tight gas system is a happy gas system.
Ripstop
I don't have my machinery handbook with me (it's at work) and the closest I can measure here and convert, it would be something like a 17 x 1 mm thread. I measured over the threads on the barrel and come up with .663" which I convert to metric by multiplying 25.4 millimeters to the inch and come up with 16.84 mm. This would be too sloppy for a 18 mm diameter and I have never seen half sizes in metric threads e.g. 17.5 mm diameter.
You would have to get a tap custom made (even up here) to chase this. I think the biggest standard tap @ 1.0 mm pitch is 10 mm diameter. I might add that 17 mm is a non-standard thread diameter.
I was expecting you would use a lathe. A 1 mm pitch would work out to 25.4 threads per inch. You will find the same problems working in metric that you do in imperial, the fine thread pitches are non-standard to the diameter of tap.
If you've messed up the threads on one side of your gas lock, maybe you can turn it around and thread it on backwards to push the threads straight. Use tapping grease when you do this so you won't just bind the threads together. If the threads on your barrel are screwed, you can get a thread file - Nicholson Thread File Type No. 8 - to straighten them or use a triangular file since the thread angle is the same between imperial and metric.
I'm sorry if I am not much help but gunsmith threads do not conform to the 'real' world and the chinese probably converted to the closest thing they could find in metric.
Ask in the exchange forum if there is anyone who would sell you their old norc. gas lock. This is probably the best way to go. I wish you good luck in your endeavours since a tight gas system is a happy gas system.
Ripstop
mikebaker1129
CGN Regular
- Location
- TEXAS, U.S.A
I checked mine with my metric gauge and I too came up with 1.0 mm with a 60 degree pitch .
Did someone by chance try to put a USGI gas lock on the barrel ? They are not the same , I learned this the hard way a few years back when I bought a box of parts that were suppose to have been USGI ,but it turned out that they were Chinese :shock: . That is when I was having a hard time getting that Chinese gas cylinder lock threaded on to a USGI barrel :roll: . I finally figured it out !
You could always just get another lock ? If you are having a difficult time locating one send me a PM and I will check with one of my local buddies that might have one .
HTH Mike
Did someone by chance try to put a USGI gas lock on the barrel ? They are not the same , I learned this the hard way a few years back when I bought a box of parts that were suppose to have been USGI ,but it turned out that they were Chinese :shock: . That is when I was having a hard time getting that Chinese gas cylinder lock threaded on to a USGI barrel :roll: . I finally figured it out !
You could always just get another lock ? If you are having a difficult time locating one send me a PM and I will check with one of my local buddies that might have one .
HTH Mike
mikebaker1129 said:
I threaded a Versa Pod M-14 spigot into the Norinco gas cylinder, used lots of oil on the threads and started threading until I felt 20-30 ft lbs of torque then backed it off then tightened, backed it off then tightened..... finally after about an hour of easy does it I had it threaded all the way down. Works fine now.
The chinese thread pitch in imperial would be 25.4 threads per inch. The USGI thread pitch appears to be 22 threads per inch. I count about 7 1/2 threads on the chinese gas lock and about 5 1/2 threads in the USGI gas lock. Either way, you aren't going to end up with one continuous thread. If it appears to be one continous thread, then the thread root has been sheared.
Ripstop
