Is there a caliper measurement you can to take to determine if a screw is a 6-40 or 8-40? Removed a base today to be replaced with a new one and I’m not sure which size the screw holes are. I have the screws that were on the old base however so I can measure those.
Rifle Scopes 6-40 Screw Diameter
- Thread starter kmc0929
- Start date
I use these inexpensive screw size / thread checkers all the time. The drill sizes for taps and clearances are handy too. Major screw diameters shown in last column.
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I have attached a thread data chart from the Machinist Handbook.
I am not second guessing you but I wonder whether you meant a 6-48 or 8-40 thread instead since these would be "typical" sizes for a base. With that said, those would not be standard unified thread sizes so they will not be on the chart that I have attached.
To answer your question about a measurement to determine a 6-40 or 8-40. The major diameter will change very slightly as the thread per inch, (TPI), change. However, there is enough difference in the major diameter between a #6 and a #8 thread to determine the size. A #6 2A will be ≈ 0.137. A #8 2A will be ≈ 0.163.
To determine the TPI, count the number of threads in 1 inch. If there is only 1/4" of threads, count the threads and multiply by 4.
Hope this helps
I am not second guessing you but I wonder whether you meant a 6-48 or 8-40 thread instead since these would be "typical" sizes for a base. With that said, those would not be standard unified thread sizes so they will not be on the chart that I have attached.
To answer your question about a measurement to determine a 6-40 or 8-40. The major diameter will change very slightly as the thread per inch, (TPI), change. However, there is enough difference in the major diameter between a #6 and a #8 thread to determine the size. A #6 2A will be ≈ 0.137. A #8 2A will be ≈ 0.163.
To determine the TPI, count the number of threads in 1 inch. If there is only 1/4" of threads, count the threads and multiply by 4.
Hope this helps
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