First of all, there are lots of threads out there that are not covered by this chart. There are Z threads, Robbins Threads, LT425 threads, Etc. With our chart, we tried to cover the API rotary shouldered connections like the API Reg, API IF, and API FH along with the API Numbered Connections. I included some of the more common waterwell threads like the Mayhew and FEDP and threw in some mining threads like the BECO.
The purpose of this chart is to give you an idea of what thread you have. I certainly do not recommend using this chart to identify the thread on your drill pipe and run out & order 1,000 feet of drill pipe based on your findings. When in doubt, send a sample to the manufacturer to be sure.
The first step is to determine the threads per inch (red column). Threads per inch is an important characteristic when trying to identify rotary shouldered connections.
The best way to count threads per inch is to place a ruler with an inch mark on the top of one thread and then count the valleys between that inch mark & the next one.
The photo shows 5 threads per inch.
I like using the female or box threads to take my measurements. Here we measure the Counterbore – yellow column. Please note the following:
1) This is a tapered area and you want to take your reading as close to the box face as possible. There might also be a bevel. Your reading might not be very accurate but that’s OK.
2) The Counterbore diameter has a tolerance of + 1/32″ / – 1/64″. If you are using a dial caliper & measuring in thousands of an inch, That can mean a range of 50 thousands.
3) once you measure your counterbore, see where it fits on the chart. The threads are listed by size.
4) See how many threads are within your range taking into account the tolerance & how accurate you think your measurement was.
5) Now use your threads per inch to elinate some possibilities. If you have more than one option, use the other dimensions to see which fits best.
To use a pin thread, we want to check the large diameter of the pin (Blue Column).
A word of caution here. If you look at the pin drawing, there is a relief at the base of the pin. The DL is the measure prior to any relief being machined. There are a lot of companies making products with these threads. I am not sure they all put in this relief with the same precision. We will adjust for this below.
1) Use calipers to measure the diameter at the base of the pin.
2) hold a straightedge alongside the pin & check to see if this relief his present by looking for a gap between the straight edge & base of the pin. If there is, add 0.110″ to your measurement.
3) Compare it to the numbers in the blue column. The tolerance on this dimension is + 0 – .015″. So the range we seek should be much less.
4) Use the threads per inch to see if you have any valid matches.
When in doubt, take it to a local machine shop & have them check it. They are used to working with measurements and could also check the taper per foot.
Please keep in mind that length of pin, ID, etc can vary widely and not good indicators of the thread ID.
Example.
I looked at a sub that I have in my office. First, I counted the threads per inch – 5. I then used a dial caliper to measure the counterbore. I noted that there was a slight bevel. (see picture below). The reading was 3.548″ or about 3 35/64. To account for the tolerance, I looked for what threads on the chart fit into the range of 3 34/64″ & 3 37/64″. Only the 3 1/2 Reg was in range & it has 5 threads per inch so it is the likely thread.
Since certain tricone bits & DTH hammers have a 3 1/2 Reg pin, it was easy to verify by screwing the sub onto these.
