What is the correct tap diameter for 3/4" studs in a locomotive boiler per code?

Related to this question, why, or is there ever, is there a difference between "code" tap thread hole size and the standard 75% depth of thread tap size?

They are tapered and suck up tight in any case.

From the BLW specs for USATC Consolidations, circa 1943 - "Boiler studs shall have a minimum diameter of 5/8" and tapered 3/4" in 12" in the boiler shell."

The only answer is "it depends". If the major diameter of the stud is 3/4 at the outside surface of the boiler, its minor diameter at the inside surface of the shell will depend on the thickness of the shell, and the thread form of the tap.

To be watertight, the thread has to be 100%. A 75% female thread like a commercial nut has will leak since the water can follow the spiral up the thread to the outside. A V-thread form needs a smaller hole than a US-thread form. Whitworth threads have the largest minor diameter size per size so can use a larger tap drill.

Also, due to the taper, the hole needs to be smaller still. A 3/4" stud going through a 1" thick shell will be 11/16 o.d. at the inside surface.

The thing to do is to measure the tapered tap you plan to use with dial calipers. Find where the major diameter is 3/4" and mark it with Sharpie. Measure down the tap the thickness of the boiler shell, and mark that spot as well. Now measure the minor diameter of the tap at that location with the caliper. That is the largest tap drill that you will want to use. Smaller is safer to get full threads the full length of the hole. A larger hole will have diminishing threads toward the inside, not so good.

Is it worth taper reaming for easier tapping? Does anyone even make a tapered reamer for this taper? I know taper reamers are available for NPT pipe threads.

Sure!

You can make your own for limited applications. I have had very good luck with home made half round reamers. We used them to make tapered holes in the ends of our staybolts (and I have used them many times for valve body reamers in non-ferrous). Here, we make staybolts with tapered center holes on both ends, and hammer them tight with a tapered plug that expands the staybolt. Two guys with two guns hammer from both sides at the same time.

You take some drill rod, turn the taper you wish. Mill, grind, or file the taper to a half round shape (mill a flat all the way down to the centerline). Harden and temper to a straw color (golden yellow). Clean up the edges with a hone and some buffing, being careful not to dull the edges. With low speeds and cutting oil it should work great.

With reference to my initial question, I am working on a CADD model of the Union Iron Works, Erie, PA, boiler for Climax CN 1551. Although I do not need to get it absolutely correct, I do not want to get it WRONG and have some rivet counter point out the mistake and then assume that the rest of the work on CN 1551 is subject to the same errors as I have been very careful so far.

The boiler courses are respectively, 19/32", 21/32", 21/32", and 5/8".

Standard Steam Locomotive boiler studs are tapered on the side that are tightened into the boiler shell. They are 12 1/2 " threads per inch and require a special taper cutting attachment for the lathe. My mistake.... 12 threads per inch. They also require that the material used to create them be of SA-36 Specifications. Cold rolled steel rod is not acceptable, has to be Hot Rolled. Any good steel supplier has this material on hand. The opposite end will be cut to accept regular straight thread nuts. Care needs to be taken to insure that the tapered end of the stud completely passes through the entire thickness of the boiler shell & a little beyond. The fact that the stud is tapered will allow it to suck up and make a steam tight seal. applying a waterproof sealant such as Indian Head Shellac will allow a better snug up & make sure to torque the studs in as tight as possible without twisting them.

Different railroads had different standards for boiler threads, some 11 TPI, some 14TPI, but the vast majority that I have come across are 12 TPI. I have never heard of 12-1/2 TPI boiler (or pipe) threads.

SA-36 is no longer recognized for boiler studs by the NBIC. They call out SA-307 grade A or B. Other grades that can cross reference to SA-307 can be used, which may or may not include SA-36 depending on the chemical make up of a given heat.

Hello: I had hoped my 1919 Machinery's Handbook would have thread data for the 12tpi tapered boiler threads, unfortunately it did not, but there was a page of data for standard sized taps. This may be of some use to the original poster or others searching the topic. By using dimensions "D" and "B", and the taper of 3/4" per foot you can calculate the OD of the tap at a point about one plate thickness back from the end of the tap, that should be in the ball park of the right tap drill size. By the way the "Briggs standard pipe" taps mentioned in the text seem to be the same as the modern day dry seal National Tapered Pipe standard.

Pat Hosford

Thanks for the graphic. That is the kind of info that satisfies my mental itch to know. I had to ask the Steel Institute in Pittsburgh to research early 20th century steel catalogs to get the profile for the channel Climax used for the frame - no longer made.

My mistake, I did not have my glasses on when I searched my antique tool box for old taps purchased at a flea market. Twelve threads per inch they are.
Just for fun dept.... I heard that " Allnut " was quite a stud back in his day ! LOL !