Motion Pins

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Motion Pins

W.I. Treadway

The North American Live Steamer, Volume 1 Number 9

In Number 6 of the North American Live Steamer Gordon Corwin explained his method for filing hex heads on valve motion pins. This reminds me of a trick I used on motion pins which has proved quite successful.

Oftentimes where a fork, or crevis, is used one side of the fork will be drilled a clearing size for the pin and the other side tapped with an appropriately smaller thread. This means, then, that the motion pin will consist of a head - round with a screw slot or head - a straight plain portion which furnishes the bearing surface for the link, rod or other mating part and a short threaded portion. Now, if the fork sides are thin, say 1/16 inch, and a 48 pitch thread is used, it means only 3 threads, at best, can be effective. If your die will go up to within 1-1/2 threads of a shoulder this leaves only 1-1/2 threads effective. So the problem resolves itself to producing a thread right up to a shoulder. Here's my solution and it's not restricted to just valve motion pins.

Chuck a piece of drill rod in the lathe, the diameter to suit the head size of the pin. Turn to diameter and length desired for the bearing portion. Do not, however, turn down for the threaded portion. Instead, do this: drill and tap for desired thread size, say 3-48 for a 5/32 inch pin. Then take a commercial steel round head screw and run it into the hole. Tighten securely. Cut off the screw head and face the remainder for the length of thread desired. This will give you full threads right to the shoulder and insure that the bearing portion of the pin seats tightly against the tapped side of the fork.

Now go ahead and cut off the pin to length. Reverse in chuck, face off the head, and file a hex or screw slot as desired. Then harden the pin - not in the chuck, of course - and draw to a light straw color. This will give you a hard, long wearing pin but the thread portion being mild steel does not harden. Therefore it will not snap off. Sound like a lot of trouble? Actually it takes no longer than turning a shank and running a die on it.

Now, in filing a hex head on a pin the indexing can be done quite nicely as Gordon Corwin has pointed out. The hard part is knowing how much to file to get a properly sized hex which will fit your wrench. Try this: turn up a bushing from drill rod, diameter to be same as distance across flats of the hex you want. make the bushing 3/8 to 1/2 inch long and drill through 1/8 inch. Harden this bushing right out - don't draw. With work in chuck ready for filing put a short piece of 1/8 inch rod in tailstock chuck, slip the busing on it and run up the tailstock until bushing just touches work. File the first flat of the hex. When the file contacts the busing you will be to size. Then index work, file second flat and so on with lathe running file a small 45 degree chamfer on hex portion. Stop lathe, remove work from chuck, and you're all ready for hardening.

Easier to start with cold rolled hex stock and case harden? Sure it is. But a really good case is necessary. Am presently replacing my motion pins made this way - once the case wears through the wear increases very rapidly. My new pins will all be drill rod. A 1 inch diameter piston with 90 pounds steam behind it puts a tremendous load on a 5/32 inch pin - better do a good job to start with and be sure.

W.I. Treadway
352 Brewster Raod
Bristol, Conn