This is a subject that comes up fairly often. It is something that is frequently done by Amateurs to connect a mast to a prop pitch rotator, or secure a mast from rotating in the mast clamp of a commercially made rotator. The basic requirement for obtaining a sound pinned connection is to obtain a tight fit between pin and hole. How good our fit needs to be depends on what we are pinning together and what we expect it to do. Some connections need very good fits, others are not so demanding.

This approach is probably the least desireable one for a strong reliable connection. It is commonly attempted by drilling a hole that is the same nominal size as the bolt. This is often done with a hand drill. This way of doing it will rarely, if ever, result in a tightly fitting connection that will remain in its original condition.

1) Drill bits rarely cut accurately sized cylindrical holes. The best that can be done is to use a new, or professionally resharpened bit in a heavy duty drill press or mill. The hole will be slightly larger than the nominal size. It is better for the bit to be ground to be center cutting in the area of the web, to eliminate the high cutting pressure required to push the web thru the material. If the bit is not center cutting, it will tend to wander around while trying to mash the material in its way out to the cutting edges on the face of the bit. This causes us to push harder, trying to force it through the material, causing it to wander more. There is a way to do a good drilling job with a non-centercutting drill, to be covered later.

If the bit is worn, it probably does not have the same sharpness on each cutting edge on the tip. This will cause the bit to wander and cut a non-round hole as one edge grabs more material than the other during rotation. This causes it to snake its way through the material.

Depending on drill condition, cutting fluid, and drilling speed and pressure, the chips coming out of the hole can cause some wear and scoring of the hole surface, and cause the starting end of the hole to get a little larger.

2) Using an hand drill motor to drill the hole increase the errors in the finished hole. We can never hold it in perfect alignment during the drilling, this will cause the hole size to increase. And, we can never be as rigid as the drill press or mill, so any tendency for the bit to bend and wander is greater, increasing hole size.

3) Bolts are usually not close tolerance pins. The ones we can get at the hardware store, or even a good industrial supply, do not have perfectly cylindrical shanks, and their size tolerance is relatively large for this application. Most bolts are meant to fit through holes with clearance and clamp things together, instead of being pins for transferring loads by bearing on the holes they are in. Close tolerance bolts can be obtained from aircraft suppliers from the "AN" line of fasteners. Another choice for a close tolerance bolt, would be precision ground shoulder bolts. Their grip length is important to get right.

All of this is not good for a connection that relies entirely on the pinning to hold it in place. What we usually get is a loosely fitting connection that has little of the available pinned surface area in intimate contact. The slop in the connection allows movement, which results in higher dynamic loads on the mating faces of the cylindrical interface. Even when the surfaces come into contact, there is not full surface contact due to the irregularities, so the pressure on the contacting surfaces are much higher than expected.

This often results in material yielding, the deformation increases the looseness of the connection, which increase the dynamic loads developed, which results in more yielding....etc. A loose fitting pinned connection will usually only get worse.

The only way to make this type of connection remain stable is to (I.E. stay as loose as it is, not get worse) is to use large enough pins to reduce the bearing loads on the holes to a point where the material will not yield when the pin is banging back and forth on the hole edges. Using more pins will help.

Now, if we are only expecting the pinning to prevent slippage of a mast clamp during the relatively infrequent times when the clamp is overloaded, it will probably work for an acceptable period, possibly forever, if the clamp is not slipping very often. The clamp will reduce the overall load on the pinned connection and eliminate most of the dynamic load peaks applied to it.

To straight pin something, we need a precision ground shoulder bolt or dowel pin, a special reamer to get the hole sized for a press fit, and the mating parts need to be fixed when drilling and reaming the holes. The hole is drilled slightly undersize, then accurately sized with a reamer to obtain a close tolerance cylindrical hole. The reamer size is selected to establish the fit to the pin. To obtain a sound connection the hole should be just a bit smaller than the pin creating a "press fit". This ensures maximum contact between the pin and hole. A dowel pin can be used as the press fit will hold it in place.

A tried and true way to get a good pinned connection is to use "Morse" taper pins. They are available from any good industrial supply or your gear & bearing guy. The "Morse" taper is one big long taper (at 11 degrees, if I remember it right, the books not here) and is divided into different sections, designated by a number. You can find info in the Machinery Handbook, probably one of the most useful general references a gearhead can have, or see your supply guy's book.

The Morse" taper is a self-locking taper, which means that once the connection is seated, by tapping the pin in place, it will not easily come loose. You can thread the small end of the pin with a die, to install a self-locking nut, for extra insurance.

This connection requires a special tapered reamer to finish the holes in the mating parts, and the mating parts need to be fixed during the drilling and reaming process.

I helped a friend pin his mast inside a tube welded to a prop pitch with two taper pins at right angles to each other. It turned a 5 element 42' boom 20 and a 2 element

When I was a youngster, an ex-pro mechanic friend and I built a 150 mph racing kart. All of the connections on the rear axle were made with small taper pins. We threaded the small ends and used lock nuts to keep them from vibrating loose. No, problems at all, worked great, and real easy to R&R during a panic.

1) Making the tapered hole is easier to do properly. Drilling and reaming a constant diameter hole requires a constant stable alignment of the cutter. Wiggling the hand drill is not good, but rarely avoidable. The tapered reamer is pretty much a self aligning cutter. You may get the hole off axis a bit, but what the reamer cuts will be true to itself, and a true conical surface. That's what the pin needs to work properly.

2) The tapered pin is easier to R&R, and more durable during service. Just tap the pin to unlock the taper, and then it comes right out. The straight press fit pin has

to be driven out for it's entire length. R&R'ing the straight pin causes surface wear, which means fewer R&R's before the connection needs to be redone.

3) The tapered pin connection can usually be renewed several times with the original tool investment. Just chase the hole with the reamer, and get a new pin (if