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CONCESSION CORNER

WARNING: DON’T THREAD ON ME!
By C.T. Cassidy

On many occasions and for numerous reasons, we have all probably needed to add additional threads to one of the control rods on a chassis. Examples, of course, would be brake, throttle and other externally threaded solid rods.

Generally, a fairly simple process, right? Just select the proper thread cutting die, run it over the existing threads, and start cutting your new threads to give you the desired additional area. WRONG!

Did you first check whether the original threads were rolled or cut? If you already know the difference and you did check, great! You now know what I am working up to and can move on to more interesting material on the website. However, if you don’t know the difference, then for your own good or perhaps a customer’s safety, I suggest you stick with me all the way on this one.

Let me first are you with the technical definition of the two different methods of threading a rod externally.

CUT THREAD: Threads are cut or chased. The unthreaded portion of the rod will be equal to the major or overall diameter of thread.

ROLLED THREADS: Threads are cold formed by squeezing the rod between reciprocating serrated dies. This acts to increase the overall diameter of the thread over and above the diameter of the unthreaded portion of the rod.

What does this mean to you? Simply, the stock used for rods that will have rolled threads is of a smaller diameter that the material used when cut threads are used. The result is that when you add threads to a rod that has rolled threads, the threads you cut generally appear all right, but in reality these new threads you cut will be very shallow and VERY UNSAFE! These threads could, and quite likely will, strip or pull loose under either severe load or simply repeated medium to light loads.

I am sure you would agree that tie rods ends are subjected to fairly loads such as going around a long sweeping high-speed corner on a road course, a tight sharp corner on a short course or the constant usage on concession karts. Here is one more steering example, and probably the most severe. This would be the almost constant stress of the short, but faster oval-type track.

Brake rods—The stress there is self-explanatory—repeated and dangerous.
Throttle Rods—Well, fortunately a failure here will not be life threatening.

What to do about it? I simply suggest that before you add cut threads to a rod, look closely at the original threaded area, and if those threads appear to be of a greater diameter than the unthreaded portion, don’t add cut threads to that rod!

About the author: C.T. (Tom) Cassidy was with GEM Products, an aftermarket kart parts manufacturer for over 30-years, until his retirement from the sport in the mid-1990’s. Tom resides in Chicago, Illinois.

END

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