The Stovebolt Site Forums The Shop Area General Truck Talk How do I properly "specify" the Pitman arm spline?

As many of you know, I am doing a project on a '55 1st Edition AD truck, called "SLURP" (Simple Lightweight Unpretentious Retro Pickup).

In adapting our steering to the new engine's wider physical presence, we have found a smaller high quality replacement steering box, but because its dimensions differ from the stock steering box in many ways, we need a custom Pitman arm to connect from the output of the steering box to the factory drag link on the '55 1st Edition truck.

As many of you know, the '55 1st Edition has a different drag link and Pitman arm than the earlier AD models. The earlier model drag link has a "ball receptacle" for a ball end on the Pitman arm. The '55 1st Edition has something that is more like a tie rod end in appearance. The end of the Pitman arm that connects to the drag link has a simple tapered hole that I have measured and recorded dimensions for.

I have found a company that makes chassis parts including Pitman arms, and they say they can either provide an existing model Pitman arm (if one exists close to what I need), or they can make a fully custom one for a VERY reasonable price.

My problem is that I know nothing about steering system types and measurements, so don't know how to describe the END that I need at the top end of the custom Pitman arm.

I need someone knowledgeable here to tell me what to ask the supplier to build.

I know the center to center length I need, and also have the tapered hole dimensions for the BOTTOM hole in the Pitman arm. But, I need to be able to describe the "splined" hole at the top of the Pitman arm.

This is what I know so far about the output shaft of the Flaming River box to which the Pitman arm must attach:

The first 5/8" of the shaft is a 5/8"-18 threaded section. This is where the nut that holds the Pitman arm in place threads on.

Then there is a 1/8" length UNthreaded portion of the shaft, to hold the lockwasher that sits bewteen the nut and the Pitman arm.

Finally, there is a SPLINED section of the shaft, which a matching spline on the Pitman arm engages. It is this spline pattern that the custom Pitman arm must have in it, and I am having trouble describing the spline pattern in terms that a machinist will clearly understand.

Here is what I have measured on that splined section:

Diameter is 13/16"

There WOULD have been 36 splines around the circumference of the shaft, i.e. one spline every 10 degrees, EXCEPT that there are four relief cuts, 90 degrees apart, each of which displaces one spline. There are 32 regular splines. (Check the math: 32 regular x10 = 320 degrees plus 4 relief slots x 10 degrees = 360 degrees total). (edited this after initial posting to correct errors)

I am thinking that this spline pattern is the way it is to ensure that the Pitman arm is correctly "clocked" relative to the steering box. That is, you cannot just mount the Pitman arm at "approximately" the right angle to meet the drag link; it can only be mounted so that the wide slots are engaged by matching wide ridges on the Pitman arm. This ensures that when the truck is driving straight ahead, the steering box is in the center, "tighest" portion of its internal gearing, where road wander will be dampened.

I am thinking that this specific spline pattern is probbaly some sort of industry standard (I HOPE!), and is probably described in some specific standard way, like, for example maybe 13/16 x 36 with 4 indexing slots???

Is there anyone out there who is a pro mechanic who works a lot on steering and can tell me what the correct "standard description" for this spline pattern would be?

The bottom tapered hole is easy:
The Pitman arm has an "inside" (near the frame rail) side, and an "outside" (away from the frame rail) side
Pitman arm thickness at the hole is 0.75"
INside diameter of the tapered hole is 0.655"
OUTside dimater of the tapered hole is 0.733"

But I really need help describing that spline pattern at the top of the arm!

Anyone?

Jim G

Sounds like you have enough info to go to your pitman arm manufacturer.
The tapered spline used in pitman arms is standard, or reasonably so.

If the folks who are going to make the pitman arm have ever made one before, or know what they are doing, they will be able to take your measurements and decide what standard size your have, and go from there.

If nothing else works have the manufacturer of the new steering box talk to the machine shop that will make the pitman arm, and take yourself out of the middle, the two machine shops should talk the same language.

When I needed a different pitman arm for my PS conversion I had good luck looking in the junk yard, as most every pitman arm I found would fit the gear box, I just needed to find the right shape for the rest of it, ended up with a chevy astro van pitman arm.

Grigg

Thank-you, Grigg.

I actually have a messgae in for my sales person at Flaming River, asking him to describe back to me exactly what I should tell the machine shop to make at that upper end of the Pitman arm. But, I thought of it too late Friday, after Flaming River closed, so won't be able to hear anything back until at least Monday. I was hoping to be able to draw and send a fax drawing to the machine shop before I go traveling on business first thing early Monday morning.

Based on what you said, I'm going to try sending that fax this weekend.

Jim G

Jim G;
That "different drag link and pitman arm" actually goes back to and includes the '53 models. That spline pattern you describe sounds exactly like the stock pattern for the '53.

Your new box was made for a particular application. Why make a new pitman arm, why not just use the stock arm for that application and modify it if need be. Did you test fit your stock arm? Chances are it fits.

I dunno, is that an oversimplification? I just don't see a need to reinvent the wheel here.

Jim G -
My sources tell me that what you want is a "Chevrolet spline". I'm told that Ford and Chrysler are different than a Chevrolet and that in 55, or earlier, and later into the 60's, steering box rebuilders builders and mechanics recognize the different splines by brand name. So, it seems that all you would need to tell them is that the spline you need is the "Chevrolet pitman arm spline". It sounds simple, but that's what I'm told.

Joe

This more complex than it seems. I simplified things a lot for this specific posting question.

The only Pitman arm available for that Flaming River box is only 4.5" center to center. The box also uses a different spline pattern than the original Chevy factory box.

The original Chevy factory box output shaft has a 1" nominal diameter splined shaft, with 36 splines = 32 "regular" splines plus 4 of what I have now learned are called "master splines". The "master splines" are female on the output shaft, to accept 4 "male" master splines on the Pitman arm!

The Flaming River box has a 13/16" nominal diameter splined shaft, with the same 32+4 master spline pattern. So, a "Chevy pattern" splined Pitman would have too large a nominal diameter.

The BOTTOM end of a Pitman arm also varies. On the stock Chevy 55 1st Edition, it is a tapered hole that fits onto a tapered shaft on the balljoint on the drag link. I have the dimensions for that tapered hole.

The LENGTH of a Pitman arm also varies, and is CRITICAL to ensure both correct "leverage" and even more importantly, to ensure no increase in "bumpsteer" when the truck suspension compresses or extends!

Finally, every Pitman arm has those 4 master splines "clocked" at different orientations to match both the steering box and the vehicle! This is to ensure that the steering box is at "center" when the wheels point straight ahead (for minimized wander and tighest steering wheel), and so that the drag link is not forward or backward of its intended position under driving conditions, as displacement of it will greatly (and badly) effect bumpsteer and turning limits left and right, and in extreme cases, could bind the steering under hard turns!

So, once you change ANYTHING, you need to ensure that you are either follopwing someone else's already proven solution, or you need to very carefully learn enough to "design" your own solution that "follows all the rules'. I can't find anyone else, at least yet, who has done an LS series engine into an AD truck AND has kept the stock front axle, so I am by necessity "a pioneer" here. I am proceeding very carefully, and Randy is very experienced, but it takes time and effort and lots of mathematics, because there is lots of 3-dimensional placement involved. I have spent HOURS on the mathematics today (including lots of trigonemtry!) to ensure we stay inside the correct dimensional envelope.

I can tell you this: those GM engineers who designed the original system were VERY good. They came up with a solutin that met all the constraints while remaining VERY simple overall once installed. The design was elegant. It just won't accept a wide engine.

Jim G

I think that a Corvair pitman arm would have the same spline as the Flaming-River steering box that you are using. I believe that the box you are using is designed after a reversed Corvair steering box. If you go to this website: www.gasalleystreetrods.com/Steering.html. They show a number of pitman arms designed to use with a reversed Corvair type steering gear.
Good Luck
Don

Anything you ever wanted to know about identifying, designing, measuring, and cutting of splines can be found in this book:

http://www.industrialpress.com/en/MachinerysHandbook/default.aspx

Buy it and when you have questions regarding tolerances of fits, gear definition, properties and treatment of metals, fasteners, dimensioning, machining, gauging, thread measurement, shear, stress, leverage, welding, plating, keyseats, v-belts, tooling, deflection tables, etc, etc, etc. you will have the info in front of you.


Only the table of contents is available online:


SPLINES AND SERRATIONS
2156 Involute Splines
2156 American National Standard
2157 Terms
2158 Types of involute spline
2160 Tooth Proportions
2160 Symbols
2161 Formulas for Basic Dimensions
2162 Basic Dimensions
2162 Tooth Numbers
2162 Types and Classes of Fits
2162 Classes of Tolerances
2163 Maximum Tolerances
2164 Fillets and Chamfers
2165 Spline Variations
2165 Effect of Spline Variations
2165 Effective and Actual Dimensions
2166 Space Width and Tooth Thickness
Limits
2166 Effective and Actual Dimensions
2167 Combinations of Spline Types
2167 Interchangeability
2167 Drawing Data
2169 Spline Data and Reference
Dimensions
2169 Estimating Key and Spline Sizes
2170 Formulas for Torque Capacity
2171 Spline Application Factors
2171 Load Distribution Factors
2172 Fatigue-Life Factors
2172 Wear Life Factors
2172 Allowable Shear Stresses
2173 Crowned Splines for Large
Misalignments
2174 Fretting Damage to Splines
2174 Inspection Methods
2175 Inspection with Gages
2175 Measurements with Pins
2176 Metric Module Splines
2177 Comparison of Symbols
2177 Formulas for Dimensions and
Tolerances
2180 Tooth Thickness Modification
2180 Machining Tolerances
2180 Tooth Thickness Total Tolerance
2181 Selected Fit Classes Data
2182 Straight Splines
2182 British Standard Striaght Splines
2183 Splines Fittings
2184 Standard Splined Fittings
2185 Dimensions of Standard Splines
2186 Polygon-type Shaft Connections

I have that book, and several other machinist handbooks, but I don't think it gives any dimensions or info directly related to the tapered splines used on steering shafts. I'll double check though, but I have looked for that info before with no luck.

Grigg

Thanks for trying to help, guys. It IS a tough one to get real information on.

Don: I checked that website. Their longest Pitman arm for the Corvair box is only 7 inches long, which is 1.5" too short for Red. Although it could be connected up (IF the "bottom" hole also fits the '55 1st Edition drag link), it would increase bump steer quite a bit - unless I lower Red another 1.5" or so.

Hmmmm.

Jim G

If you find one that works other than the taper for the drag link being to small, that is easy to fix with a tapered reamer sold for that purpose.

Grigg

Grigg: That's good to know! That makes things a bit simpler at least!

Jim G

Output shaft on the Corvair is much Smaller than reg. Chev.Pitman arm will not work.

Yes, that is correct. The output splined factory shaft is about 1 inch nominal diameter. The Flaming River one is a nominal 13/16 inch.

Jim G