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Thread: F100 pitman arm

  1. #1
    Sidewalk Spectator

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    F100 pitman arm

    Has anyone out there modified or made F100 pitman arms? Just about to dig into the steering on my '39 pontiac and never attempted either. Is it safe to modify/weld to stock pitman arm ( ei use the large splined end and weld new arm to )? any advice/insight greatly appreciated

  2. #2
    Harvey's Front Row

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    There's a long history of hot rodders using F100 steering boxes in early Fords, and the use of dropped axles, suicide mounts, etc. occasionally requires that the pitman arm be heated and bent or, in extreme cases, cut and welded. The arm itself is forged steel and can handle modification if you've done your homework on keeping the heat to a minimum and using the proper technique. Now I'm curious...

    - First, do you know what year your box/arm is? (Just thinking of how the arm is attached to the splined output and if there are already some aftermarket arms that might help...)
    - What specifically are you trying to do? (Get rid of bump-steer? Clearance issues? Lengthen, shorten, offset, etc.?)
    I think we can do better than that...

  3. #3
    Sidewalk Spectator

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    I believe its off of a 56. splined on half the shaft and locked on with a bolt through. I do not have a box or column in my 39 Pontiac and it is a straight axle car which is proving to be difficult to find as all I can find is early independent stuff. This box fell in my lap and is still in very good working order so I do what any self respecting hot rodder should do....make it work

  4. #4
    Harvey's Front Row

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    Totally. So are you just looking to change the end so that it will adapt to the factory Pontiac draglink?
    I think we can do better than that...

  5. #5
    ♠ LBCC Nor Cal ♠

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    Quote Originally Posted by Tinman View Post
    There's a long history of hot rodders using F100 steering boxes in early Fords, and the use of dropped axles, suicide mounts, etc. occasionally requires that the pitman arm be heated and bent or, in extreme cases, cut and welded. The arm itself is forged steel and can handle modification if you've done your homework on keeping the heat to a minimum and using the proper technique.

    Yep, it's not uncommon. I've welded forged suspension parts a handful of times, it's easier than it looks. Weld prep is super important.

    You really want to maximize heat when welding forged mild steel, not minimize it. Especially if you're potentially welding together two different arms, it's going to be tough to keep it even without pretreatment. I'd highly recommend preheating with a torch & cutting deep v-grooves before welding to maximize penetration. The critical thing is how you cool it, don't quench it or it'll become brittle, let it air cool. Prep, Preheat, air cool, and you'll be golden.

  6. #6
    Sidewalk Spectator

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    I am going to have to modify to adapt to drag link for sure. Thanks for the input guys. Will let you know how it works out, or does not

  7. #7
    Harvey's Front Row

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    Just to clarify, I say "heat to a minimum" as a general guide to maintaining strength in critical (and the odd heat-treated) components. Off course you'll want the weld to be strong with good penetration, but excess heat will negatively affect the part - as when dropping an axle or bending steering/pitman arms, you want the part just hot enough to move, not firey orange with carbon pooling and flaking off...

    If you're adapting a new end to the arm, there are ways of attaching it that will maximize surface area and minimize shear forces prior to welding. Just use your noodle is really all I'm saying... the pitman arm is more than capable of handling a little surgery.
    I think we can do better than that...

  8. #8
    Sidewalk Spectator

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    I am hoping to get into the shop tomorrow night. as soon as figure out how to post pics, I will. thanks again!

  9. #9
    ♠ LBCC Nor Cal ♠

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    Quote Originally Posted by Sparky View Post
    I am hoping to get into the shop tomorrow night. as soon as figure out how to post pics, I will. thanks again!
    Excited to see your car come together. If you click "Go Advanced" at the bottom there's a Manage Attachments button, you can upload pics that way, or use the IMG code from an image hosting site. I should probably copy-paste this to the Board Feedback category, it's the same deal for our site as on this one: http://www.myth-weavers.com/wiki/ind...nts_and_Images

    I'd like to hijack your thread and geek out on metallurgy for a minute, I love this stuff. Y'all got me excited


    Quote Originally Posted by Tinman View Post
    Just to clarify, I say "heat to a minimum" as a general guide to maintaining strength in critical (and the odd heat-treated) components. Off course you'll want the weld to be strong with good penetration, but excess heat will negatively affect the part - as when dropping an axle or bending steering/pitman arms, you want the part just hot enough to move, not firey orange with carbon pooling and flaking off...
    Sperry, my friend, I agree with you if you're talking about heating and bending a part. You'd absolutely minimize the risk of dramatically altering the performance of a metal part by keeping the heat you use to deform it to a minimum. Welding, that's a different story.

    Anyone ever wonder why this is? Anyone interested in how to check?

    First you need to know what kind of steel you've got.

    I found this handy chart- I don't trust it 100% but it's a decent place to start:


    I'm going to assume it's like an axle so probably 1040 or lower. There's not a huge difference between heat treating low carbon steels, so 1040 is probably a decent guess.

    Cool! Ever wonder what that means but didn't care to google it? 1040 is the AISI/SAE code for steel

    10: plain carbon steel, no alloys
    40: .4% Carbon content.

    There are a bunch of other types: http://www.engineeringtoolbox.com/ai...em-d_1449.html

    Why is knowing that it's .4% carbon useful? The Phase diagram! I stuck together two cool charts:



    A phase diagram of steel shows the different phases of steel at different temperatures as a function of the carbon content. This is actually pretty useful in thinking through heat treating a pitman arm. The chart I added to the left is pretty cool too. It shows the different colors of heated steel at specific temperatures. This is pretty much independent of the type of steel (or material in general) because it depends on a fundamental property of matter: blackbody radiation! And all matter in nature emits something close to blackbody radiation, so cool- there's a way to measure temperature that doesn't require any tools besides your eyes, and this amazing fact of the universe is helpful in you building your car safely. Radiation is neat, J. Frank Parnell was right, most of what you think when you hear "radiation" is PERNICIOUS NONSENSE!



    Anyways, back to the chart. Take a look at where .4% carbon falls on the X-axis (hint: it's pretty close to .5). I drew a dotted purple line there to drive the point home. To keep this thread hijacking short just pay attention to the temperature that the purple line touches the black lines between phases. Sperry's scary sounding fiery orange? That's about 1600F, Austenite! Neat! I'll leave you to google it if you're interested in learning more about it. It doesn't really matter in this conversation exactly how different it is from the Pearlite & Ferrite we started with, but it matters a LOT that it is different, and local difference in material properties: potential for bad times. This Austenite is going to start growing as soon as the steel hits that transition temperature (somewhere around 1500F, a duller red color) and it's growth isn't going to be super different until the steel is heated to the next point the purple line crosses the black line (where it then becomes Austenite in Liquid). The color chart at that point ~2700F isn't useful any more, this is white hot, and just a few degrees past that at ~2800F it's all liquid. That liquid point is where you're welding at, at least for very brief periods of time. There are tons of other charts of varying complexity that can help you calculate annealing times, but they're not super useful in your garage. A fundamental understanding of what's going on is usually good enough. This chart can show you part of why Sperry is correct when it comes to heat treating to bend steel.

    Bending steel:

    Bending steel you don't need to get close to the melting point, in fact I'm willing to bet that fixtured sufficiently you could probably bend a pitman arm with the right press and no heat. That does a lot of other cool stuff to the microstructure, mostly work hardening, and is a whole other can of worms. Heating the pitman arm to bend it allows you to reduce the force necessary to bend it. By how much? A lot, I'm sure there's a chart for that out there somewhere. That's the crux of the issue with bending- how much can you heat steel to reduce work hardening without changing the microstructure of it, and where Sperry's advice on color comes from: avoid the fiery orange, and it's is pretty close to correct. If you look at the chart, you can see the purple line crosses the phase boundary at 1333F (at nearly every carbon content, interesting huh?). Look at the color chart and you see that correlates with a dull red color. Awesome! Now we can say what Sperry said with a little more confidence: To avoid changing the mechanical properties of the pitman arm significantly, we shouldn't heat the steel past the transition temperature at 1333F, so we shouldn't heat the steel much further than a dull red color. Hitting fiery orange for a short period of time is probably fine, but I'd avoid it.


    Things get a little hairy when you pass the transition temperature, and when you weld steel elevating it to a liquid phase is pretty much unavoidable (unless your shop is much MUCH fancier than most), so that's where the discussion of heat treatment and annealing comes in. Annealing steel is a pretty different conversation, there are a lot of other charts for that, but the basic idea can be summed up pretty simply based on the phase diagram. When you heat steel into a new phase it crystallizes differently. If you take steel back down from that phase quickly you'll have more of the Austenite phase left when you go back to low temperatures, and again, a local difference in material is a bad thing. Of course, sometimes you want to achieve this. All types of steel have their uses, sometimes you need it soft, sometimes you need it hard, but in the garage more often than not you need to end up with a part that had exactly the same material properties that it did before you modified it. That's one good reason to not quench welded steel, any why I advice air cooling: you are trying to end up with exactly what you started with and taking it slow gives you the best shot at achieving this..

    Welding steel

    When you weld steel you do so into a puddle of molten (liquid), and carbon doesn't like to stay in that puddle. Despite your best efforts you'll have carbon boiling out of it no matter what. That sucks. That region is now heat affected, the zone of which is called the Heat Affected Zone (HAZ). A HAZ is important for a number of reasons- it changes the carbon content locally, it changes the microstructure, and changes the hardness. ANY one of these things being different locally on a weld joint can be bad news, and can mean weld failure. That's why you'll sometimes hear welders talk about the heat line- it gives you an idea of how even the metal was heated, welded, and cooled, and the more even the HAZ or heat line the more even the weld. Take a look at this string of welds, and pay attention to the heat line (the blue area around the weld):


    See how uneven it is? You can clearly see where the weld started and stopped, you can see where the weld got too hot, and you can see where the weld was even. This bluing gives you an idea of how potentially different the microstructure of the metal is near the weld along the joint. If you did a bend test on this I'd assume it was going to crack at the region with the most difference in HAZ (looks like 175 to 150 to me). Anyways...

    A good way to minimize local changes to the microstructure when welding is to minimize the rate of heating up or down. On a big chunk of forged steel your welder probably won't be enough on it's own to heat evenly. That can be done easily by preheating and post annealing. Again, there are charts out there, but the basic idea is to either increase the time it takes to get to a certain temperature or reduce the difference in temperature for a fixed time, both ways you are trying to minimize how much you change the temp as a function of time.

    I think this is super interesting, and people have spent their lives studying this, but the basic thing to remember here is that again, any localized difference in a material's mechanical properties can lead to bad times. If you're going to heat steel to bend it, stay out of the transition zone Think about the color chart to know where you're at with temperature. If you have to weld steel on something critical like a steering arm, be very conscious of how you cool it.
    Last edited by Nic; 01-13-2016 at 03:45 PM.

  10. #10
    ♠ LBCC High Priest ♠

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    That was a fascinating post Nic.

    But I must admit that the primary thing my abstract-thinking brain came away with was that someone needs to build a car called blackbody radiation.
    Tradition didn't end in 1964

  11. #11
    Harvey's Front Row

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    Quote Originally Posted by Rasputin View Post
    ...someone needs to build a car called blackbody radiation.
    Ha... a good take-away.

    And Nic, there are few people I enjoy setting the stage for more than you... go get 'em, Professor!
    I think we can do better than that...

  12. #12
    Sidewalk Spectator

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    Thanks for the hijack! Very useful info. Took me back to 3rd year in school when we did metalurgy. You made me dig out all my scool books and revisit it all. I still dont have it put together yet though....

  13. #13
    Carlow
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    Holy shit Nic!, this is some serious content!

    Something I should have probably looked into before the last Map gas blueish/red color that I thought was the only appropriate color for bending..

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