Piston Pin Material - Unexpected

Discussion in 'Small Block Mopar Engine' started by nm9stheham, Nov 24, 2018.

  1. nm9stheham

    nm9stheham Well-Known Member

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    Nothing earth shattering here, but this was interesting and also perplexing: As part a little project to modify pin weights, a spread sheet was set up to compute wrist pin weights from volume and material density. EDITED to correct for a spreadsheet error.

    The first check of this spreadsheet was for factory 273 2 BBL pins. Measure the pins' dimensions, plug those into the spreadsheet, use a standard density for tool steel of 7.85 gr/cc, and it spits out a weight 0.25% off from what was actually weighed.... well within the scale's accuracy; so far so good. EDIT: Corrected to 7.78 gr/cc.

    Next was some small Chevy pins from a modern V6. Put in the numbers and WTF..... the pin material density had to be reduced to 6.6 gr/cc to get the spreadsheet weight to match the actual weight. ...???? EDIT: Corrected to 7.78 gr/cc...... normal!

    The last check was for some 340 wrist pins. Now, the materials density has to increase to 8.55 gr/cc to get the spreadsheet to agree with the actual pin weight. EDIT: Corrected to 8.03 gr/cc......

    The scale's calibration has been checked in 25 gram increments and that looks right on the money. Can't see/or find any inside tapers in the pins to upset the dimensional computations. So the pin material's densities appear to vary....

    For the 340 pins, the materials that match close to the density found are high nickel and/or chromium steel.

    Pix of pins, L to R: 273, 340, Chevy V6

    DSCN2522.JPG
     
    Last edited: Nov 30, 2018
  2. 451Cuda

    451Cuda Well-Known Member

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    Makes sense. Chevy tends to use cheaper materials and wall thicknesses, usually just barely on the safe side of what's needed for an application. Chrysler was always over-engineering driveline parts, especially in the '50s and '60s. If you want to see something even more over-engineered than Chrysler, look at the nickel content of AMC blocks - it wrecks tooling if the machinist isn't careful.
     
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    • pishta

      pishta I know I'm right....

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      273-2 piston pin is a heavy wall to emulate the 318 polys piston bobweight as the crank was basically the same casting. Just measured a generic chevy size pins weight and area. The pin weighed 140.7g and the displacement was 16ml of water, so density of this pin was 140.7/16= 8.793g/cc

      ha, that aint right. let me try something else....
      .927 width*3 inch tall -.627 hole*3 inch=1.06 square inch total, weighing in at 140.7g...17.86cc= 7.877g/cc
       
      Last edited: Nov 24, 2018
    • Marcohotrod

      Marcohotrod Well-Known Member

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      good info. you are correct, the density of steels varies. there are dozens of different grades of ferrous alloys. varying percentages of iron, chrome , vanadium, molybdenum, nickel, carbon etc.
       
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      • nm9stheham

        nm9stheham Well-Known Member

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        Tnx very much .....that is even further outside of the range of standard steels than the 340 pin. How did you measure the displacement? If you wold care to share the dimensions, I'll put that into my spreadsheet and compare the results. 16 ml seems in the 'typical' range for volume.

        Yes, but 6.6 is way outside of the range of common steels; that is the one I'd like to figure out.
         
      • pishta

        pishta I know I'm right....

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        specs are correct, but I was half in English and half in metric. Its 1.09 inch/3 that converts to 17.8619cc. 140.7g/17.8619cc=7.877 g/cc pin material. I was 1cc off in my liquid displacement test rig...(it was a 10ml graduated baby bottle filled with blue water!)

        Pistons were a Chevy no-names that were given to me thinking they were 318's! I saw a huge pop up on them and thought they were something special.

        for volume, I took the cylinder volume then subtracted the hole volume. there is a very slight 45 bevel on the pin end, maybe .040 if you want to further drill down on the mechanical volume of the pin, subtract a .040 X .040 X .927 diameter ring.
         
        Last edited: Nov 24, 2018
      • famous bob

        famous bob mopar misfit

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        And people wonder why chevys blew up so much back then.
         
      • nm9stheham

        nm9stheham Well-Known Member

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        Ah OK... 7.877 gr/cc is right close to the density number for tool steel and right in the range for 'standard' steels. Makes sense for plain ol' Chevvy pins. Many tnx!
         
      • MOPEkidD-3

        MOPEkidD-3 Torsional Member

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        Sadly I don't think enough people even know Chevies used to blow up all the time or there wouldn't be so many Chevy bozos nowadays. The LS craze doesn't help either.
         
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        • pishta

          pishta I know I'm right....

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          More people choke on hotdogs (Chevies) than any other food...why? Because so many people eat hot dogs (Chevies)!!!! Take 100 360s, 100 350s, and 100 351Ws...run them 100K at 5000 RPM. What percentage would fail from each pool?
           
        • nm9stheham

          nm9stheham Well-Known Member

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          Edit to correct this info!

          Ooops....I found a mistake in my spreadsheet.... It's been corrected in the original post and now the densities makes a lot more sense.

          273 pin - 7.78 gr/cc as originally found
          340 pin - 8.03 gr/cc...... probably a high nickel or chromium steel
          Chevy V6 pin - 7.78 gr/cc..... a tool steel or similar

          Please accept my apologies for the misinformation!
           
          Last edited: Nov 30, 2018
        • Marcohotrod

          Marcohotrod Well-Known Member

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          similar to a forged steel crank is heavier than a cast iron=why the '72-'73 340s with cast cranks were externally balanced