how far can one turn down a crank?

Big Block A body Tech

  1. Cope

    Cope Fusing with fire

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    My buddy had some hack build him a motor (500 CI) and it spun a rod bearing right out the gate....
    He gave me the crank. It's a forged 440 source 500 cubic inch stroker.
    It don't look to bad, I would like to know haw far I can have it turned before it's junk?

    Should I have all the journals turned or just the one?

    Thanks again for all your time and advice.
     
  2. cudascott

    cudascott wish I had a Hemi

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    Common practice is to turn all journals to the same dia.
    If the mains are good leave as is, .010 is common when turning the journals Anything more than that it's pretty much junk as far as I'm concerned.
     
  3. 67Dart273

    67Dart273 Well-Known Member

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    The "technical" answer is "whatever you can find bearings for." I believe .030 is accepted, some cranks maybe older engines may have had .040

    For some special "stroke" builds, different rods, etc, some cranks have been turned a LOT more than that. Found this post in a similar discussion:

    "Before 350 journal sized stroker cranks were available, it was common practice to turn a SBC 400 crank .100" under when building a 383".

    And someone corrected:

    "Nope. Turned .200, not .100.

    400 main size is 2.65", 350 main size is 2.45". .200" different"
     
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    • Cope

      Cope Fusing with fire

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      Yeah the mains are fine. I'm gonna drop it off at the machine shop and see what they say. I just wanna have an idea of what's ok and what's not.
      Thanks.
       
    • Marcohotrod

      Marcohotrod Well-Known Member

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      machine shops say a lot of various stuff. cranks are heat treated to a variety of depths. many 440 cranks have been ground down to fit in 383-400 blocks. I would not be afraid to grind only 1 journal. as for surface hardness-depends on how hard she is going to be abused, and in a good world there is a layer of good oil for protection.
       
    • RustyRatRod

      RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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      You can get bearings for most popular cranks all the way to .060 and more on special order.

      People have been turning 440 cranks down to go in B motors for decades. Plus, it's been common practice to offset grind and turn the rod journals down to the Chevy size for decades too. I guess they are junk too.
       
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      • Double M

        Double M Legandary Member Legendary Member

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        It's .200 different, therefore you cut .100. When you cut .100, you're cutting .100 per side, which makes it .200. Think about it.

        Russ.
         
      • RustyRatRod

        RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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        While you are correct, that's not how it's measured. Chuck it up on a crank lathe and turn it .100". All you got is .100". You don't measure diameter "one side at the time".
         
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        • 72bluNblu

          72bluNblu FABO Gold Member FABO Gold Member

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          You guys are making my head hurt.

          What you set the lathe to and what you actually end up with are different by a factor of 2. But like Rusty says you measure the whole diameter, and that's how it's defined. A crank that's .100" undersize means the machinist set his lathe to remove .050". In this case the crank is .200" under so the machinist set the lathe to remove .100", but that's not the size. It's turned to .200" undersize.

          As far as how far you can turn them, really the only limits are getting bearings and the strength of the crankshaft. If you're building big horsepower you don't want to turn a huge amount because the strength of the crankshaft will come into play at some point. But on a forged crank that's a different amount than a cast crank, the forged crank is stronger to begin with. The only other thing to consider is how deep the hardening of the bearing surface goes. Again, if you're building big horsepower you might want to consider having the bearing surfaces re-hardened if it needs a big cut. But even that will start plenty of arguments. If you're turning it .030" or less you don't have to worry about anything. And if you're not racing you can go a lot further than that.
           
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          • MOPARMITCH

            MOPARMITCH Mekong Gold Member FABO Gold Member

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            I used to sell .040 and up to .060 bearings , back in the day. No Problem !!
             
          • RustyRatRod

            RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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            And you made my head hurt, but you splained it better than I did. Thanks.
             
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            • Cope

              Cope Fusing with fire

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              Cool, like I said it don't look to bad so I think it will clean up.

              Next question is can I use my LY rods or do I need a special rod?

              With the crank being free I'm not afraid to spend some cash to get the stroker crank in...
               
            • RustyRatRod

              RustyRatRod 30 Degrees Outta Whack FABO Gold Member

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              Measure a good rod journal and see. It's a very popular mod to grind or have the rod journals made to the big block Chevy size. If they are the Mopar size the the LY rods will work fine.
               
            • Double M

              Double M Legandary Member Legendary Member

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              Ok, I see how you guys are translating it. No problem.

              Russ.
               
            • AndyF

              AndyF Well-Known Member

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              Measure the rod journals to see if they are Mopar size 2.375 or Chevy size 2.200. Once you know that then you can look for bearings. You can turn the crank to whatever you need to as long as there is a bearing that works.
               
            • Cope

              Cope Fusing with fire

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              Next question, how will a stroked crank affect my pistion choice?
              Thanks again.
               
            • AndyF

              AndyF Well-Known Member

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              There are plenty of stroker pistons available. You just need to know what stroke your crankshaft is and which rods that you are going to use and then you can pick the piston.
               
            • famous bob

              famous bob mopar misfit

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              agree w/ andy on this one, he beat me to it .
               
            • barbee6043

              barbee6043 barbee 6043 FABO Gold Member

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              also check out the quality of the origin of that crank. some say 440 source stuff comes from China. maybe not an issue.
               
            • Cope

              Cope Fusing with fire

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              [​IMG]
               
            • Cope

              Cope Fusing with fire

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              Dammage..

              [​IMG]
               
            • AndyF

              AndyF Well-Known Member

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              Just need to take it to a shop and have them check it for cracks. Sometimes the cranks will crack if they get hot enough to turn blue. If it isn't cracked then the shop can turn it down until it is good again. You should be able to get undersize bearings without too much problem. Not really a big deal since the guy gave you the crank. If you can fix it then you're good to go, if not then you aren't out much.
               
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              • IQ52

                IQ52 Well-Known Member

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                At first glance, if that's my free crank I take it to my machinist and have him grind it to a 4.250 stroke with a Chevy 2.20 journal. Use a 7.100 Chevy rod and a Mopar piston with a .990 pin and about a 1.485 compression height and I'm at 505-512ci.
                 
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                • Cope

                  Cope Fusing with fire

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                  Thanks for the crack checking idea, i had not thought about that.

                  Im looking for a shop that turns cranks around the san jose, Ca area.

                  The machine shop i use is cheep but i dont trust em with much.....


                  Thanks IQ. I know you know your chit and value your reply highly.
                   
                • Cope

                  Cope Fusing with fire

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                  Just running numbers through a calculator this postion with my specs gives me 10.746 SCR

                  http://m.summitracing.com/parts/uem-ic828-030

                  Thats if this crank turns.

                  Thoughts??

                  This is a track motor that will see a few beer runs...

                  Thanks again!
                   
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