SBM Main Cap Material

Small Block Mopar Engine

  1. gzig5

    gzig5 FABO Gold Member FABO Gold Member

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    I have the "opportunity" to upgrade the main caps on a 340 block I was hoping to build, pending sonic checks. Opportunity comes from the main caps not being original to the motor, so they will need to be bored/honed to fit and they are also 318 caps as far as I can tell so i might as well put a "stronger" cap on while I'm doing it. My build goal is 550+ hp with a 4" crank and I don't think I feel comfortable with the supposedly inferior 318 cap which is supposed to be not as thick. So the question is, what material to use? The ductile caps a FABO member was selling are out of stock so I will be making my own. I've heard of folks using aluminum on big blocks, as well as alloy steels of various types. I can't recall hearing of them being used on a SBM and the only commercial caps I've seen referenced are ductile cast iron. I can get ductile cast iron as well, but it will probably be more expensive for the material than 4140 steel or 7075 aluminum. Regular grey iron probably isn't appropriate but I haven't verified the strength numbers on it. I don't think my HP goal is out of line for any of the three materials so what do I choose? Aluminum is easy to machine. Ductile iron would have same thermal properties as the block, but does that matter for something that will be street driven during the week and road course stuff every few weekends? Steel is the strongest. I tend to overthink these types of things. Depending on how complicated it is to make, I may end up using the stock rear cap and I've been told that is acceptable. Looking for whatever insight the group can provide.
     
  2. yellow rose

    yellow rose Overnight Sensation FABO Gold Member

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    Where are you getting ductile iron caps? I can’t find any. I wouldn’t use aluminum on a small block. You can find billet caps put there, but they ain’t cheap.
     
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    • gzig5

      gzig5 FABO Gold Member FABO Gold Member

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      Member Rocket had a couple sets for very reasonable price. I hemmed and hawed too long and they are gone now and I'm not sure there will be any more. Making the main caps is not rocket science, the shape is simple to do in the mill and there are only a couple dimensions that need to be very accurate. They can be bored in pairs in the lathe or stacked up in the mill vise. Haven't found a set of orphaned 340 caps yet for reasonable money. No matter what I put on, they will require a line bore and hone and to make it a viable proposition I need to keep the price of the caps down or it makes more sense to get another block. Any of the other aftermarket caps would put me out of range of sensible budget. I can get a hunk of 4140 for about $75 or less and my time is free to me.
       
    • abodyjim

      abodyjim Well-Known Member

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      I had sd concepts make me some. They are steel and was $400 + shipping

      20210131_105249.jpg

      20210131_105007.jpg
       
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      • gzig5

        gzig5 FABO Gold Member FABO Gold Member

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        Those look nice. Pretty much what I plan to do. $400 isn't a bad price for something that is essentially a one off. If you wanted to sell a bunch they could be done a little cheaper.
        Are they in the motor now or can you get a couple measurements for me?
         
      • brian6pac

        brian6pac Well-Known Member

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        I think Rockets are around $250 if I remember correctly.
         
      • 1Badcolt

        1Badcolt Well-Known Member

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        Pro-Gram engineering
        did u check this company out ?
         
      • yellow rose

        yellow rose Overnight Sensation FABO Gold Member

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        They don’t have any.
         
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        • brian6pac

          brian6pac Well-Known Member

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          They are just down the road a ways from me in Barberton.
          Ken sold out to one of his employees a couple years ago.

          About Us

          Billet SpeedWorks
           
        • yellow rose

          yellow rose Overnight Sensation FABO Gold Member

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          I understand how to make a main cap. The question is is it worth it. My answer: nope.

          I’m stunned how many blocks don’t have caps. Who looses caps?

          At any rate, unless it’s a numbers matching 340 block or a race block making caps isn’t worth it to me.
           
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          • 1Badcolt

            1Badcolt Well-Known Member

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            U nailed it !
            Who losses caps ?
            I guess retards [many motorcycle owners loose titles also] never could understand how that happens
             
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            • 1Badcolt

              1Badcolt Well-Known Member

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              I bought a set yesterday for future 360 [$230] shipped
               
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              • Garrett Ellison

                Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                Has Milodon ceased production of their caps? Their website is dated 2017 Milodon Main Caps
                 
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                • Garrett Ellison

                  Garrett Ellison Amateur driver on public roadway, do not imitate.. FABO Gold Member

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                • abodyjim

                  abodyjim Well-Known Member

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                  There was some delays in shipping and I missed my window at the machine shop so they are still on the shelf.
                  Let me know what you want and I'll grab some measurements
                   
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                  • gzig5

                    gzig5 FABO Gold Member FABO Gold Member

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                    For someone doing it for business or whose spare time is critically limited, I would agree with you. But in my case I love jobs like this. It’s part of the reason I have the machines in my garage. I got this standard bore block cheap enough that even with material and line boring costs I should be into it less than $500 all in ready to drop the crank in. Pretty hard to find a decent 340 block for that anymore.
                    If you had seen the shop this block came out of you would understand how the caps could get lost. It was a complete disaster two-stories deep.
                     
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                    • Bewy

                      Bewy Well-Known Member

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                      Alum main caps? And with a stroker crank? I would use factory caps before I used alum.
                       
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