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Narrowing Main Bearing Caps

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  • Narrowing Main Bearing Caps

    I found a standard bore 340 block with the correct date code for my 73 Plymouth Cuda. I got it cheap, but the down side is the main caps are from another, later motor. That means it will have to be line bored, which is fine, but I need to get these caps to fit the block. The block has a recess for the cap to set into and the caps are about .005 too wide so I need to remove about.003 off each side so they are a snug fit. I'm assuming they need to be a snug fit. That's a little much to stone off. Trying to figure out how to fixture the five caps together so I can do them all at once but I'm struggling. Only thing I can come up with is to stand them on end on a thin parallel in the notch, but the vise jaws would have to be 5" high. Originally, they were cast and machined in one block and then cut into individual caps. Appreciate any suggestions and if I have to do them one at a time, it is what it is.
    edit...the bores line up better with the cap flipped 180.
    Attached Files

  • #2
    There are aftermarket caps available but they need to be machined as well. If I can use these, I'll save ~$400.

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    • #3
      My Mopar had pressfit caps. I'd guess at least .002 interference, maybe more..........I could not get them in; had to use the main bolts to pull them down. And they were the originals!

      I'd be reluctant to make them a slip fit.

      Mike

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      • #4
        Your right about needing it line bored due to the caps. However there is no point in you doing it. The machine shop that will do the line boring has a machine called a cap grinder that they use to adjust the fit of caps. Before they line bore they will have to fit your caps. After boring will come honing where the caps may have to be fine-tuned to achieve the proper clearance. Most times caps need a little tuning/grinding to bring them all into dimension.

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        • #5
          They have to fit very tightly as mentioned or the caps will "walk" at the first sign of stress
          Also as mentioned above get them ground when line bored. Find a reputable automotive machine shop to do this.
          Home, down in the valley behind the Red Angus
          Bad Decisions Make Good Stories​

          Location: British Columbia

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          • #6
            Allot of them go in and out with scoring they are so tight, I would not take anything off unless there's scoring on both pieces then stone down the highs and beat em in for align boring, use anti-seize on the surfaces so you don't have to stone them too much again when going back in - they should be tight as all hell but not too tight to install or totally distort the cap...

            last but not least - get it close - it's a dinosaur and will go slow no matter what you do, kinda like a harley... but not quite as bad...

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            • #7
              Originally posted by gzig5 View Post
              I found a standard bore 340 block with the correct date code for my 73 Plymouth Cuda. I got it cheap, but the down side is the main caps are from another, later motor. That means it will have to be line bored, which is fine, but I need to get these caps to fit the block. The block has a recess for the cap to set into and the caps are about .005 too wide so I need to remove about.003 off each side so they are a snug fit. I'm assuming they need to be a snug fit. That's a little much to stone off. Trying to figure out how to fixture the five caps together so I can do them all at once but I'm struggling. Only thing I can come up with is to stand them on end on a thin parallel in the notch, but the vise jaws would have to be 5" high. Originally, they were cast and machined in one block and then cut into individual caps. Appreciate any suggestions and if I have to do them one at a time, it is what it is.
              edit...the bores line up better with the cap flipped 180.
              Line boring isn't going to make that cap and block in the photo work together. It's waaay too offset to one side. Unless it fits closer
              turned around.

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              • #8
                Okay. In a previous life, I worked in a automotive machine shop. The engines I align bored were mostly Diesels. We fit replacement (service) caps, tightened loose ones, Installed used ones, and once I fit up a rear half replacement block on a 16v71 Detroit Diesel. The machine shop you take this to will have very definite ideas about how the caps need to be fit. Take it to them and pay the bill when they're done.
                I cut it off twice; it's still too short
                Oregon, USA

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                • #9
                  I've built and raced a lot of 340's. Do not turn the caps around. If the cap you have pictured is sitting in its correct direction, it will never fit in that block. Find another set of caps most 318's and 340's use the same caps.
                  I almost, always used aftermarket main caps.

                  THANX RICH
                  People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!

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                  • #10
                    Do not use 273-318 main caps.
                    They will fit but are much shorter and weaker. You will need shorter main cap bolts and a healthy 340 will toast them quickly if you make use of it's power.

                    Like a set of speedos, just because they fit doesn't mean you should wear them.

                    Edited to add: A good set of splayed Milodon 4 bolt mains are a wise investment for a hot 340 and can be installed at home. They have the benefit of of using a much thicker and stronger portion of the block for the splayed bolts. Plus their design locks the cap ends into the block's recess thus eliminating the dreaded cap walk.
                    Last edited by Willy; 05-08-2020, 10:34 AM.
                    Home, down in the valley behind the Red Angus
                    Bad Decisions Make Good Stories​

                    Location: British Columbia

                    Comment


                    • #11
                      Originally posted by v860rich View Post
                      I've built and raced a lot of 340's. Do not turn the caps around. If the cap you have pictured is sitting in its correct direction, it will never fit in that block. Find another set of caps most 318's and 340's use the same caps.
                      I almost, always used aftermarket main caps.

                      THANX RICH
                      As I mentioned, the pic was taken with the cap rotated 180. I got in a hurry. It lines up reasonably well with the right orientation. I didn't realize they would be would be such a tight fit. I'll leave them be and work with the shop I choose. I just wanted to make sure these are viable. I'm leaning towards an aftermarket set of caps but if I didn't need to go through the extra expense, I'd put the money towards something else. This is going to be a stroker and targeted around 550hp so I'm probably past the point where these 318 caps would be comfortable. 340 caps are a little thicker than 318s and the aftermarket are thicker still.

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                      • #12
                        Originally posted by Willy View Post
                        Do not use 273-318 main caps.
                        They will fit but are much shorter and weaker. You will need shorter main cap bolts and a healthy 340 will toast them quickly if you make use of it's power.

                        Like a set of speedos, just because they fit doesn't mean you should wear them.

                        Edited to add: A good set of splayed Milodon 4 bolt mains are a wise investment for a hot 340 and can be installed at home. They have the benefit of of using a much thicker and stronger portion of the block for the splayed bolts. Plus their design locks the cap ends into the block's recess thus eliminating the dreaded cap walk.
                        My understanding is that the four bolt or splayed caps are really only viable on an X or R aftermarket race block which have solid pan rails and thicker webs. The stock 340 motors don't have a lot of meat where the extra bolts go so they don't add much security. Four bolts would be a bit overkill on a stock block. Mopar small blocks already use 1/2 main bolts so they are stronger than other makes out of the box.

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                        • #13
                          https://www.manciniracing.com/mimacapki1.html
                          Home, down in the valley behind the Red Angus
                          Bad Decisions Make Good Stories​

                          Location: British Columbia

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                          • #14
                            I ran an automotive machine shop for years and we did fit main caps to blocks where they didn’t match either to repair damage or as an upgrade. There are a number of ways to do this and I agree with the above comments to find a reputable shop used to doing this and, yes, pay the price. The stock caps shouldn’t be an issue if you’re just generally rodding around with the vehicle Friday night for a bit of fun. If you are going to be serious with it, an aftermarket set of billet caps is more than highly advisable. The cost won’t likely be much different than fitting stock caps.
                            The cap reversal shown in the picture is a BIG no-no.

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                            • #15
                              I'm not a Mopar guy, but have always bought and paid for the best internal parts on my Ford engine builds since if/when internals fail, it is REAL expensive. If it were my engine, I'd spring for the extra cost of high quality caps that fit.

                              Its been a lot of years, but when I was building, my machine shop guy always fit the caps, the torqued them down, then line bored. Never had a main fail, but wonder whether he had it right to fit and torque then bore?
                              S E Michigan

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