Will Rod swap require balancing?

Small Block Mopar Engine

  1. C2ndLTpigeon

    C2ndLTpigeon Mopar or no Car! FABO Gold Member

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    Im building a 318, std bore stock style pistons, will using floating pin 340/360 rods in the 318 require that I get the engine balanced?
     
  2. moparmat2000

    moparmat2000 FABO Gold Member FABO Gold Member

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    Im no expert, but it probably depends on what they weigh compared to what was in there. If they are close in weight, and your not spinning it above 5k probably work just fine.
     
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    • Locomotion

      Locomotion Well-Known Member

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      It's a gamble. BUT......since you are rebuilding the engine and using "stock style pistons", which sound like aftermarket, you will probably need to rebalance the whole rotating assembly anyway. You might get lucky there too IF the weight is the same. But even if the overall rod weight is the same, the ends may not weigh the same! Piston or either end of the rod, the further away from the crank centerline it is, the more the weight difference will affect balance.
       
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      • RustyRatRod

        RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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        If you want it dead right, yes.
         
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        • belmateo

          belmateo Well-Known Member

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          You will be much happier with the outcome if you spend a little extra to get the rotating assembly balanced.
           
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          • toolmanmike

            toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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            340/360 rods are heavier than 273/318 rods. 273 pins are heaver to compensate for the difference in 273/318 piston weight. So, yes it will throw the balance off.
             
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            • Mopar44134

              Mopar44134 Well-Known Member

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              Depends on the weight. I bought a harbor freight scale...searched you tube on how to balance rods at home. 360 non bushed rods i weighed were around 758 grams. I haven't weighed 318s.
               
            • toolmanmike

              toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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              Rods are weighed and balanced on each end and not the rod as a whole. You will also need a balancing fixture.

              rod balance.jpg
               
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              • 66fs

                66fs FABO Gold Member FABO Gold Member

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                It depends on what year 318 you have. Newer 318's used the same rod forging as 340's and 360's. You can compare rod forging numbers. Past that It is worth the money to have your engine balanced, since it runs smoother and will last longer. You do not need a fancy fixture, I used a home made tool to balance rods, I'll try to take pictures next week. The ends need to be parallel, and as a check the ends should add up to the total weight of the rod.
                 
              • Ottmundr

                Ottmundr 68 Fastback

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                • JDMopar

                  JDMopar Well-Known Member

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                  If you're going to have to get the rods reworked and buy new rod bolts, you can buy brand new Scat rods cheaper than you can get the old ones redone. I bought new Scat rods that were end balanced and fitted with new ARP rod bolts for $331. I'm getting the rotating assembly for my 273 balanced, and feel that it is well worth the money.
                   
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                  • moparmat2000

                    moparmat2000 FABO Gold Member FABO Gold Member

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                    I bought new Eagle forged I beams with ARP waveloc 2000 bolts and bushed little ends for full floating pins for $301 shipped off evilbay. You cannot recondition the originals for that, and still have a weaker rod. The eagle I beams are rated for 500hp, and 6,000rpm. I plan on reusing the stock 318 cast crank, and using KB167s. Bec as use of the piston and rod changes, I will be getting it all balanced.
                     
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                    • toolmanmike

                      toolmanmike FABO MODERATOR Staff Member FABO Gold Member

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                      I think so too.

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                      • moparmat2000

                        moparmat2000 FABO Gold Member FABO Gold Member

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                        Eagles. Mine were $301 with sales tax

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                        • pishta

                          pishta I know I'm right....

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                          Why not get stock 273/318 floaters? they are already in the ballpark of the 318 castings (same) and wont require a balance to get it back to factory specs. The 318 pins in the pistons you have are not the heavy wall 273-2 pins. Factory specs were a bit loose to facilitate production so you dont have to be spot on to get it back. As for weighing rods, its pretty easy to weigh the rods split on a standard gram scale. The fixtures obviously get you a big end/little end weight on both ends but since the balance pads are at the extreme ends, weighing the split rod parts on a scale is very close to actually jigging them up as reducing weight on either end is not likely going to produce any noticeable deviation on the other. The lever effect of a larger balance weight on the big end effect on the little end in a fixture is so small to the <6000 RPM street motor. It is very hard to get repeatable weights on a jig wiithout a dedicated scale as the sensors on a cheap scale are 4 corner and any deviation of the angle or even the placement of the fixture on the scale will give you a different weight +- 2 grams. I think the $$$ professional machine shop balance scales are center weighted to provide a single sensor and will not deviate if the load is slightly off center. Mark the mating pairs and get all the caps to the lightest weight and then repeat with the little end side. The bobweight data the jig provides is for the balancers sheet but your only interested in getting the rods equalized to the original rods. they are probably within 5g already if they are stock parts.
                           
                        • C2ndLTpigeon

                          C2ndLTpigeon Mopar or no Car! FABO Gold Member

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                          The engine is a 1973 cast year 318. Upon further inspection the rod and piston assembly that I pulled out have the part number 3418645. The rod set that I planned on using that have the bushing for a free floating wrist pin have the same part number 3418645. The original rods with a pressed pin have a back number of DF116 also MS 304. The ones with the free floating wrist pin say DF56 as well as MS 304. Are these in terms the same Rod other than one being a pressed pin and the other being bushed for a free-floating pan?
                          The Piston set I am using are stock style sealed power cast pistons.
                          The engine has a will have a hughes whiplash cam and will not see constant 5000+ RPM its a daily/street car.

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                          • 66fs

                            66fs FABO Gold Member FABO Gold Member

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                            Should be close then.
                             
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                            • pishta

                              pishta I know I'm right....

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                              any casting can be bushed. your good. Cast replacement pistons (ie. Badger) are usually within specs in weight as OEM pistons, even if oversized.
                               
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                              • Ottmundr

                                Ottmundr 68 Fastback

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                                Before I go off topic here, I think I paid either $250 or $350 for some Scat rods about 2 years back, from a member here.

                                You're in Indio? Waaaaay out in the desert, with loooong stretches of road? You're going too slow. :poke: How else do you expect to catch them Roadrunners? We can't let them B bodies have all the fun. Beep!Beep!

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                                g%2Fsites%2Fdefault%2Ffiles%2FGreater%2520Roadrunner_KK_APA_2011_20015_158198_ChristopherSchwarz.jpg
                                 
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                                • C2ndLTpigeon

                                  C2ndLTpigeon Mopar or no Car! FABO Gold Member

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                                  The 318 setup is not as bad as I thought it would be. Anything is better than the dodge Neons I was into before I got into older cars. Matter of fact I drove my 03 Neon RT the other day, (my summer car since it has AC lol) and such a disappointment. But maybe down the line im might put a 383 in the Valiant, it just takes money :)
                                   
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                                  • C2ndLTpigeon

                                    C2ndLTpigeon Mopar or no Car! FABO Gold Member

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                                    That's what I figured, I just wanted to be sure. Thanks!
                                     
                                  • pishta

                                    pishta I know I'm right....

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                                    skip the 383, get a warm 360 with big valve heads and don't look back...at the 383's.
                                     
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                                    • 66fs

                                      66fs FABO Gold Member FABO Gold Member

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                                      The 383 will be in front of you...
                                       
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                                      • pishta

                                        pishta I know I'm right....

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                                        Even stock J heads will outpump 383 heads. Add a cam and headers and a 360 will be way up from a 383 with actual headers that fit. I think an LA/Magnum is a much better fit than a B in an A.
                                         
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                                        • Ottmundr

                                          Ottmundr 68 Fastback

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                                          :lol:
                                           
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