Custom Engine Build

Discussion in 'Engine, Fuel and Exhaust' started by lesr389, Sep 28, 2015.

  1. lesr389

    lesr389 Member

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    Hi my name is Les, I have been trying to research my bike for quite a while. If anyone has some input please comment. my engine is Frankensteined together. My cylinders are S&S 124" and heads are 120R screamin eagle. The crank case is original, bit in order to make this work the crank case had to be modified. I am trying to find out how a crank case can be modified to accept different rods (ie: bigger diameter piston and/or different rod). If I can understand how to modify a crank case maybe i can understand how my engine was built.
     
  2. Tramp60

    Tramp60 Member

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    Someone correct me if I'm wrong but to accept a larger cylinder they the bore the cases to accept the larger diameter cylinder. If your motor is stroked, which means a larger diameter fly wheel assy. to increase the length or distance of your piston travel. Then they have to bore the cases to accept the larger dia. wheels. I however, do not know how much room is in the cases of the twin cams and how much larger diameter a wheel they will accept without boring them out. I hope that gives you a better understanding of what you have and how it works.
     
  3. Breeze3at

    Breeze3at Well-Known Member

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    As stated above, the cases must be bored for the larger (4 1/8" bore) cylinder spigots to fit. The crankshaft has the same outer diameter wheels, but the crank pin is located closer to the edge to get the longer 4 5/8" stroke. The new cylinders are taller to accommodate the stroke. No case mods for the crank.
     
  4. Jack Klarich

    Jack Klarich Expert Member

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    Unless you are building a monster motor your cases should be OK, after market cases have raised deck height and are made from better material
     
  5. lesr389

    lesr389 Member

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    I believe my case was bored to accept the 4 1/8 piston. After this is done will that change the stroke? Am i right to assume the stroke will be the same as the 124" engine the cylinders are made for.
     
  6. Breeze3at

    Breeze3at Well-Known Member

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    No. Bore and stroke are independent of each other. A 4 1/8 bore can result in different displacements based on the stroke 117" 120" & 124" that I know of. Here's some examples of 4.125 bore with different strokes.
    Cube = Bore x Stroke
    124 = 4.125 x 4.625
    120 = 4.125 x 4.500
    117 = 4.125 x 4.375
    Harley 96", 103" and 110" all use a 4.375 stroke crank, with the largest bore being 4." So, if you DO have a 4 1/8" bore, you almost certainly have a longer stroke crank.
    My head hurts from all these numbers. :bigsmiley22: And there are combinations of bore & stroke that I don't have any idea about.
     
  7. dolt

    dolt Senior Member Contributor

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    Not really a "Frankensteined" motor. If the bike is the '07 FLSTN in your avatar, it's a pretty straight forward combination. Your cases were bored, as has been pointed out, to allow the larger diameter cylinder spigots to fit; most likely the left side case was machined to replace the left side crank bearing with a tapered Timken bearing and the piston oilers were probably changed out as well.

    All TC rods, excepting the SE120 rods are the same length; 7.667" for all 88", 96", 103" and 110" TCs but 7.575" for the SE120 rods. As Breeze3at points out, the stroke increase comes from placement of the crank pin in the wheels and all TC wheels are the same diameter. Do you know if the crank is an HD crank or aftermarket? Do you know if the crank is 4.375" or 4.625"? You either have a 117" motor or a 124" motor.

    The fact that the heads are SE120R heads has nothing to do with the bore and stroke of the motor. Those heads will fit any TC motor.

    Did you intend to pose a question in your OP?