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Thread: Problem with Reusing Original Pistons, Pins, Rings, etc. for Excelsior Rebuild

  1. #1

    Problem with Reusing Original Pistons, Pins, Rings, etc. for Excelsior Rebuild

    I've been asking a few questions about rebuilding a 1917 excelsior motor . I have nice resleeved (I was told) cylinders measuring 3.3545" (measured by myself using a bore gage). I also have a few original pistons 3.3285".
    a few questions...
    1) why not use the original pistons assuming they are crack tested and checked over. how about the rings and pins ?? Im not sure if I have a complete set of pins , wristpins etc. If either rings, or pins, wrist pins etc should be replaced, can anyone recommend a provider ??
    2) are the cylinders the original bore size ?
    3) would the pistons have the proper tolerances for the cylinders ??
    4) Since all else is original , I assume I would not have to rebalance the flywheel and be concerned about where to remove the weight- only true it ??
    5) this will only be a rider and don't plan on any grueling rides like the cannonball.
    your thoughts ??
    thanks and stay safe,

  2. #2
    Join Date
    Feb 2011
    Sarasota, Florida

    Re: Problem with Reusing Original Pistons, Pins, Rings, etc. for Excelsior Rebuild

    Hello, Dan. Original bore on a Big X was 3-21/64" (3.328") so I think your cylinders are too far over size for the pistons you have. I agree with you that cast iron pistons are best for a ride-around antique, but not for an endurance motor. I believe the cast iron pistons were originally fit rather close because the piston and cylinder had a similar heat expansion ratio, so they were quiet, and gentle on the cylinders. Also they didn't wear like aluminum pistons. I've had a number of bikes with cast iron pistons and the problem is always the rings. Old rings are usually double the width of modern rings, and no where close to being of modern quality. Hopefully, Gene Harper will respond as he is doing this right now with a Series 20 X. I think you will be forced to go to a piston manufacturer to have aluminum pistons made for your cylinders. I don't think re-balancing will be needed if you go to aluminum pistons. Again, Gene has done a lot of the heavy lifting on Excelsior motors and can better answer your questions.

  3. Re: Problem with Reusing Original Pistons, Pins, Rings, etc. for Excelsior Rebuild


    I doubt that your cylinders are re-sleeved, picture of that?

    If your bore and piston measurements are correct, that is way too much piston clearance, .026". Sure, the motor would run but it would sound like hell, lots of piston slap there and it would do some serious damage pretty quickly! As I've mentioned before, HD UL pistons work well. That standard bore is 3.3125, or 3 5/16". I've done all the math and using a +.020" over UL piston will fit a standard bore X cylinder that gets honed about .005" oversize. The compression height is the same but the HD wrist pin is larger. That can be used by boring and honing the X wrist pin bushing to fit the new pin. You will also need to increase clearance inside the piston a bit for side play of the rod.

    If using UL piston, bore sizes should be as follows, giving about .005" clearance for each oversize.

    Piston size Bore
    Std 3.310"
    +.010 3.320"
    +.020 3.330"
    +.030 3.340"
    +.040 3.350"
    +.050 3.360"

    Based on this, I would get +.050" pistons and hone your cylinders about .004-.005" OS. If you don't want to hone, guess you use +.040's and listen to it knock.

    Also, you will never find any original pistons bigger than .+020" OS. X didn't even offer oversize pistons until 1920, and then only in .005. .010 and .020 OS.I always thought they were too thin to bore bigger than that but not true. I know one cannonballer who successfully went all the way and still rides the bike with +.060" OS high compression pistons.

    Yes, you will need to re balance the motor......

    I'm currently dealing with pistons right now on my 24 X. I"ll be using EL pistons and the numbers above are exactly the same, as EL and UL bores are the same. Of course using EL pistons will raise compression significantly. I did volumetric measurements and calculated the stock X compression ratio at 4.4/1. Using an EL piston gives 5.45/1 compression. That and a new cam will make the old X wake up and go down the road much better and should also increase fuel economy. Building this bike for a cross country trip in 2021.....


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