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Thread: Harley J Engine Balance Factor. Again!

  1. #1
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    Default Harley J Engine Balance Factor. Again!

    Hello folks, I have not done an update on my 20 F for a while but all has not been still.

    Hopefully I will soon be able to post an update on my project thread soon but my next job is to balance the crank. First I need to determine the balance factor that I need.

    Cotten did suggest to me last year that I should determine the existing factor and use that. However it occurred to me that the balance factor of my engine is not the Harley Factory factor because my engine was rebuilt in 1989 and had a new crankpin, oversize wrist/gudgeon pins and various other new bottom end parts but was not rebalanced at that time.

    I have read the various posts on the forums and there seems to be various opinions ranging from 50% to 64%. Now I know that there is a wealth of knowledge and experience on here and so I have a simple question.

    What factor do you recommend for a 61" Harley J model and is your recommendation based on experience? (Is that 2 questions?)

    Thanks guys.

    John

  2. #2
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    Attached are extracts from the book "Questions and Answers" on balancing vee twins.
    In balancing the most important thing is that your bed ways are perfectly level and parallel.
    No dents on the knife edges either
    Near enough is not good enough.
    The seven J's we rode across America in 2003 were all balanced in this way.
    Attached Images Attached Images
    Peter Thomson, a.k.a. Tommo
    A.M.C.A. # 2777
    Palmerston North, New Zealand.

  3. #3
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    John!

    Was it re-balanced in '89?

    If not, then I would bet it would still back-calculate to less than 50%, by far.
    This would owe to the frame differences with more modern 'cradle' models, which used higher factors.

    Crankpin weights are negligibly different, and even larger pistons as long as they are cast, yet the oversize wristpins may have lowered the factor, 'a skosh'. That means minimally.
    No matter what, single-throw V-twins are enormously forgiving of factor (assuming everything else is in order...)

    You must still determine where your flywheel assembly factor for piece of mind, as it is.

    Although I swear by knife-edges, if you can find a lathe large enough between centers where the rods will not strike the ways, it should do.

    ....Cotten
    Last edited by T. Cotten; 04-10-2017 at 02:52 PM.
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

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    Tommo, thanks for the book extract. That is interesting as it doesn’t talk about a specific factor. That helps explain why there are so many opinions on what factor to use.

    I will post a more detailed update once I have made some progress.

    Thanks again.

    John

  5. #5
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    Cotton,

    It was definitely not re-balanced in 89.

    I can determine what the factor is as it is now. However I donít have decent knife edges and an accurate balance so I am enlisting the help of someone who does to speed things up.

    It had cast iron pistons before so I have had new cast iron pistons made to suit the new bore sizes. The old pistons had the old style large rings but I am using a more modern style thinner rings in the new ones. I am hoping that using cast iron new pistons will mean that there will be less rebalancing to do.

    I will let you know how it goes.

    John

  6. #6
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    Very recently this subject was discussed and it was decided to test the options.
    I balanced my set as described in my previous post.
    A local engine reconditioner used the method posted here and a specialist company dynamically balanced a set.
    Once we were all done we dynamically balanced my set and the reconditioners set so as we could compare all three sets together.
    The specialists response was "Why did I waste so much money on an expensive machine when you get the same results on a pair of straight edges?"
    There were very small differences between all three sets but we all agreed that they were so small you wouldn't worry about them and that no matter how experienced the rider was he wouldn't notice the difference.
    The "Balance for Speed" method described in the attachment to this post also comes from the "Questions and Answers" book.
    Attached Images Attached Images
    Peter Thomson, a.k.a. Tommo
    A.M.C.A. # 2777
    Palmerston North, New Zealand.

  7. #7
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    On reflection I thought it prudent to add a word of caution.
    When I first started balancing engines years ago I was scrambling 350 and 500cc single cylinder AJS's which were a dime a dozen at the time and I have to hold my hand up and say I ruined more than one set of flywheels in my efforts to balance my race motors.
    Please be very careful if you haven't done this before as flywheel sets for the bikes we are talking about don't grow on trees anymore.
    If you're new to this lark have someone who knows what they are doing teach you if you're dead set on learning the art, otherwise pay the money and have an expert do it.
    Any parent metal removed is gone forever.
    Peter Thomson, a.k.a. Tommo
    A.M.C.A. # 2777
    Palmerston North, New Zealand.

  8. #8
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    In agreement with Tommo's post,

    I had extensive experience with dynamic balancing on a Stewart-Warner apparatus, as well as knife-edges.
    In my opinion, although it may have its place, a strobe is also a blind-fold.

    And also in agreement, a rider probably will not feel any differences.
    Engine balancing is about the motor, not the rider.

    ....Cotten
    AMCA #776
    Dumpster Diver's Motto: Seek,... and Ye Shall Find!

  9. #9
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    If anyone is going to get into Balance Factors and all the can of worms that go with that when you alter any motorcycle engine away from the factory recommended balance factors you need to read both of these books and thoroughly understand what you are reading.
    I don't know if either of these two books are still available.
    I got my copy of the Phil Irving book in about 1964 and the Repco book came out as a result of the development of the Repco V8 Formula One engine somewhere in the early 1970's.
    Both books explain balance factors in detail and explain how to determine a balance factor for smooth running in the rev range you wish to use.
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    Peter Thomson, a.k.a. Tommo
    A.M.C.A. # 2777
    Palmerston North, New Zealand.

  10. #10
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    Tommo and Cotton,

    Thanks very much for your very helpful and informative replies.

    Tommo, thanks for the word of caution. I do (did) want to balance them myself but, as I said earlier in this thread, I am enlisting the services of someone who has experience. He comes recommended by someone else who is very experienced in early motorcycles including these early v-twins.

    I have spoken to the balancing guy and it was he who asked me about the balance factor. His specialty so I am told is building speedway engines but has a lot of experience of all sorts including v-twin Harleys. I am therefore getting him to do the balancing but I am going to try to absorb as much knowledge from him as I can.

    He has static facilities only, no dynamic balancing equipment.

    Both of your comments regarding the fact that these v-twins don't need to be dynamically balanced correlate with most of what I have read in other sources about the issue so i am glad that your practical experience backs up what I have read elsewhere.

    On the subject of dynamic balancing I do have to comment that my 38 Triumph was initially statically balanced (not by me) but it vibrated badly over 50 mph. I then had it dynamically balanced and it has transformed the bike. From this I can state from the the very limited experience of one engine only that parallel twins would seem to benefit from being dynamically balanced. Again this correlates with what I have read elsewhere that parallel twins have a noticeable improvement when dynamically balanced over static balancing.


    Thanks again,

    John

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