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Thread: '27 J cracked flywheel / bottom end rebuild

  1. #1

    Default '27 J cracked flywheel / bottom end rebuild

    I have just split the crank cases on my 27 J. The cases are in good shape overall.

    When I came to inspect the flywheels however I could see the damage shown in the attached photos. Looks as though the con rod may have broken at some time by the marks / crack. Having used a small mirror I can see the crack does not extend the full width of the rim but its pretty bad

    My first question is whether this flywheel is usable or repairable or whether I need to find a replacement? If I need a replacement do I need to just one or would it be better to find a pair ?

    I have no experience of how durable the flywheels might be. All I can say it that the motor was running when I got the bike albeit not doing many road miles.

    The sprocket shaft measures at 0.997.5 / 0.998. My book says replace if its worn more than 0.001. Pinion shaft is 0.935. So it looks as though both shafts are worn about 0.002

    The sprocket shaft was quite tight on the bearings though. The rollers measure at .250 but Im not sure my mic is accurate enough to detect if they are oversize. Could well be .0001 oversize which might account for the tight fit.

    The connecting rods are in good shape and there is only slight discernible lateral movement in the female rod , less still in the male and no movement in either up and down. My guess is that the crank shaft bearings/bushes and con rods were replaced after the damage was done to the flywheel.

    My second question is about replacing the sprocket and pinion shafts. Can I safely use what I have or are they actually worn out? I want the engine to run as well as possible.

    I cannot imagine I will find an OEM set that are in better shape that what I have. I see repro sets available I would welcome hearing from anybody who has rebuilt the bottom end using repro parts. What are the pitfalls, things to look out for.

    Thanks for your help

    in the uk

  2. #2
    Join Date
    Sep 2005
    Sarasota, Florida


    I suspect that damage was done by a hamfisted builder in the past, who used a steel hammer, and bad judgment when attempting to true the flywheels. Common practice is to use a lead, or soft brass hammer, wedges, and 'C' clamps to true wheels. Also, excessive whaling on the wheels should be avoided, as they can only take so many hits. As for repair, I would weld up the crack, provided that damage is as isolated as it looks in the picture. If the crack is worse than that, and you can't fix them, Truett & Osborn makes J wheels. Magnaflux would show any additional cracks, and those would be most critical around the taper bores for the crank pin. In other words, if there are any cracks around the crank pin bores, you will not be able to true the wheels. In regards to your sprocket, and pinion shafts; it is always better to be a bit loose, than tight. You're in the ambiguity zone and only you can make the decision to run them, or replace them. A new pinion bushing can always be made to fit the current shaft (provided it's not stepped). Same for the sprocket shaft, as oversize rollers can be found. It's all about the condition of the bearing surfaces on the shafts, and sprocket bearing housing, (i.e. no roller paths, or steps).
    Eric Smith
    AMCA #886

  3. #3
    Join Date
    Nov 2015



    I am doing a bottom end right now on my J so I am in a similar situation. I'd replace all of the shafts with new Easterns shafts. Then you can upgrade to the VL 3 hole crank pins. Truett and Osborn makes a great flywheel I hear. Otherwise original J flywheels are 100 bucks usually. You never know if the tapers are good though which is the tough part.

    If you want to use your original flywheels see if you can knock that piece off and rebalance them. Or edge the flywheels to the break point. It'll lighten your load and put a bit less load on the engine itself too.

    If you want it done right, send it all off the Truett and Osborn and have them put it all together. It won't be cheap but that's the cost for peace of mind. I too thought about using the original shafts, then a few friends hit me with the common sense stick and said get new shafts and bearings. Go through every bearing with a mic that reads .0001 and see if they all match up. Usually they don't....

    Moral of the story, use Easterns parts as much as you can. They seem to nail it in the quality department.

  4. #4


    Thanks Guys.

    I have had a welder look at the damage and he didn't advise a weld repair - in fact he would not even consider attempting it and I trust his judgement. Looks like a new set from T & A and a big hole in the budget

  5. #5


    Truett & Osborn wheels the only way to go

  6. #6
    Join Date
    Jun 2001
    Central Illinois, USA


    Quote Originally Posted by Martin Pettingell View Post
    Thanks Guys.

    I have had a welder look at the damage and he didn't advise a weld repair - in fact he would not even consider attempting it and I trust his judgement. Looks like a new set from T & A and a big hole in the budget

    Its really weird how so many pro welders totally ignore the advantages of silver-solder.

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

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