General & Model Specific Technical Forums > Twin Cam

Trueing Harley-Davidson Crankshafts

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Max Headflow:
I'd bet that you may not get any comments from HD engineers in this forum.. I've seen TT324 which says that the max allowable error is 0.012" when checking runout in the cases and I think that it may have some errors in it... It states that the flywheels can have 0.012 TIR either side which I think is in error.. I think the Brian justified the runout on the drive side correctly.. Since the splines are splined (broached or blanched as Brian says) the process cannot accurately cut the splines on center.. Because of this process, that end of the shaft and sprocket can wobble some... Technically that would be fine if the sprocket locates on the shaft and only used the splines for drive.. If the sprocket rides on a hub that fits on the splines you could get a tightening loosening of the chain that could still be acceptable (but drive the bagger types crazy, eh hardtail?  :wink: ) Brain states that everything else can be made to run dead nuts based on the sample of flywheels that he's seen.. Back to TT324.. I suspect that there in an error in that only the drive side should allow 0.012 due to the manufacturing process but it shgould keep the pinion side runout to less than 0.004. Also total runout of any part of the flywheels should be limited to less than 0.004.. While we can pick on Brians wording, I think that he did a great service to the HD community in presenting his information to all.

BTW TT324 states that TIR in a truing stand (I assume between centers) should be less then 0.005. I suspect this is because the chamfer on the hole is cut with greater accuracy then the spline.. The splining process may tweek the shaft some..  From what I've seen centers will usually only give about 75-80% the error of V blocks or cases.


  First, I really didn't think that it was all that bad. But he didn't explain anything either. So here I go. All my thoughts come from my XR-750 days, they ran pressed together cranks also.
  I think the main thought behind the press crank is to keep the crankpin at a 90 degrees with the flywheels and keeping all shafts true, tappered pins are hard to machine and stay at a 90 degrees. When the crank pin is not on a true with all shafts, it beats out the wrist pin keepers.
  Now, to true a pressed flywheel, you can't true it the same as a bolt together flywheels, put it in a stand and smack it with a hammer to true it.
When you put the pressed crank together, it is installed in a jig, and when it is pressed together its true. Here's how I did it.
 The jig  is two thick slabs of steel,with a pin on each end, with the pins apart enough so the flywheels fit between them. One slab had the pins pressed in it and the other slab could slide on the pins. Both slabs have holes in them , the pinion shaft went through one slab, and the sprocket shaft went through the other slab. The slab with the pressed pins and pinion shaft hole, had a extra pin that went through the center of the crank pin to keep it straight as you pressed it in the pinion side flywheel. All holes and pins are machined to a perfect fit.
  1) Lay pinion side flywheel in jig, press crank pin in flywheel with oil holes alligned. 2) install rods and bearings on crankpin. 3)C-clamp or have someone hold the sprocket flywheel to the other slab and press the sprocket side on the crank pin. As you press the flywheels together you use a feeler gauge to get rod side end play. Press the crank plugs in. Thats how I did it on the XRs. Hope it helps.

 That is some real good info. While you are here lasmitty, I have never heard of the XR's having trouble with the wheels,were they pretty stout or were tolerances a bit better?

 Not as bad as I've read on this site. But your talking about a engine with a short stroke. I remember Smitty, the guy I worked for, yelling at the young
riders to stay in the throttle on rough turns. There was a track with a jump in it, that the riders would twist cranks, if they didn't stay in the carbs.             I tell guys that I build their engines, that if they do a holeshot and the rear wheel gets loose,ride it out or pull the clutch in, don't just close the throttle.

Thanks Sir Garfield,

It looks like the harley engineers knew what they were doing after all, they just never told anybody how to check the cranks.



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