Crankshaft

Discussion in 'General' started by Monstar, Jun 23, 2019.

  1. Monstar

    Monstar Member

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    Is honda the only place to buy a new crankshaft? Checked CHP but they don't sell them.

    Thanks
     
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  3. kirrbby

    kirrbby Well-Known Member

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    For what bike?

    Dratv has good aftermarket cranks for 4speeds. If you need a 3speed, you could contact racerx about rebuilding yours to like new condition.

    I think Honda only has 12v cranks still available.
     
  4. Monstar

    Monstar Member

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    Sorry, 71 ct70
     
  5. jrrjr71

    jrrjr71 Member

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    Why does crankshaft dictate alternator voltage? All because of length/taper of the alternator-side of crank? That's it?
     
  6. racerx

    racerx Administrator
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    It doesn't.

    The reason for being specific about which crankshaft you have/must have has to do with it being part of the alternator assembly: crankshaft, flywheel, stator. There are 3 different crankshaft types: 3-speed/6v; 4-speed/6v; 3-speed/12v w/CDI. The difference between crankshafts is the "nose", i.e. the portion on which the flywheel fits. Each of the 3 types has a slightly different taper & length, which only fits its intended flywheel. Match the flywheel to the crank and it fits perfectly. One last detail...the flyweels are each specific to their stator configuration.

    It is possible to build a "phantom" engine combo: 4-speed (w/centrifugal advance) alternator & crank into a 3-speed, 3-speed/6v (fixed timing) crank & alternator into a 4-speed engine; or, even retrofitting a 12V CDI crank + alternator assembly into an earlier (pre-1991) 6v engine (with minor mods and a 12v flywheel cover.
     
  7. jrrjr71

    jrrjr71 Member

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    Good info as always, Bob. Makes me want to pickup a used example of each of these cranks. Just so I can see the actual physical differences. I thought I read that someone had modified a 6v crank to accept a 12v flywheel? not sure if that was a honda 12v flywheel or one of the cheap chinese ones.
     
  8. OLD CT

    OLD CT Well-Known Member

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    This is a 12v 54mm stroker crank. Check this compared to all DSC00018.JPG the used rusty 6v ct70 cranks on ebay.
     
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  9. jrrjr71

    jrrjr71 Member

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    the flywheel side of that crank looks quite a bit longer than the 6v ones. so the 12v flywheels are that much "taller" than the 6v ones?
     
  10. Monstar

    Monstar Member

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    On a side note, is there anyway to clean up a little bit rusty crankshaft? So i can just use my old one?
     
  11. kirrbby

    kirrbby Well-Known Member

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    Hecks ya. Just clean it up. If it works, it works. It would have to be pretty bad, or rusted under the seal, to really cause a problem.

    Wait tho. Rusted where?? Rusty bearings are a no go...generally, no rust in-between the center cases.
     
  12. jrrjr71

    jrrjr71 Member

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    that's another question i had about the cranks. are they 2-piece? i saw an exploded diagram somewhere that made it look like 2 separate halves. can you put left half from one crank to right half of another?
     
    #11 jrrjr71, Jun 27, 2019
    Last edited: Jun 27, 2019
  13. racerx

    racerx Administrator
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    These are "assembled" crankshafts. Parts list: LH side, RH side, crankpin, needle bearing, connecting rod, 2 main bearings, timing sprocket.

    Typically, it's the flywheel side half that gets the damage, most commonly stripped threads, less commonly, rust. As long as the rust doesn't leave pitting in the area that contacts the oil seal, they're almost always reusable...this, of course, assumes no damage from a crash, or ham-fisted "mechanic". IOW, the reason a crank needs to be rebuild is wear to the connecting rod, at either end. Too much lateral clearance (i.e. sideplay) at the big end, or any radial clearance that can be felt, by hand, and it's a choice between rod knock, rebuild or replacement.

    Cranks can be rebuilt, multiple times. However, it's an involved...high-precision...process, which requires specialized equipment an experienced touch and a LOT of patience. Make no mistake, a properly rebuilt crank can be, literally, good-as-new. It's a question of cost. Seriously, $150 + parts is a bargain, anymore.

    So...if one is considering building a performance tune (i.e. enough power to be roadworthy) it makes sense to factor-in the cost of a crank rebuild. That's where a sub-$300 stroker crank becomes only a minor premium, compared to the other choices...and...a TB 51mm crank roughly the same, or less, money. My preference would be 52mm, if going for an outwardly stock appearance and minimal cost; ideally, however, a 54mm arm will deliver a lot more torque and register a bigger hit on the seat-of-the-pants dyno...albeit for another $150+/-, all-in, to cover everything (mainly the required 69mm tall cylinder + piston & longer studs, the rest is unchanged).

    BTW, the 12v crank is definitely longer on the flywheel side. The 12v/CDI alternator assembly extends farther outboard. For that reason, a 12v flywheel cover is the easiest way to get flywheel clearance. It is possible to add custom spacers to a 6V cover, but far more involved.
     
  14. OLD CT

    OLD CT Well-Known Member

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    The 54mm 12v crank I pictured is made in Japan Clipping Point. Comes with the special oil pump shaft and directions in Japanese.:LOL: I would let it go for $280 shipped to anyone interested.
    Lower 48 states only, of course.
     
  15. Monstar

    Monstar Member

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    There is a little side to side play, maybe about a 1/16. And I guess the rust isn’t too bad it’s just were some water was sitting.
     

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  16. racerx

    racerx Administrator
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    The seal contact area looks like it should clean up well enough. Ideally, polish it; if that's not possible crocus cloth is a good alternative. Failing those, ~2500-grit sand paper and oil will leave a smooth enough finish. The surface rust on the counterweights is more fugly than anything else. A rotary wire brush ought to make quick work of it.

    Biggest issue is the actual condition of the connecting rod, including the bearing. First off, it should feel completely smooth...no notchiness or other uneven movement. Second is the side (lateral) and radial clearance. Use a feeler gauge; if there's more than ~0.018" between the rod & crank, it's probably going to rattle. If you can feel any radial play, the bearings are toast.

    Judging by the photo, this crank appears to be rebuildable, at least...if need be.
     
  17. Monstar

    Monstar Member

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    .013 is all I could get in, I’m thinking is going to be usable.
     
  18. racerx

    racerx Administrator
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    IMO, 0.014" or under is excellent. New, they're usually around 0.010"+/-0.003".

    I would recommend scrubbing away as much of the rust as possible, then spraying-out the rod bearing...from the clutch end of the crank, that's the main oil passage. Placing a clean sheet of paper towel under the crank will highlight any dirt that might be lurking inside. Oil it right after the solvent cleaning, to prevent flash rust.
     
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  19. Monstar

    Monstar Member

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    I was just thinking, if I put my crank in evapo rust would it ruin the crankshaft bearings? My guess would be it should be fine if I clean it good then dip it in oil.
     
  20. Monstar

    Monstar Member

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    I ended up putting the crank and cylinder in evapo rust and it came out great. The cylinder was the worst since the piston was stuck in there with water sitting on top. Had it in there a couple hours and then cleaned it up and coated with oil.
     
  21. racerx

    racerx Administrator
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    I'd be concerned about the bearings, as they are high-precision/close-tolerance, items. The mains can be replaced easily enough, if one has the proper puller and a press. It's the rod bearings you have to consider. Those needle rollers are made from hard, brittle, steel; it's durable material but, prone to rust. Let's assume there's no rust in them, now. Exposure to a water-based acid solution presents two disturbing possibilities....etching and flash rust, when the part is washed. Cranks can be rebuilt. But if a rebuild is going to be done, then it'd be simpler to just slide lengths of heater hose over the crank ends and glass bead blast the halves...before having it disassembled, the rod, bearings & crankpin replaced with new parts.

    Just before you oil it, spin the main bearings. They should be silky-smooth. Any roughness, or notchiness, is a very bad sign and the bearings should be replaced.

    Not saying, categorically, that this won't be successful. It probably will work out okay. It's not a process I'd use for this, specific, assembly. If there's a problem, you should know about it soon enough. Reach the 100-mile mark, without a rod knock developing and you should have verification of a successful result.
     

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