Side play on Crankshaft Bearings

Discussion in 'Tech Area' started by Wheelyking, Oct 7, 2018.

  1. Wheelyking

    Wheelyking New Member

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    Crankcase 1.jpg Crankcase 1 Close Up.jpg Crank Bearing Side Play.jpg Need some insight on this one please...

    I finally got to the point of putting the engine cases back together this evening. I'm rebuilding a CT70H engine and I'm going with a TB Stroker crank. I just assembled everything inside the cases and put the two case halves together. I wound up with .015 thousandths of side play from the outer races of the main bearings and the case halves. I don't believe this is good news. The only remedy I can think of would be to make round shims out of gasket material and put it in between the sides of the outer bearing race and the case halves. Check out the pictures I attached

    You can see a gap between the bearing race and the engine case.

    Thanks in advance. Looking to hear for Ideas...

    Jim H.
     
    #1 Wheelyking, Oct 7, 2018
    Last edited: Oct 7, 2018
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  3. cjpayne

    cjpayne Well-Known Member

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    I would tend to think there is something wrong with the crank.

    Btw, how did you get the cases SO clean on the outside?
     
  4. kirrbby

    kirrbby Well-Known Member

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    I'm pretty sure the side to side play is normal/typical, and is not a problem. I'm sure someone will second that, so you can be sure.
    I don't think you have anything to worry about.
    Your engine looks awesome :)

    Vapor blast??
     
  5. cjpayne

    cjpayne Well-Known Member

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    I cant remember exactly, but I thought it was supposed to be tight in there. Maybe on the stroker cranks there is supposed to be a little spacing?

    I thought maybe the cases had been polished. Looks great from here. They almost look like fresh castings.
     
  6. Wheelyking

    Wheelyking New Member

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    Thanks for the quick replies. I'll see if I can measure the distance between the two outside surfaces of the bearing races on the stroker crank and compare it to the original crank I pulled out. I don't think I can let that much side play go and feel good about it. I should have seen this coming. A few weeks ago when all the parts came in I was just seeing how things fit together. I had just the crank installed (no gasket) and the fit was right on.

    I spent countless hours wet sanding the case halves. Concentrating mainly on what will be viewable when the engine is mounted. Very tedious work. A true PIA. Then I buffed the two pieces. The buffing only took an hour or so. Thanks for the comments.

    Jim H.
     
  7. kirrbby

    kirrbby Well-Known Member

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    I'm pretty confident. I just went and checked 3 different engines. None have ever been split (I don't think) 2 have obvious side to side play. The third, I can't say for positive, but it moves at least a smidge.
    I think it would be a problem if it DIDN'T have some movement.
    I'm sure racerx or OLD CT will reply tomorrow.
     
  8. kirrbby

    kirrbby Well-Known Member

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    When you think about it, every single shaft in these engines is allowed some side to side movement, except for the shift drum (Not really a shaft, but kinda.)
     
  9. cjpayne

    cjpayne Well-Known Member

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    Maybe the clearance is for heat expansion.
     
  10. Wheelyking

    Wheelyking New Member

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    I measured the distance of the bearings between the original crank I pulled and the aftermarket crank that's going back in. The aftermarket crank is only .002 thousandths narrower. So I guess some side play is normal for the crankshaft bearings. I really wouldn't think it is but it must be. Thanks Kirby for taking the time to check some engines you have on hand.

    Jim H.
     
  11. racerx

    racerx Administrator
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    There's no endplay spec, of which I am aware. The play isn't in the bearings, it's how they fit inside the cases. I agree with the others who have already replied in this thread; there's normally/typically a substantial amount of endplay, just as you've described. FYI, the new-gen motors have virtually none of this...the clutch-side bearing is pressed onto the crank, the RH case half heated to install the bearing - which seats solidly inside the case half. If anything, bearing clearances expand with heat. That includes the outer races, which tend to fit the case halves more snugly (while the inner/outer "grip" on the ball bearings loosens). Between the typical 0.009-0.015" rod-to-counterwieght clearance of most crank assemblies and (by comparison) acreage between the small and of the rod and the piston, it all works well enough. If there were an inherent issue, we'd have know about it decades ago.

    So, if you want to use a shim to tighten the clearances, it is possible. The question, imho, is one of practicality. Excessive endplay, could have more than one cause. A ~0.005" shim is not only going to be thin, it's going to be narrow and not all that easy to work with. It will also be easily overlooked, the next time the engine is disassembled. Who would notice, let alone remember, an unusual custom part like this? It'd probably be better to shim the clutch side bearing, on the crank. The problem is that then you have a crankshaft that may not be usable in another set of cases, unless the shim is removed...somewhat of a hassle and re-e-e-e-e-e-eally easy to overlook. It'd be more accurate to almost impossible to notice. If you know, with 100% certainty, that the excess clearance is a result of the case half/halves being incorrectly machined, the next step is figuring out which half...if it's not both. This is where impracticality rears its head. How do you accurately make that determination? For most owners, the logical method would be to source another RH case half. If that "cures" the issue, may as well just use that part and call it a success. OTOH, the LH case half is the one with the SN and for most, that will be an issue. Only way to correct that would be welding-up, then re-machining the bearing bore...a high-precision job that must be done perfectly, the first time, and it won't be cheap. The actual machining will be the "easy" part. The setup requires a seasoned pro who cares greatly about the work.

    My suggestion is to take a close look at the crank & case halves. Is there a ton of sideplay (more than ~0.018") at the big end of the rod? Any radial runout? Take a look at the small end. Does the wrist pin fit loosely enough that you can feel it rocking inside the rod? How about the bearing seats...any "witness marks" signs of the bearings dancing around, causing wear? If all of those parts check out, in acceptable condition, well..."if it ain't broke..." An extra 0.002" due to a narrower assembled crankshaft length shouldn't be an issue. FWIW, the trailbikes engine parts...including their stroker cranks...have been very reliable for a number of years now.

    The one thing that would motivate me to have a shim made would be flywheel-to-stator contact. That'd also dictate where the shim goes, the clutch side.
     

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