Could this be a reason for wrist pin bearing failure?

Discussion in '2-Stroke Engines' started by 45u, May 19, 2016.

  1. 45u

    45u Active Member

    The first and second picture is of a crank and piston out of motors like we run. Notice how much of the bearing load is outside of the top of the rod taking away a good bit of the bearing surface and the bearing is not supporting the rod like it should. This is not good what so ever. All so see how much side to side play the wrist pin bearing has.

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    The reaming pictures of some of my radio control 1/5 scale motor parts. Notice the two washers which in close the wrist pin bearing on the ends making it where it does not move side to side much making a much better bearing load and is always very close to center. The upper rod end is supported all the times and the rod is not hanging off one side like on the motors we run. The washers cover the end of the wrist pin bearing where it cannot move side to side much as well as the rod cannot move much. The washers are made flat on one side, piston side and the others has a recues that allow the wrist pin bushing to stay centered much better than the set up on the motors we run. The rod does not fit over the washers. You will see on the RC piston there is still a little play and has to be but the ones we run have way too much side to side play.

    Being on the motors we run the bearing load is much of the time on one side of the top rod not giving not near as much of a good bearing load? What do others have to think?


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    FurryOnTheInside likes this.

  2. FurryOnTheInside

    FurryOnTheInside Active Member

    Makes sense.
    I wouldn't particularly want to add extra mass of steel to the piston since I already need to find someone with a drill press to lighten the wrist pin for me (or let me use his drill press).
    If someone were to produce a set of washers to fit and they were fairly light would certainly be interested in acquiring a set. :)
     
  3. 45u

    45u Active Member

    I would think a little more weight would be better than a wrist pin bearing going bad as when they do it really messes up the inside of the motor. I am going to drill my pistons and wrist pin. I just wonder how hard the wrist pin is and good chance will need a cobalt or harder bit.
    If will be a while but I am making plains and try to make some wrist pin washers. My lath with are at my friend’s house taken apart and need to get them here at the house and set up.
     
  4. FurryOnTheInside

    FurryOnTheInside Active Member

    Of course I agree it would be better than the bearing breaking up, but before it does so, if it does so, it would be nice to minimise vibration as much as possible. Can a light metal be used i.e. aluminium rather than steel or would it need the strength?
    Carbide 9/32 is what's recommend on mr Jaguar's dragonfly site for wrist pin drilling.
    Also I just found a source of a good bearing was recommended there too: Pistonbikes has one with the smooth cage looking like a single piece, I think it was $5
     
  5. butre

    butre Well-Known Member

    it doesn't need the strength, it needs the hardness. titanium works reasonably well if you throw away the needle bearing and use a bronze bushing instead.
     
  6. jaguar

    jaguar Well-Known Member

  7. 45u

    45u Active Member

     
    Last edited: May 19, 2016
  8. 45u

    45u Active Member

    I have some bushing but the bearing if you can keep it from destructing would be better. Many older 2 cycles use bushing not the all seem to be using bearings.
     
  9. 45u

    45u Active Member


    I have looked at that bearing and not so sure that it is any better the many others. Sorry to say the

    I have looked at that bearing and not too sure it is any better then many others. I am sorry to say the Stihl chain saw bearing is about 1.5 mm narrower then the stock bearing which would have more room to walk and decrease bearing surface.
     
    Last edited: May 19, 2016
  10. crassius

    crassius Well-Known Member

    look back on the boards about 4 or 5 years - you'll see much more of this discussion back then before the shorter small end bearings started to go away - some folks even tried nylon spacers, but they melted - bearings in most kits got much longer after those days
     
  11. 45u

    45u Active Member

    In my opinion they are still WAY to short. Now my RC engines are only up to 32cc but they turn well over 12k plus RPM and I have never had a wrist pin bearing go bad.
     
  12. jaguar

    jaguar Well-Known Member

    with the assembly in hand just move the rod so that it is centered and then you will see that no matter how much you move the bearing that there will always be bearing rollers betwwen the full width of the rod and the wrist pin.
     
  13. skyash

    skyash Active Member

    45u has your Bearings ever gone bad be for. I have never had it happen plenty of crank one gone bad and if you like what's wrong with steel washers thay stop the shaft movein in my 4 stroke transfer box haven't worn out in 1000km just an idea what about 2 skateboard bearings put next to each other or one fat one abec 7 should fly
     
  14. 45u

    45u Active Member

    Thanks for the reply you are the first I have read about the big end bearing going bad I am sure others have as well but not like the wrist pin bearings that many talk about. What brand and type of oil are you using?
     
  15. skyash

    skyash Active Member

    castrol active 2t for motorbikes .i only get chepo motors and most of them have bad scratching noise in the bottom end and it wears in or away but like 3 got worse then loss of power open it up and burn marks on the crank Wait normally left side i wood say the load side out of 10 motors i have never had a wrist pin go bad some bikes go through 2 stets of rings till it just works with out bracken them eny more. but more bottom end problems then eny thing vibrates on some revs pretty bad lot of kids hav stuff come loose and bad noise on certain parts of the cranks rotation like its not balanced or the bearings worn or loose
    scratching noise
    scratching noise
     
  16. aceofspades

    aceofspades New Member

    So how about green loctite?
     
  17. FurryOnTheInside

    FurryOnTheInside Active Member

    To prevent the bearing wandering around? Im not certain which one is green now. But my main worry would be that it would be permanent and stop the wrist pin ever being removed.. Unless heat plus a lot of vibrations would be enough to break it up. But then it wouldn't do any good.

    For myself, I think I will have to try to not worry, and just rely on the bottom end bearing to keep the con rod upright all on its own. Hopefully moderate compression and good ignition timing will be enough to protect the bearings.
     
  18. Rob H

    Rob H Member

     
  19. Rob H

    Rob H Member

    Wrist pins are chrome moly and case hardened the will drill easily but they are brittle they are more likely to crack than get tore up
     
  20. Steve Best

    Steve Best Active Member

    Wow, excellent find, I didn't notice that much "walk" on the rod bearing.
    Jag is right, the crank should center the rod, but you are right Jeff, the rod bearing will walk out of the rod.

    Steve
     
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