Wrist pin bearing on 2/stroke

Discussion in '2-Stroke Engines' started by geebt48cc, Sep 11, 2012.

  1. geebt48cc

    geebt48cc Member

    Guys, have you ever heard of a original wrist pin bearing that can last for a few thousand miles on motor? I've got a 2010 Skyhawk 66 that's still hanging in there, and has never been touched at 650 miles. I also milled the head, and upped the compression at about 400 miles. I just wonder every time I ride just how long it will continue to run like it should? Using Amsoil synthetic with a splash of Castor oil, but from what I understand, it really doesn't matter considering where original wrist pin is made?

    So, like I said, just wonder how much longer it's going to last? Da"", I don't want to be 15 miles away from house when it rips up!~

    Anyway, it's doing great at this point.


  2. crassius

    crassius Well-Known Member

    got a girl customer running 17 miles each way to work 5 days a week, plus stores and things as needed - I figure her for 750 - 850 miles a month

    she's still going well after 5 months
    Timbone likes this.
  3. Skyliner70cc

    Skyliner70cc Active Member

    I've got several years on a HT engine with stock wrist pin bearing. 4 oz of oil per gallon (2 oz castor, 2 oz cheapo stuff)....its the castor.
    Timbone likes this.
  4. geebt48cc

    geebt48cc Member

    The reason I'm bringing the wrist pin bearing up, is because so many people on here talk about the way these engines are eatting them up. Uno, on top of knowing that, I've also upped my compression by milling, so I know that it can be a lot of work on these little bearings. From what I gather, it doesn't matter what mix or what oil combo is used............because they still just have a fixed life span?

    Uno, because when the wrist pin goes, engine goes to trash.
  5. Fabian

    Fabian Well-Known Member

    I have "never" had a wrist pin bearing fail and i've covered nearly 40,000 kilometers on my bike, although having had a number of replacement and rebuilt engines failing from the older style caged needle roller connecting rod big end bearing.

    The newer crowded roller big end bearing seems bullet proof.
    Timbone likes this.
  6. geebt48cc

    geebt48cc Member

    So, Fabian, I've yet to hands on check mine. I wonder if being a 2010 66cc GTS Skyhawk, if it would be considered newer style bearing? I know it's told that it's a wrist pin bearing...................
  7. Fabian

    Fabian Well-Known Member

    Some of the early engines used a brass bushing in the connecting rod small end and some used a caged needle roller.
    I would think that all current engines use a caged needle roller assembly.

    Even when the caged big end needle roller bearing failed; miserably vibrating and rattling it's guts out, the small end caged needle roller bearing appeared to be perfectly ok with no discernible free play between wrist pin, "small end" caged needle roller assembly and the connecting rod small end I.D.
    Even to this day, i don't understand how other people have had the small end bearing (repeatedly) fail when i've never experienced this kind of failure, over the course of 8 engines of which 3 engines were rebuilt with the newer crowded needle roller big end bearing assembly.
  8. Purple Haze

    Purple Haze Active Member

    I've had 4 engines, none of which had any problems with either small or big end bearing failure. All had the crowded needle big end, so I totally agree with Fabian. I did have 1 engine shipped without a small-end bearing (boygofast) that I rode about 10 miles before I noticed a problem. I installed an INA bearing (don't have part #) that had a plastic cage and used the old wristpin (probably not a good thing) and it still lasted for many hundreds of miles. I think the old style bushed end would work better, but would be more tricky to install. The main forces on the wristpin and bearing are lateral and really don't need a full-floating pin, but is needed for ease of assembly-disassembly. Any slop should be avoided, and the bushing has more surface area to absorb high compression and high RPM tensile shock. In normal cruise conditions, a stock bearing is plenty safe as long as lubrication is good.
  9. crassius

    crassius Well-Known Member

    as a rule of thumb, if your bike will go at 30-35mph on level roads, then your cruising speed should be about 20-25mph - this is the speed at which the bike runs smoothly & quietly & comfortably - running it up to higher speeds will make it vibrate uncomfortably, and shorten the life of the engine by a very large amount (typically shaking the main bearings apart)

    I suspect that this is also the cause for many of the stator coil solder joints failing.
  10. Fabian

    Fabian Well-Known Member

    I somehow suspect that a good portion of the small end bearing failures and crowded roller big end connecting rod bearing failures are caused by people revving their single speed bikes to 7,000 rpm.

    I'm also in agreement with crassius on the stator coil solder joints failing and possibly from high frequency vibration, caused by excessively high rpm.
    My bike has traveled 40,000 kilometers, yet in that time i've only had one stator coil failure at the 20,000 kilometer mark.
    Having heard about all the failures, i was curious to see how long the thing would last, and it was transferred from every damaged engine to new replacement engines until it died.
    It gave surprising reliability, even with multiple river crossings where the stator was drowned good and proper. More surprising is that my bike is wired incorrectly to the kill switch; with the white (accessory power) wire being shorted to ground.

    Good to hear about Purple Haze's experiences with the reliability of the crowded needle roller big end assembly, which mirrors my own experience.
  11. philski

    philski New Member

    i know this is an old post. but may help someone in the future. i swapped my small end roller bearing for a stihl chainsaw cage roller the same size and the motor never felt better. cost $15 and rated to 70,000 rpm.
  12. jeffuehrer

    jeffuehrer Member

    jug3.jpg piston3.jpg wrist_pin_and_bearing.jpg I just had my wrist pin bearing fail. Mileage - 2,896. This is the second engine that had a wrist pin bearing fail and the shards damaged the piston as well as the cylinder. At least it's an easy fix. Hopefully I'll get many more miles out of the engine.
    Last edited: Mar 28, 2015
  13. jeffuehrer

    jeffuehrer Member

    HAHA, easy!? In the motored bike world? Nonsense! Well, it turns out I don't have any spare pistons for the long crank arm hole position(21mm from the top edge to the center of the wrist pin hole.). All my spare pistons have the shorter crank arm hole position(26mm from the top edge to the center of the wrist pin hole.) Blah! I was fortunate enough to have the other three replacement parts, cylinder, wrist pin, and wrist pin bearing. I've been riding for 5 years now and this is the first time I've known that there are two different pistons for the same friggin' size engine. Can anyone tell me what, if any difference there is in the performance between the two different designs? Anyhoo, I was able to order the correct piston on ebay for $10 shipped.
  14. Purple Haze

    Purple Haze Active Member

    I think one has a longer rod, changing the compression height (distance between top of piston and pin location) you could change the cylinder height with thicker gaskets, if the ports match up. Of course, the ports never match up anyway. Some builders swear by a long rod, while others swear at them!
  15. jeffuehrer

    jeffuehrer Member

    Well, seeing how this motor (longer crank arm) has lasted longer than the two spare parts engines (smaller crank arms) I'll stick to this design for now. I was going to mess with the gasket height but why work around something if I can get the right part – these things are hard enough to get right as it is let alone trying to jimmy-rig the set-up. Thank you for the input.
  16. jeffuehrer

    jeffuehrer Member

    How do I install a wrist pin bearing on a 66/80cc motor?

    Last edited by a moderator: Dec 15, 2015