Floating piston wrist pin

Discussion in 'The Garage' started by svejkovat, Apr 9, 2012.

  1. svejkovat

    svejkovat Been here awhile

    Joined:
    Jun 23, 2009
    Oddometer:
    856
    I'm replacing the bronze bushing in an old two stroke. Somethings got me curious. I've pressed the bushing in the rod and honed it so that the wrist pin is just right in it. I install it and notice that it floats/rotates as freely in the aluminum piston as it does in the bronze bushing. Seems odd. Is this right? It must be since there are evidently oil access holes under the piston skirt to the wrist pin where you'd expect them to be if this was a bearing surface. You'd think that it would have been engineered so that the wrist pin presses into the aluminum snugly so that it was forced to rotate within the replaceable bronze connecting rod bushing.

    What's up?
    #1
  2. Tosh Togo

    Tosh Togo Long timer

    Joined:
    Nov 21, 2010
    Oddometer:
    1,594
    What bike/engine is this?.
    #2
  3. Benesesso

    Benesesso Long timer

    Joined:
    Jul 29, 2008
    Oddometer:
    9,898
    Location:
    West of Phoenix, Arizona
    The pin stays nearly stationary in the piston because of its inertia. The rod has no choice but to swing back and forth and rotate on the pin, but the pin resists rotating back and forth so fast.
    #3
  4. mudgepondexpress

    mudgepondexpress Long timer

    Joined:
    Jan 11, 2007
    Oddometer:
    1,175
    Location:
    Spokane, WA
    You have a full floating pin...not uncommon.

    You can float either or both...but full floating has the least friction (but possibly the most wear).

    Kenny
    #4
  5. Beezer

    Beezer Long timer

    Joined:
    Oct 26, 2004
    Oddometer:
    6,005
    Location:
    Anchorage, formerly Spenard (hub of the universe)
    Every aircraft piston engine I have worked on (hundreds) has them. Even a light press fit into the aluminum piston becomes a loose fit at operating temps. On aircraft the pin is prevented from hitting the cylinder walls by aluminum buttons... some are pressed in with an interference fit and some are just a plug.
    #5
  6. ragtoplvr

    ragtoplvr Long timer

    Joined:
    Oct 18, 2007
    Oddometer:
    6,724
    Location:
    central USA
    that is the way they are supposed to be. Wherever the lowest combination of friction and inertia occurs determines who and how much, in practice the pin moves more on the wrist pin bearing but there is movement in the piston. This combination is lowest in friction loss, and under poor lubrication conditions which ever bearing has the most lubrication moves. Having seen several 2 strokes run without oil, I have never seen the wrist pin bearings fail. Usually the rod or the piston galls up.

    Rod
    #6
  7. Wolfgang55

    Wolfgang55 Long timer

    Joined:
    Dec 24, 2006
    Oddometer:
    3,531
    Location:
    Next to Rio Bravo
    This info is good stuff.

    Why did you go to a different material for the bushing?
    #7
  8. svejkovat

    svejkovat Been here awhile

    Joined:
    Jun 23, 2009
    Oddometer:
    856
    [​IMG]

    The innards of a 2 speed Puch 50cc moped engine. Wonderfully simple. Two centrifugal clutches inside the bell and a sprag clutch inside the first gear ring. First gear drives the second gear clutch until it expands, takes over duty on the bell, sprag releases the first gear ring. The thinner bell is driven by the starter cone clutch when you pull the handlebar lever. Wish I'd added a quarter or a coke can or something for perspective. That piston is about the size of a small aspirin bottle.

    I didn't go to a different material on the piston wrist. This has bronze sleeve on top and needles on the con rod bottom end. At 2HP the bronze was adequate for 11,000 miles and takes, literally, 20 minutes to replace. All the work was in lathing one to fit. 12mm ID 14mm OD is not stock anywhere I could find.

    Ordered replacement bearings for it via this site....http://www.locateballbearings.com/Search
    ...which (so far) I highly recommend. If you have part number, they've got your bearing and at good prices. Phone help was quick and good. They even stocked the STEYR (might as well keep it OEM) originals. The STEYR's were actually less than the no-name chinese bearings stocked by the moped vendors.

    Here's to another 11,000 miles of fun! I ride this thing as much as any of my other bikes during the summer. I'm blessed with almost no need to ever look "cool". But ironically, I get more admiring questions and thumbs up when beating around on this thing than anything else.
    #8
  9. ericrat

    ericrat Long timer

    Joined:
    May 18, 2007
    Oddometer:
    1,230
    Location:
    Cincinnati, OH
    Nice to see that old piston with a proper, long skirt for modesty. Those new fangled pistons, why the only thing modest about them is the amount of material in the skirts. They go around flashing their pin bosses to anybody who cares to glance.
    #9