Now I'm screwed... what spare motor have I bought

Discussion in '2 smokers' started by bower100, Feb 12, 2013.

  1. bower100

    bower100 ex-racer

    Joined:
    Dec 15, 2007
    Oddometer:
    5
    Location:
    wilmington, de
    Hello advrider members,
    I'm a new poster here... proir to now I've just read stuff. Hoping I can find some help however. I'm fixing up a 1980 Yz250, ( frame and motor prefix 3R4), which required major motor repairs. I found and bought a complete bottom-end from a 1981 yz250, ( a prefix 4V4 ). Opening up both motors I find the 1980 is a 6 speed.... the 1981 a 5 speed and parts lists support that.
    First question... can anyone verify that?

    Now what I'm really stuck on is the two cranks have different connecting rods in that the size of the wrist pin bearing. The 1980 con-rod, ( marked 5x5... a yamaha part #), measures 23mm ID. The motor I'm told was a 1981 yz250 has a con-rod measures 22mm ID ! !
    Short story is : I can't fit my 1980 Weisco piston onto the 1981 motors crank... the wristpin bearing is too big to fit into the con-rod.
    A concern is... the 1981 con-rod has YAMAHA cast onto it's side where-as the 1980 cod has, like I said, 5X5... a std. yam part #. Maybe the rod is aftermarket ? ? Weisco doesn't list one. Or maybe the crank,and /or whole motor isn't a 1981 yz250 at all.

    Anyone have some insight into this?
    Thanks, dave
    Is there a better place to source this info?
    #1
  2. Rockcat

    Rockcat LDA

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    Oct 2, 2005
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    Location:
    Portland, OR
    The '81 YZ's were very different from the '80's. The 250 went from a 6 speed to a 5 speed. The '81 is the last year of the air cooled engine and shares very few parts with the '80 or '82 engines.

    #2
  3. bower100

    bower100 ex-racer

    Joined:
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    5
    Location:
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    Thanks for info. It just seemed weird they'd go from..... '80 a 6 speed to '81, a 5 speed.
    As for the crank... all dealer online parts lists I've found claim the crank, connecting rod, wrist pin, AND the pin bearing to be same from '73, (1st year of YZ250), all the way up to '82.
    My '80 motor con-rod has 5X5 on it, (a prefix of the full part #). The rod on the '81 crank has YAMAHA cast into it in big letters down one side of it. The '80 rod small-end measures 23mm ID ...... '81 rod measures 22mm ID.
    There's my problem... I have no wrist pin bearing for this crank.
    #3
  4. Rockcat

    Rockcat LDA

    Joined:
    Oct 2, 2005
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    Location:
    Portland, OR
    Have you tried Speed and Sport?

    Www.speedandsportinc.com



    #4
  5. bower100

    bower100 ex-racer

    Joined:
    Dec 15, 2007
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    Location:
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    Problem solved. Thought I should post findings here for future .
    From 1st YZ250 model 1973....thru 1981 crankshaft essentially same, ( maybe/maybe not a minor revision or two). Then in 1982 model year the YZ250 con-rod and small bearing are re-sized. Bearing O.D. .... yes O.D. goes from 23mm to 22mm. It's in fact the same bearing that the YZ400,YZ465, YZ490, IT425,IT465 of around '79-'84 years has. They changed to the smaller bearing on O.D. to allow small end of con-rod to be thicker without being bigger.
    Now I figure my motor was near end of '81 production run so it got the '82 crank.....OR someone rebuilt it with '82 kit prior to me. The Yamaha part #' don't give bearing dimention... I found that on Wiseco website under crankshaft parts tab...sizes just as I measured.
    Last,... the bearing I.D. 's didn't change. So wristpins all same as well as pistons...( in-so-far-as related to wrist pin hole goes.)
    dave

    PS...
    WANTED: anyone know of a '80 or '81 yz250 CYLINDER for sale. (I'd like one close/closer to STD bore than one I have)
    Thanks.dave
    #5
  6. YZEtc

    YZEtc Feel lucky?

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    Connecticut
    That's funny.
    Every Yamaha parts catalogue I see lists the very same connecting rod assembly for 1980 (G), 1981 (H), and 1982 (J) models.
    #6
  7. Foot dragger

    Foot dragger singletracker

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    Location:
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    Doesnt Wiseco make scads of oversize pistons for these bikes? They do for many old crocks. Might be easier to bore and go oversize then buy another cylinder.
    #7
  8. bower100

    bower100 ex-racer

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    Location:
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    2 numbers listed at dealers website I go to: con-rods : # 5X5-11650-00-00 and # 5X5-11651-00-00. And '81 to '82 bearings are different #'s. The parts list doesn't give bearing dimentions but I found that on Wiseco's site. That was helpful.
    dave
    My cylinder has a whole cooling fin broke off it.... otherwise ok.
    #8