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79 xs650 piston pin and crank questions

Discussion in 'The Garage' started by Braap2TheFuture, Nov 8, 2015.

  1. Had been sitting for quite some time when I got it, got it running and at higher rpm it started making an awful rattling sound. Had to ride it 100 miles for a build competition, sound got a lot worse (go figure). By the end, anything a bit over idle would bring the noise, Nothing at idle. Beforehand, I checked what I could (cc tension, valves, etc) and everything was good and it doesn't sound like a top end noise anyway.

    I've got it torn apart now and big end clearance of the connecting rods are ~.5mm, which I believe max to be .6. Not great but probably not the cause of my noise. Small end was about 1mm. Great.

    Now when I slide the pin into the small end to check for play I understand there should be no play whatsoever. My question is when I slide it all the way in centered, there doesn't appear to be play, but when I slide it in so that one side is flush with the piston, there's a barely detectable amount of wiggle. It's so minor I'm almost not sure if I'm really feeling play or if I used thicker oil to coat it that there wouldn't be play. The inside of the small end look good and still has all the coating. Is this really not acceptable and could cause a serious, deafening rattle? To me the noise sounded like a con rod, so this is definitely what I'm scrutinizing the most but I'm surprised that it could be so close/or is "good" and could cause such a racket

    Might be worth mentioning I'm not trying to get away with a "marginal"crank, I've got a low mileage parts motor with hopefully a good crank. Im more concerned with determining if this could cause my noise since Id like to try to only take it apart once. Might be worth mentioning also that the cam chain slider was coming apart and there were plastic bits inside. I also found small metallic pieces (looked like gear teeth, stuck to a magnet) but I couldn't find the source, everything else looked real nice
    Last edited: Nov 8, 2015
  2. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Follow the measurement specs in the '79 service manual.

    A few more things to check:

    Crank main bearings.
    Crank runout.
    Starter and crank gearteeth.
    Scrub/scrape marks on crankcase bearing webs.
    Signs of rotor/stator contact.
    Camchain stretch.
    Piston slap.
    Primary gearset.
    Clutch basket absorber springs.

    Usually, worn conrod big/small ends produce knocking sounds that change with loading and rpms...
  3. I don't have what I need to check runout. Starter and related gears/everything else you listed were in good shape.

    I got the crank out of the other motor and checking it it was .55mm big end and .8mm small end, and it's a rusty mess. Pretty disappointed at how bad shape everything in the motor was, almost nothing worth salvaging.

    I checked the small end of the original again and I'm convinced now there is no play. The noise really sounded like a rod knock, and changed with load/rpm like you say. At this point I'm leaning towards throwing it all back together with the original parts and hoping for the best

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