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Piston head ruined. Need a completely new engine or just put on a new piston head?

Discussion in 'The Garage' started by Vigeoman, Jul 19, 2018.

  1. Vigeoman

    Vigeoman XS400 New Member

    Hello everyone! My name is John and I am from Metro-Detroit.

    I bought an XS400 last year but finally got to working on it this summer. I just took apart the engine and found the left piston head completely destroyed. Everything else looks okay inside of the engine though. I am wondering if others have had similar problems and if just replacing the piston heads is adequate, or if others think there might be more damage. The engine block looks great despite the piston.
    Thanks! I am excited to get this bike running :) IMG_20180715_165345.jpg IMG_20180715_170540.jpg
  2. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    How are the cylinders? If theres ANY scoring, they are probably toast unless your machinist is able to bring the bore out enough. Id look for a used jug and new pistons. Better off starting with a fresh bore.

    If finding parts is difficult or too expensive, you can always find a good used jug with matching pistons and you might be able to get away with just new rings and a re hone.
  3. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    That kind of damage on a piston...that engine was beaten within an inch of its life.
    Or there was a casting defect.
    I'd do a bottom-up rebuild but that's my OCD
  4. BBS360

    BBS360 XS400 Guru Top Contributor

    Where did the pieces go???
  5. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Well now, an entire rebuild is easily in the $1200-1700 range lol. Good luck finding new parts too... been there done that.

    Honestly, If the crank moves freely and the rods don't have any up/down movement, Id run it. You may have some valve damage though, id look at the top end very carefully. I wouldn't even split the case, drain/flush the oil out and dump some new stuff in. Make sure to check the oil trap at the bottom of the case. That will give you an indication if anything made it to the bottom end. Don't be scared of a small chunk or two lol
  6. kshansen

    kshansen XS400 Addict

    My thoughts are what you do would be depending on what you want from this engine. Is this going to be a bike you will be putting say 10,000 miles on a year or attempting a 48 state Iron Butt ride?

    Or is it something you will ride short local trips on weekends and maybe take in a bike-night show now and then?

    Are you using the bike to get you to work daily in heavy traffic?

    Or is this one of a dozen old bikes and you ride which ever one starts on first kick down to the ice cream store on Wednesdays in the summer?

    My point as you can tell is the only answer that makes sense is "Depends...."!
  7. Vigeoman

    Vigeoman XS400 New Member

    The cylinders don't seem to have any scoring inside of them surprisingly.I might try to get away with just buying the piston heads and new piston rings.

    and to kshansen, it would just be something for around town so it doesn't need to be a top runner; you make good points.

    I picked them off, they came off pretty easily, I couldn't see any inside

    Attached Files:

  8. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    If you don't have any decimal stamped on the piston crown (.25, .50, .75) then they are the original size. You need to bring the jug to a machine shop and have them spec the bore sizes out to see if they are in tolerance for the original size.

    Those bores seem to be a bit glazed over and Id have them rehoned. Then you can get stock size pistons and new rings and be fine. This would only be the case if your mechanic says the bores are within tolerance. If you can feel a lip at the top of the bores where the rings stop, then you need to bore to the next size. IMO, its not wise to skip bore sizes unless you really have to because of a bad score.

    It should actually be easier to find stock size pistons since everyone goes after the next overbore sizes. Hopefully this is the case for you.

    Getting the jug honed, stock piston sizes and new rings, would be the cheapest and easiest route to take by far...

    The next big deal is checking the valve train. Seeing as the piston crown is still intact and it was just the ringlands that broke... The head could very well be in usable condition because the broken bits were contained within the piston and the rings.
  9. xschris

    xschris XS400 Guru Top Contributor

    Those are 1.00 over pistons which would be fine if you could find 1.00 over rings. That's the hard part.

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