Slapping noise, possibly piston slap. Clearance question

Discussion in 'Engine' started by Conner Jones, Jan 14, 2020.

  1. Conner Jones

    Conner Jones Member

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    Okay so I had another thread open but it started out as a different issue that I ended up fixing, and then I started talking about this problem but I figured I should start a new thread specifically about this problem. So recently I rebuild my blaster. I took the cylinder to a machine shop and had them bore and hone and chamfer it. They told me to get a piston made for 67.5mm bore. I bought a pro x piston kit and a wiseco wrist pin bearing and all new gaskets. I didn't check the clearance or ring end gap or anything because I figured the machine shop probably did it right. I assembled the motor, and now I am noticing a loud slap/pop noise than happens about 3 or 4 times rapidly as I am at about 3/4 throttle and then start to decrease throttle a bit. I checked for air leaks with leak tester and no leaks. I just took off the head, and it looks like the piston has a little bit of a gap toward the intake side, and almost no gap on exhaust side. I don't know if this is normal or what. Maybe my rings did not sit properly and were slightly on top of the pin marker thing. Or maybe the piston or cylinder is not the right size. I can put my finger on top of the piston, and it wiggles a tiny bit back and forth. Do I need to take off the cylinder and measure the cylinder with telescopic gauges and the piston with a digital caliper to see if it is in the clearance limit? And should the clearance be the same as stock or is it different because this is an oversized piston and bored out cylinder?
     
  2. Larry's Shee

    Larry's Shee Active Member

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    Here’s how it should go
    1.
    Measure cylinder
    2. Buy piston
    3. Bore/hone to fit that piston

    Look to see if you have unusual wear marks on intake side of cylinder, although unless it was way off sized, I don’t think you can get into the cylinder unless the rings are right
     
  3. Slydog

    Slydog Member

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    You have any pics
     
  4. Slydog

    Slydog Member

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    What's cylinder look like with piston all the way down?
     
  5. Slydog

    Slydog Member

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    The prox is a cast piston i believe. Could it have been bored for a forged piston?is exhaust tight?
     
  6. Conner Jones

    Conner Jones Member

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  7. Larry's Shee

    Larry's Shee Active Member

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    Yup, sounds like piston slap.

    Somebody made a mistake somewhere
     
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  8. Slydog

    Slydog Member

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    Guess pop cylinder off and measure everything and see where your at.:mad:
     
  9. Conner Jones

    Conner Jones Member

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    Alright, im gonna get a digital caliper to measure the size of piston. Can I use the caliper to measure the cylinder too? I know people usually use bore gauges, but if the cylinder is bored the same all the way through, shouldnt i be able to measure the top of the cylinder with the caliper?
     
    Last edited: Jan 15, 2020
  10. Larry's Shee

    Larry's Shee Active Member

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    That will be “close enough” for now. Measure top and bottom of cylinder. Measure piston in multiple places front/back , side/side top/bottom. Most pistons are what’s known as “cam ground” , basically a tapered oval. Smaller at top and inline with pin. That’s so it’s round when it expands from heat
     
  11. Slydog

    Slydog Member

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    Pics of piston and cylinder with measurements if possible
     
  12. Conner Jones

    Conner Jones Member

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  13. Conner Jones

    Conner Jones Member

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    I got a dial caliper. When measure the cylinder, do i need to apply lots of pressure to stretch the caliper as hard as possible, or do i just apply a tiny bit of pressure? When I apply lots of pressure, it gives me a higher number than when i apply a small amount of pressure, so I don't know which number is accurate.
     
  14. Slydog

    Slydog Member

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    You don't need alot of pressure.does the caliper have a thumb wheel on it.
     
  15. Conner Jones

    Conner Jones Member

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    Yes, kinda hard to get to as its on the bottom side of the caliper, but it does have one
     
    Last edited: Jan 18, 2020 at 5:18 PM
  16. Slydog

    Slydog Member

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    Thats what you use for your tension
     
  17. Conner Jones

    Conner Jones Member

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    Ok so i just reoved the cylinder, and i found some bad news. My piston has 2 wear marks, and my cylinder does too. I will attach pictures. I measured the clearance and it was 0.002 inches so good clearance. I dont know why this halpened. It only had about 5-6 hours on it.

    https://drive.google.com/file/d/1ig4dnDuV-XsGglq9vFguATPAxtE86d_p/view?usp=drivesdk

    https://drive.google.com/file/d/1ihf0pbubIyXsYzBRUGsdm6gStrXGDl1z/view?usp=drivesdk

    https://drive.google.com/file/d/1ipsgz2vyJ46JIRQh_X6aqmnF3inctkIo/view?usp=drivesdk

    https://drive.google.com/file/d/1ip21-cSDJDqAZNDwk9wG06JVW3hVyL2K/view?usp=drivesdk
     
    Last edited: Jan 19, 2020 at 1:43 PM
  18. Slydog

    Slydog Member

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    Did you do proper heat cycles?and break in before going wot?ring seal don't look to good.kinda looks like cold seizure is happening.you have to warm the engine up to operating temp.piston is aluminum. Expands alot faster than cylinder.
     
  19. DKavcak

    DKavcak Member

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    Whoa, last time I saw a piston do that was on my buddy Jon's KTM 300 EXC.
    So he ended up having a wrist-pin bearing go bad on him. I would check the cage of your wrist-pin bearing.
    If your tolerances are good, I'd say either wrist-pin bearing was totally wiped or it was possibly heated up too quickly.
    Also, have you checked your crank bearing clearances?
     
  20. Conner Jones

    Conner Jones Member

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    I replaced the wrist pin bearing with a new wiseco one when i rebuilt the top end. Im not sure how to tell if it is bad or not. And I did 3 heat cycles exactly like on ken o connors video. And i let the quad warm up by riding it around slowly and keeping the rpms down for a good 3 or 4 minutes before going hard on it. How do i check crank bearing clearances? And I was thinking, could it be possible that my rod is just ever so slightly bent? Im not sure how i would check to see if it was perfectly straight.
     

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