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  1. #1

    1 oversize piston???

    I have a 1997 Kawaski 900stx 3cyl.
    Does anyone know if it will damage my engine if i bore and oversize the #1 piston to 1MM=.040 over ?


  2. #2
    steve45's Avatar
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    It can be done, but generally isn't. It might possibly introduce vibration because of different piston weights, but you can weigh the pistons while you have it apart and solve that problem by removing a little metal from the heavy piston. Any power difference between cylinders is more likely due to how well the rings seat than such a small change in piston size.

    That said, I wouldn't do it simply because it will confuse things the next time you have to do a top overhaul.

    And, Waz, a lot of people have different size feet.

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  4. #3
    so your saying it should not be done. is it because of the 1MM or just because its a different sized piston? i was told .5mm would be ok but since its 1mm i have to get a sleeve. but that does not make sense. if i have to get a sleeve an have it bored to standard size why do they make 1mm and a 1.5 mm????
    they make oversized pistons for a reason. do you work in a engine shop? or are you a engineer of internal combustion engines?

    does anyone have any real world experiance with pistons up to 1mm or .040 over the standard bore size?
    the defferent compresion ratios should be compansated for with the new piston? or milling of the head?
    is there anyone out there with real answers not just here say?
    i dont wan to send a extra $250 + if i ont have too.
    Last edited by lasportsmn; 08-13-2012 at 01:20 PM.

  5. #4
    Thank you steve45 for the information. i did not see your post until after i responed to the last guy. i was thinking of doing the weight and balance of pistons also.

  6. #5
    thetolleyman's Avatar
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    I work in a shop... and would say "don't do it". I can't say I've ever had to rebuild an engine because it was running different pistons (most likely because its rarely done), but it is a general rule of thumb that you don't mix pistons. Even the same size, but different brands, will not weigh the same or give the same compression. Will it work, sure, you may not even notice the vibration. The bearings will likely be the ones to suffer down the road. If you are looking to just get another season out of an engine before you do a total rebuild, perhaps you will get by.

    Just my 2 cents....

  7. #6
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    Quote Originally Posted by kruptimmyk View Post
    so your saying it should not be done. is it because of the 1MM or just because its a different sized piston? i was told .5mm would be ok but since its 1mm i have to get a sleeve. but that does not make sense. if i have to get a sleeve an have it bored to standard size why do they make 1mm and a 1.5 mm????
    they make oversized pistons for a reason. do you work in a engine shop? or are you a engineer of internal combustion engines?

    does anyone have any real world experiance with pistons up to 1mm or .040 over the standard bore size?
    the defferent compresion ratios should be compansated for with the new piston? or milling of the head?
    is there anyone out there with real answers not just here say?
    i dont wan to send a extra $250 + if i ont have too.
    Waz has no real world experience other then working and wrenching for one of the biggest names in racing. Building high hp motors. working with oems and still doing this on a daily basis. You might want to sit up and listen when that man speaks he probably has more real world experience the. 98% of the people on the form. I know someone who did exactly what your wanting to do in a truck it lasted 8000 miles. Good luck.

  8. #7
    this is not a race motor and is not a 4stroke truck motor. and by his post it is very hard to tell anyof those of those things you are claiming about him. i meant no harm in my response so i apologize to you and Waz. simple saying its not a good idea because of shoe sizes is not sufficent.
    back to the point.

    how much will compression be different by? why is .5mm ok but 1mm bad ? will simple moding the head fix the compresion ratio?

  9. #8
    RLACEMAN's Avatar
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    This is not a real high output application so it should work, I have done this on engines with individual cylinders without a problem, however I believe this motor is a mono-block & think it would be better to do all of them oversize, as putting a monoblock engine together is a PIA! If you are going to run it hard its worth the extra $$. Mixed pistons may or maynot cause a future problem. Compression & weight of pistons can differ slightly even between pistons of the same size. If you are going to do it, use the same brand piston as the other 2. .5mm ok but 1mm bad, the larger they are up to even 1.5mm there is a bigger weight difference & there is no way to know what the tipping point will be that could cause a vibration or early bearing failure.

  10. #9
    Thanks for the input RLACEMAN.

  11. #10
    I have seen many 2 stroke outboards with different size pistions. You are not going to hurt anything.

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