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Info ZXR250a oversize pistons

Discussion in 'Kawasaki 250cc In-line 4's' started by stevodude, Dec 3, 2017.

  1. stevodude

    stevodude Active Member Premium Member Contributing Member

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    HI,

    the zxr250a has 48mm bore 34.5 stroke.
    I picked up a zx2r a while back, but i am now stripping it back and rebuilding.

    all bores are 48.20xmm so from standard 48mm bore I need a first oversize piston and rings which is 48.25

    found a few standard ring sets for 48mm, and further searching returns NO zxr250 48.25mm pistons.

    But there are quite a few fzr250rr os piston and ring sets 48.25mm, I am wondering if the pistons are similar, as the images on aliexpress for the fzr look VERY similar to the zxr250.

    The other option is to bore it out to 49mm and fit std bore zxr250c pistons.

    Looking difficult, but not impossible to rebuild...
     
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  2. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    Going to 49mm would be your best bet, if the liners have enough meat left on them.
     
  3. Andych

    Andych Well-Known Member Premium Member Contributing Member

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    You would also need to ensure crown height, wrist pin diameter, ring height, any wrist pin offset etc were all exactly the same... my bet is they wont be... but you never know your luck... If it were me I would do an awful lot of investigation before laying out $ for a rebore and new pistons and rings.
     
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  4. ShaneP

    ShaneP Well-Known Member Premium Member Contributing Member

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    @GreyImport did I see a comment on Facebook in the zxr group about this very topic, with your 'name' on it?
     
  5. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    Not guilty :cool:
     
  6. ShaneP

    ShaneP Well-Known Member Premium Member Contributing Member

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    Screenshot_20171207-074200.png
    Might be an avenue to try.
     
  7. Mr_Muddle

    Mr_Muddle Active Member

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    Yeah that was me.

    The problem with oem 49mm pistons is that the gudgeon pin is really low on the piston so you have to beef it way up using barrel gaskets to avoid valve strike. I have been running this for a few years now and I hate it. So I have spent some toime and got oversize and standard ones sorted from germany.

    The price is reasonable. new piston and rings will set you back $110 for oem 49mm and pins and circlips say an extra $10-$20. These are being custom made on limited run by wossner in germany so the quality will be there.
     
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  8. stevodude

    stevodude Active Member Premium Member Contributing Member

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    Yep, I've put my name down for a set of pistons.
    Cheers, S.
    Next is if I need new valve guides, they seem a little loose, but not to bad. or just throw in some new seals.
     
  9. Bryn duFresne-Mann

    Bryn duFresne-Mann Active Member

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    Just to avoid Starting yet another thread on this very similar and well rehearsed topic!-
    Has anybody looked into What oem cylinder liners may be able to be retrofitted into our wee jems? were you to re-sleeve them you have the potential to add almost 8mm (ill do a proper measure on one tomorrow) worth of 'meat' or bore
     
  10. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    Out of curiosity, what valves do you think are in the head on the right of the following picture? I've only ever seen flat face valves in any ZXR250 engine I've had apart, but all of them were A model engines. I'm wondering if other models (C) have a recessed dome in the valves?

    Valves.jpg
     
  11. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    I know the fizzers have a dish on the valves and domes on the pistons.

    The only thing I can think of is that with the dishes and domes, it increases the available surface area in the combustion chamber for better fuel atomisation.
     
  12. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    @Bryn duFresne-Mann - I'm going to be putting the La Sleeve FL-52, from the all purpose flange sleeves, into an FZR250 in coming months.

    There may be a suitable candidate from amongst the multitude of sleeves available or the common mutli purpose sleeves they have available. They're very helpful also when it comes to questions

    https://www.lasleeve.com/tech/all-purpose-sleeves

    If you can give me some dimensions of the ZXR piston I can probably nominate a few candidates for pistons in the 52mm - 54mm range also I've done way to much reading on the subject and have a lot of links
     
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    Last edited: May 4, 2018
  13. my67xr

    my67xr Senior Member Staff Member Premium Member Contributing Member

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    Dished valve's are a touch lighter, better for high rpm's and gives the valve a low pressure area beneath it to help the intake draw fuel/air in.
    Flat exhaust valve's help the gas flow out of the combustion chamber too
     
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  14. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    So, should the intake valves be dished and the exhaust valves be flat for best results?
     
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    Last edited: May 8, 2018
  15. Bryn duFresne-Mann

    Bryn duFresne-Mann Active Member

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    This appears to be the consensus of both old school builders and more modern schools of thought on the internet, my 10 second fluid dynamics thought experiment also concurs! So all told it sounds good enough for me!
     
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