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  1. #1
    PolarisNut's Avatar
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    Whats the deal with 80mm flat top pistons?

    As some of you may know, I've recently acquired a Speedwerx triple pipe GPR. It was always a rec boat, but I have now found that it has flat top pistons in it. I pulled the race gas domes and was going to swap them to pump gas ones, but the squish angle on the pump gas ones looks too steep for flat tops. The pistons are in perfect shape (pulled two cylinders), and I will leave them if I can find domes to work. I know what advantages flat tops have over domed pistons in general, but why are they being used in this case? I have receipts from the seller that show them as "Speedwerx cut pistons". What pistons are these? They are a cast piece. Any durability issues? I noticed that they have very thin rings. I really don't want to be running any odd ball setup. If I ever need a piston quickly, I want to be able to easily acquire one. I know that flat top pistons can also change port timing, but I dont know how much in this case.


  2. #2
    btldigger's Avatar
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    If you look on the side of the piston next to the pin hole does it happen to have HF2 on one side and 3731 on the other cast into it?

  3. #3
    CHICKA WAH WAH bren's Avatar
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    have you measured them to 80 ?

    some run 85's if it was a SS boat

    you may be looking at cut kx500 pistons, same as mine was after much searching. they re drill the pin holes from 19mm to 22mm and cut new clip seats.

  4. #4
    PolarisNut's Avatar
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    Quote Originally Posted by bren View Post
    have you measured them to 80 ?

    some run 85's if it was a SS boat

    you may be looking at cut kx500 pistons, same as mine was after much searching. they re drill the pin holes from 19mm to 22mm and cut new clip seats.
    Yes, 80mm...it was never an SS boat. The KX500s are larger than 80mm correct?

    Don't see any markings on pistons, but I haven't removed them from the rods to see the underside.

  5. #5
    CHICKA WAH WAH bren's Avatar
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    alot of the boys on the race circuits cut the oem pistons to make flat tops.

    you may be looking at 66v pistons

  6. #6
    btldigger's Avatar
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    It would be visable on the outside if they are there. The reason I asked about the #s is that I had heard they used kawi pistons in some of the yamahas. I know speedwerx did flat top motors for the ultra/ stxr so I thought that the pistons and domes might be the same. I have speedwerx cut pistons in my ultra now.

  7. #7
    kham383's Avatar
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    Most 80 80.5 & 81mm flattop setups are Prox or wiesco setups Kawasaki JS750/1100 pistons.
    Have the cylinders been decked on the bottom & the top, if they are cast they are most probably Prox kawi js750/1100 pistons.
    Compare your cylinders to a stock set to figure out if they have been shaved.
    The best way to sort it out is to relieve some cc"s from your race gas domes to achieve 155/160psi compression.
    Standard 1200/1300 domes wont cut it.
    I wouldn't change the squish angle just relieve the actual dome cc
    Flattops can handle a little higher compression than domes without getting into detonation problems

  8. #8
    Moderator beerdart's Avatar
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    Anyone have any more info on this set-up?? We are working on 1200 with this set-up. Can the stock head be used uncut?

  9. #9
    PolarisNut's Avatar
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    Quote Originally Posted by beerdart View Post
    ...Can the stock head be used uncut?
    No, you have to use one cut for flat tops, or you will have issues with the squish clearance being too high. Mine ended up being cut, stock 80mm ART pistons. FWIW, my ski is running a higher MPH and more RPM with lower compression and standard pistons than it was when it had the flat tops, with nothing else changed on the hull.

  10. #10

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    I have a set of these pistons if your interested just PM me.
    Thanks, Travis

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