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  1. #1

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    1200 Damaged Piston Swap Question ?

    Tore down my GP1200 today to find #3 cylinder has been a little hot. Down to 90 psi. The previous owner pinched the oil line to #3 in between the flame arrestor base and the carbs. Duh. The cylinder isn't too bad so I'm thinking hone and re-ring. I have 120 psi on the other two cylinders. My question is, I am thinking I should also replace the #3 piston as it has some major pits in it from heat.

    My block has 64X pistons.(like from a 760) If I buy an OEM piston to replace it, should I get a proper 65U or order the 64x so they are all the same? The pistons also bare a 1.50 marking, wondering the significance of this as well..?

    Can I replace the one stock with an aftermarket piston and if so what would be balanced well with the other two stock pistons.

    Should a Re-ring all 3 cylinders or leave 1 and 2 alone since they are at 120 psi?

    I really appreciate any advice...

    Thanks
    -Brian
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  2. #2
    urugol's Avatar
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    The 1.5 means that it is an oversize piston, make sure you get the equivalent replacement. I would go with aftermarket, do the re-hone and check your head should be some pitting there as well you may replace it as well depending how bad it is. I would leave the other pistons alone if on a budget. If not I would do all 3 at the same time so you know it's done you can always have a couple of spares for later to get you out of trouble. Make sure you verify those all lines are running right as well.

  3. #3

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    Thanks Urugol... Yeah the head is a little pitted too on that cylinder, but Its just a for fun ski, my only concern is reliability not necessarily speed. So the 1.5 means its a stock std 64x piston + 1.5 oversize? How does that compare to a 65u? Trying to figure out what to order. thinking stock.

    Thanks again...

  4. #4
    Yamaha artisan Cutlass's Avatar
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    If you're at 1.5 over you're pretty much maxed out. Some people run 2.0 over but some people have overheating issues at 2.0mm over. If you wanna stay 1.5 over then make sure the cylinder can be honed out to keep a reasonable piston clearance (ie not too big of a clearance). I usually prefer to do all 3 at the same time but if you want to keep the other 2 pistons untouched, then try to stay consistent with what piston you choose to replace the third with.

  5. #5
    Yamaha artisan Cutlass's Avatar
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    I think the 64x (760) piston is exactly the same as the 65U (1200) as far as weight and stock diameter.

  6. #6

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    Thanks cutlass... I was surprised at the 1.50 on the pistons meaning its been bored out. It only has 150 hours on the ski, I was expecting stock bores etc... Looking for a OEM 64x piston at 1.5 oversize sounds like is the best route...

    So just to confirm.... whether I order a 65U or 64X the 1.50 means 1.5mm oversize thus 85.5mm bore ?

    Looking at OEM Yamaha fiche the biggest I see the pistons go oversize is .5mm (Am I missing something) My pistons are OEM 1.50's so where do I find them?

  7. #7
    Yamaha artisan Cutlass's Avatar
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    Yes I think its a 85.5mm bore either way (99.99% sure)

  8. #8

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    Ok thanks man... I see you are from Madison,WI ... I'll be travelin up that way in June to Oshkosh. My band is playing at Country USA opening night...Pure Grain... if your into music festivals... Good to meet you and thanks for your input....

  9. #9

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    OK so now for the mystery, since we are pretty sure my pistons are 1.5mm oversize(1.50 marking on the piston) After doing some hunting it appears OEM are only available in .50mm and .25mm oversize... Where did these pistons come from? how can I get a new one that matches? They have 64x on them so they are obviously Yamaha right... The 65u pistons are also only available in .50mm and .25mm oversize from Yamaha...

    Anyone have any thoughts?

    Could I mix a Pro X piston with my other two stockers or would this present a balancing issue?

    Sorry guys this should be a simple thing but its confusing as hell to me...Thanks

  10. #10
    Yamaha artisan Cutlass's Avatar
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    OEM and ProX are made by the same company (at least they were). So ProX does stamp their pistons 65U and 64X. You probably have ProX pistons

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