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  1. #1

    Polaris 750 Standard size piston availability?

    Hi guys I have a 94 Polaris 750 installed in a 15' jon boat and am trying to rebuild it. I sent the jugs off and they nykasiled them to standard bore size (69.75mm) I believe. I also got a weisco piston kit that was supposed to fit but as it turns out they are 0.020 over. Now the shop I ordered from says that standard bore pistons are no longer available and the pistons cant be sent back.

    My first question is this: Are there standard size pistons still available?

    I would rather keep the jugs I've got and find pistons to fit, then if I have to bore it later I can. I plan on keeping this thing going for a while so I want to keep my options open. I do have another set of jugs but I think they are too big already for the pistons I have, they measure 70.358.

    It is just a shame to bore these jugs out bigger after I paid to have them plated.

    What would you do?


  2. #2
    Banned User
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    Why would they plate an iron sleeved cylinder? Standard size pistons are available from wsm and wiseco I believe. You may even be able to still get Polaris oem.

  3. #3
    BlueFishCrisis's Avatar
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    Quote Originally Posted by DT7 View Post
    Hi guys I have a 94 Polaris 750 installed in a 15' jon boat and am trying to rebuild it. I sent the jugs off and they nykasiled them to standard bore size (69.75mm) I believe. I also got a weisco piston kit that was supposed to fit but as it turns out they are 0.020 over. Now the shop I ordered from says that standard bore pistons are no longer available and the pistons cant be sent back.

    My first question is this: Are there standard size pistons still available?

    I would rather keep the jugs I've got and find pistons to fit, then if I have to bore it later I can. I plan on keeping this thing going for a while so I want to keep my options open. I do have another set of jugs but I think they are too big already for the pistons I have, they measure 70.358.

    It is just a shame to bore these jugs out bigger after I paid to have them plated.

    What would you do?
    Bryan is right, they are still available, and the cylinders shouldn't have been plated.

    Also, be sure you don't mix up metric and standard sizing when selecting pistons. .010 = .25 mm, .020 = .50 mm, and so on. Standard bore is 69.72 mm. If the piston kit is .020 over, it is then 69.72 mm + .50 mm or 70.22 mm. This leaves 70.358 mm - 70.22 mm = .138 mm. Since the piston is centered in the bore, split the remaining difference in two to get your cylinder to piston clearance or .138 mm / 2 = .069 mm. Spec calls for .11 mm - .16 mm with a .20 mm limit. Double check your bore measurement to make sure you are finding the absolute largest diameter of the cylinder. The other jugs you have may work.....

  4. #4

  5. #5
    Wow thanks for the fast replies.

    OK, so is there going to be a problem if I run these jugs as they are plated? Should I just have them bore the jugs to match the pistons I have?

    The shop that I'm working with is a snowmobile shop and plating is pretty common but they may not understand the nuances of these older ski motors and obviously I don't either. Please advise.
    Last edited by DT7; 04-29-2014 at 03:48 PM. Reason: spelling

  6. #6

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    sbt now has standard 750 pistons also

  7. #7
    LaveyT's Avatar
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    I don't see where Ni-com (plating) would hurt anything?

  8. #8
    john zigler's Avatar
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    I have standard pistons IN Stock. Please be sure to read up on the Polaris Fuji engines, upgrade the fuel system, and determine why your engine failed in the first place. ( lean condition from fuel related issues, crank seals, possible overheat, lack of oil etc).

    I also agree, there is no need to plate a steel bore cyl. Who ever did knows nothing about engines.

    Also, it is important to know, pistons are not always the "exact" same size. Especially different manufactures. Even when you re-do a plated cyl, you plate the cyl, then finish hone it to match the new pistons. This is even true with snow mobile engines. Your shop should have advised you of this before taking your money. I think you need to find a different shop....

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  10. #9
    Thanks for the advice guys I appreciate it. I have been reading here on the hulk about the Fuji problems (Awesome resource). Mine was lean burn down on the last cylinder on the fuel line, combined with a weak single fuel pump. I completely went through the carbs with an oem kit. I now have two fuel pumps and will plumb them in proper and make sure the pop-off is functioning properly. I replaced crank seals while I had it torn down for insurance.

    "Even when you re-do a plated cyl, you plate the cyl, then finish hone it to match the new pistons." This is what I thought I was getting. This was the first and last time to work with this shop.

    I think I am going to order a new set of pistons and try to sell this set I have (they won't take the other pistons back) or save them for a future rebuild.

    Thanks again for the advice and help, there is a wealth of knowledge here

  11. #10
    john zigler's Avatar
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    You could bore your cyls to match your new pistons. BUT honestly I do not recommend Wiseco's in a recreational application. They are an excellent piston, very strong, and light, BUT they are a forged piston, and due to the silica content of the aluminum expand at a higher rate than an OE cast type piston. Due to this, you need to run a much larger piston to cyl wall clearance, and will be closer to max wear specs. They are good in a high compression, high revving race motor, that is rebuilt often any way.

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