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  1. #1

    WSM Piston Clearance

    Hi y’all, long time lurker, fairly new poster here.

    Got a question that I’m getting conflicting info on when I use the search button. I picked up a basket case ‘07 RXP, 215hp, for cheap, and am rebuilding the engine. Got it all apart and it actually looks great, but minor rust in the bores will necessitate a bore/hone job, and new pistons. So I picked up a new set of WSM’s, .020” over, on eBay, for 200 bucks, which is fine for me since I just want a stock rebuild. There is no spec sheet that came with the pistons though, and I see anywhere from 1.5 to 5 thousandths clearance thrown around on the forums.

    Now I get that there are different alloys out there with different expansion rates, and different clearances, as well as to run ‘em looser for more boost. That all said, can anyone tell me 1) if the WSMs are 2618 or 4032 alloy?, and 2) your firsthand recommendation on clearance with these particular pistons in a stock application?

    And no, I’m not buying CPs or Wisecos. This one’s staying all stock, lol.

    Thanks,
    Kevin

  2. #2
    ptscon's Avatar
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    Stock RPM I'd do 3.5 but if you want to have a bit more margin if in the future you want to run higher than do 4. I don't think you'll notice any difference between the two.

  3. #3
    Actually, those were the numbers I was “thinking”, based on other engines I’ve built before. Good to know from someone that’s built 4TECs and used these particular pistons, though - thank you.

    BTW, I got to thinking, we have an XRF machine at work (I work in Engineering). That machine will scan a material and give the elemental breakdown. Going to compare the OEM (Mahle?) piston material to WSM. With the super tight clearance that the OEM runs, I am guessing that it is a 4032 alloy. I know one of the materials has a telltale flag of greater than 12% Silicon content. I’ll post back results.

    Thanks again,
    Kevin

  4. #4
    ou812's Avatar
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    I went 3.0 on the 1st build then went to 3.5-4.0 depending on the setup. Done many builds using 4tec WSM.
    Here's an old post showing some visual diffeneces between CP, OEM and WSM. Some good input by some fellow members too..
    http://greenhulk.net/forums/showthread.php?t=221362


  5. #5
    Good info.

    So I took a look at what I got. I can say that they are NOT WSM pistons. eBay seller said that they were WSM in his description. However, carefully comparing them directly to the OEM Mahles that came out of the ski, they look exactly the same as those. Interestingly, these all came in plain packaging with the only label “made in Taiwan” on the ring pack. No other numbering on the parts.

    Wondering if I got a set of early WSMs that were a direct knock off of OEM. Or a knock off of OEM, or ???

    I guess I’m gonna XRF them tomorrow to check material, and if they’re 4032 alloy I’ll run clearance to the high end of the Rotax spec. That’s after I mic them all over to make sure they aren’t machined all wonky.

    Thanks,
    Kevin

  6. #6
    sounds like you got a set of SBT clean skin, look underneath to see if theres a label ground off
    Quote Originally Posted by lightningtruck View Post
    Good info.

    So I took a look at what I got. I can say that they are NOT WSM pistons. eBay seller said that they were WSM in his description. However, carefully comparing them directly to the OEM Mahles that came out of the ski, they look exactly the same as those. Interestingly, these all came in plain packaging with the only label “made in Taiwan” on the ring pack. No other numbering on the parts.

    Wondering if I got a set of early WSMs that were a direct knock off of OEM. Or a knock off of OEM, or ???

    I guess I’m gonna XRF them tomorrow to check material, and if they’re 4032 alloy I’ll run clearance to the high end of the Rotax spec. That’s after I mic them all over to make sure they aren’t machined all wonky.

    Thanks,
    Kevin

  7. #7
    Quote Originally Posted by winterstick View Post
    sounds like you got a set of SBT clean skin, look underneath to see if theres a label ground off
    No label ground off on these. So this is an SBT piston? Wonder who makes it for them.....they look identical to the OEM Mahle. Do you know anything about them?

    By the way, the XRF shows this as a 2618 alloy.

    Thanks,
    Kevin

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