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  1. #1

    '93 SL650 piston tolerences all screwed up!

    This is gonna be short ans to the point. Rebuilt part of my sl 650. PTO was burnt. New PTO piston and rings. Put it it the water and only center piston firing. Checked compression (yes, I should have done that before I put it back in!) PTO 60, Center 90, Mag 80. Pulled top end back off. Now.....read this carefully...... By the way, I've owned it since it was new! No prior rebuild.

    New rings and PTO piston from WSM and cylinders were honed. Ring end gap is approx 3/8" !! Suppose to be .008-.016 max! Both are brand new! Checked the piston-sleeve clearance and it's .006, so the bore is not oversized. How can the piston be correct, the bore be correct and new rings be so out-a wack? The rings have the markings WSM-931 on them. The ring end gap from the center piston that is original in all aspects is .024.

    Bottom line, will that end gap on the PTO cause a 60 PSI compression?


  2. #2
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    I would think so yes. Maybe they sent the wrong rings or the end was broken off.

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    All your readings seem low. Were you holding the thottle wide open when you checked compression?

  4. #4
    The ring is not broken! The cylinder never even fired with such low compression. Wrote an email to WSM re the marking, but no response! Can an imporper gap cause that low compression

  5. #5
    Polarisitis loonatik's Avatar
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    Pistons installed backwards and ring gap is on exhaust port side letting air escape hence low compression numbers? I'm just throwing this out there, we all have our 'Duh!' moments.

  6. #6
    Pistons are correct orientation. When I took everything apart and held the old and new rings together, the "uninstalled ring gaps" are clearly twice as large on the new rings, but the rings are identical in size! They have to be the wrong ones! It was my stupid mistake. I had someone here that could help me install the jugs and I rushed the job and didn't check the end gaps. Big mistake! I also see that the new rings are actually slightly narrower than the originals; ie. narrower in the groove depth aspect, not the thickness of the rings. WSM still hasn't emailed me so I ordered 3 more sets from STB. In order for these to achieve the proper end gap, I'm guessing these rings would have to be installed in a 60mm jug, not a 65mm?

  7. #7
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    Are you measuring the end gap with the ring in the cylinder? Comparing rings that are not in the cylinder bore has no meaning. You check end gap by sliding the ring into the cylinder and sliding a feeler gauge into the gap. A new ring is going to have a wider uninstalled gap because of the tension on the ring.

  8. #8
    Yes. After removing everything, I checked the gaps with them in the cylinder! 1/2" from the topa and at the bottom to verify any taper. The reason I described them when they were held up together (old and new) is with the new one having a gap twice as wide as the old one that is out of tolerance but their uninstalled circumference is identical, I should have easily recognized that the new one could NEVER be correct when installed! I just finished doing a quick hone in all 3 in hopes that my rings will come today! I'm just surprised that the WSM rings were so far off! There is absolutely no way that the rings could be for a cylinder even close to 65mm. I'm not blaming WSM. I know better! I just had someone to help at the time and I forgot to do the ring clearance check! I just wish WSM had responded to the Markings on the rings so I would know for sure what the rings were meant for! I told them in my email that it was my mistake and I wasn't asking for anything other than info!

  9. #9
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    Try giving WSM a call instead of an email.

  10. #10
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    OK...Here are some pics. I usually have a problem figuring what order the pics appear on the thread, so here is the info.

    The one picture shows the OLD, ORIGINAL ring gap (the smallest gap). It's my '93 sl 650 that I bought new.

    The other picture is with the NEW ring from WSM (markings can be seen on the ring) A search of the WSM site revealed that the markings WSM 931 are for a Polaris 650, which is what I ordered!

    The gap on the 19 year old ring is approx .040 (should be .008-.01 The NEW ring gap is approx .090

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