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  1. #1
    kmaher's Avatar
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    Piston Opinions!

    Hey everybody, I'm thinking about doing a pre-emptive top-end rebuild on my 1300r. When I winterized it this year I noticed that my compression in the rear cylinder had dropped by about 8psi from the beginning of the season. It's still within tolerances of the other cylinders, but it's a moderately modified machine running higher compression and I've got about 160 hours on it now, so it's about time, and I imagine I just caught the beginning of a ring that's losing its life.

    Anyway, I'd rather rebuild now than allow the ring to wear further and potentially cause a catastrophic failure. I haven't pulled the head in a while, but I'm guessing the pistons themselves will still be in OK condition. Despite that, I'm thinking I may as well replace them along with my rings (tell me if you think I'm in the wrong here and should keep the original stock pistons in there if within spec).

    Now, as for pistons, what are everyone's preferences and why?

    - I would consider replacing for stock, but I'm curious if any aftermarket are truly considered "superior" and for what reason.

    - I replaced a prior 1300r's pistons one time with SBT pistons when doing a rebuild, but a lot of folks seemed to chastise me for that, bemoaning the quality of SBT. Other than anecdotal reasons about a friend of a friend who knew a cousin's sister's dog whose prior owner blew up because of SBT's horrific engineering, is there anything empirical to support the claims of inferior quality? Any personal negative experiences with them? Or positive? The 1300 I replaced it on ran fine, but I sold it within 10 hours of rebuild. The current owner still hasn't reported any issues 2 years later, and he says he runs it hard.

    - Heard Wiseco is best left for "race-machines", and not suitable for rec use. Truth to this?

    - WSM I have no experience, but have heard is junk because of expansion/contraction. Experiences here?

    - Other brands worth considering?

    As you can see, everything I've heard is even anecdotal, other than my personal indifferent experience with SBT on a completely stock craft, so I'm looking forward to hearing personal experiences with brand(s) of your preference!


  2. #2
    jeffg426690's Avatar
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    My personal two cents:
    If you want to be accurate:
    Pull the jugs, take them to a shop and have them measured. Procedure and tolerance is listed in the service manual.
    Yamaha 2 stroke cylinders/pistons have a spec size A, B, C, & D - which size you need is dependent upon your cylinder measurements. The difference from one to another is a few thousands.
    In aftermarket, I've had great success with ProX pistons and rings - they sell according to Yamaha specs of A,B,C,etc.
    Measure them, get appropriate size pistons, measure ring gap make any adjustments that may be necessary and put it back together.
    There is a possibility that you may only need new rings, accurate measurements with the proper micrometer tools will tell you if you need pistons or not, they can measure the pistons and the cylinders to see if they are in/out of SM specs.
    Any good motorcycle shop can do this if you don't have a pwc shop nearby that you trust...
    I don't have a say on WSM, SBT on then other hand is well known for sloppy specs and tolerances. From what I know, Wiseco are forged and have tighter tolerances and different heat expansion factors that make them more favorable for high performance vs typical recreational use, others can give greater insight there.
    Best advice I can give you is get good measurements and go from there, just my opinion... (we all know what they say about opinions!)

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  4. #3
    butterbean_29512's Avatar
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    ProX pistons are the equivalent of OEM pistons. I have them in my 1200r, with great results. They do come in A,B,C, and D sizes. Correct measurement is key. WSM pistons are ok in stock form, but not modified form.

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  6. #4
    kmaher's Avatar
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    Just curious, if I go ProX, has anyone simply not matched size and gone with Size A despite their cylinder designation? I feel like I may prefer the extra ten thosandth of an inch or so clearance, and can't imagine that small of a tolerance impacting my compression.

    Thoughts?

  7. #5
    jeffg426690's Avatar
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    Sounds like a potential recipe for disaster to me. The specs are there for a reason, 10 thousands is bigger than you think.
    The cylinder, piston, and ring are a married set that all work together. Small pistons means your rings will be floating around a lot in the ring groove on the piston, too much clearance and you'll sling a ring. Wanna see major carnage? - look at pics of when that has happened to folks (for various reasons...) Extra piston clearance is not a measure of safety by any means.

    Best longevity is to stick within the tolerances outlined in the service manual.
    Again, just an opinion... I wouldn't do it. You'll have the jugs off anyway, give me a good reason not to have them inspected and measured the way Yamaha tells you to do it...

  8. #6
    kmaher's Avatar
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    Well, my thought is that a new Size "A" piston in the largest diameter cylinder would still meet specs for a "good" piston per the service manual, so ultimately, if I were evaluating a motor for a rebuild using specifications, the smallest piston in the largest cylinder would still give you a thumbs up (by a decent amount).

    Now, my thought process applies more to the Wiseco pistons since they're forged, but with the different expansion and contraction between cylinder and piston, that extra ten thousandth of an inch may be beneficial during warm-up.

  9. #7
    Tool Bag water worx's Avatar
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    Do halfass work & expect halfass results.

  10. #8
    jeffg426690's Avatar
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    Quote Originally Posted by kmaher View Post
    Well, my thought is that a new Size "A" piston in the largest diameter cylinder would still meet specs for a "good" piston per the service manual, so ultimately, if I were evaluating a motor for a rebuild using specifications, the smallest piston in the largest cylinder would still give you a thumbs up (by a decent amount).

    Now, my thought process applies more to the Wiseco pistons since they're forged, but with the different expansion and contraction between cylinder and piston, that extra ten thousandth of an inch may be beneficial during warm-up.
    Your service manual must be different than mine, or maybe you should look and read it again. The smallest piston size does not meet within spec to the largest cylinder size regarding piston to cylinder clearance - if that was the case, there would be no need for different sized cylinders.

    Again, extra 'slop' in piston to cylinder clearance is NOT a means of extra protection, safety, longevity, breathing room, precaution, blah, blah, blah.
    And... again, whatever extra space you introduce to the piston/cylinder clearance - you also introduce that same slop into the way the rings fit in the grooves of the pistons when the pistons and rings are installed. Think about that for a bit...

    Do as you wish, good luck.

    If you still feel 10 thou in a spec doesn't mean much, gap your plugs an extra 10 thou and get back to me w how that little bit doesn't matter.

  11. #9
    Oem pistons and rings. Nickasil cylinders really don't wear much

  12. #10
    Adam's Avatar
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    The latter GP1300R used forged pistons , Oem or Wiseco forged pistons only . Strong and reliable. I have Wisecos in my GP1200R for that reason.

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