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  • question about SBT cylinder sleeve

    i would like made a big bore 750 kawa cylinder,
    with 85mm pistons (840cc).
    so about your sleeve, can i bore them to 85mm?

    i saw you have only max 81.5mm pistons set, do you have some 85mm in special order?

    thx.

  • #2
    Hi;
    Bryan can correct me if I'm wrong, but I think that the SBT sleeves are the same as OEM. 83mm is the absolute maximum "safe" late model SXi cylinder bore size, but such a large bore tends to reduce cylinder strength and rigidity, plus leaves no room for a rebore.

    Wiseco offers 83mm pistons, but when removing that amount of material from the sleeve, I recommend that your machinist takes great care! Insist upon boring the cylinder with a torque plate and the manifolds torqued in place to stress the cylinder casting. Otherwise you may find that the bore will be less than round once the engine is assembled. Running Wiseco's forged pistons in a bore that isn't perfectly round is asking for trouble.

    I also highly recommend using a girdle-style aftermarket cylinder head with a big bore OEM cylinder to avoid cylinder casting cracking problems. You'll need to have the chamber squish profiles and volumes modified in whatever style head you select to accomodate the big bore pistons and displacement.

    If you really intend to go to 85mm, again Wiseco forgings are the only source I know of, other pistons manufacturers only offer up to 82mm slugs. I suggest that you avoid using an early small pin engine as a starting point. The late model crankshaft assembly is considerably more rugged, and slinging big pistons around greatly increases the stress on the crank.

    L.A. Sleeve or Wiseco are your best bets for finding big bore sleeves. The 750 crankcase will need attention before a big bore sleeved cylinder can be used.

    Dan

    [ September 28, 2002, 09:38 AM: Message edited by: Dan DePardo ]

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    • #3
      thanks again Dan

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      • #4
        When boring a 750, the new sleeve is actually much larger an thicker than the original which is more of a liner than a full sleeve.

        Whatever you could bore an OEM to, you can at least do with an aftermarket.

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        • #5
          Hi Bryan;
          Just for my own knowledge, does SBT usually resleeve all their 750 rebuilds, or just on a case by case basis...for instance if the OEM liner is obviously trashed, or won't clean up at your normal (1.5mm?)overbore?

          Dan

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          • #6
            Just case by case.

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            • #7
              Hi;
              One final note, as I previously mentioned, you'll need to match your cylinder head combustion chamber diameter and squish profile to the big bore displacement, but at the moment all the aftermarket 750 heads that I know of are designed for a nominal 80mm bore, and will work okay for the first few 750 overbore sizes.

              Depending upon the design, 80mm domes can be massaged to work with 82-83mm bore, but removable style "O" ringed domes may not have enough dome material left around the bore to seal against combustion pressure if you open them up beyond 82mm.

              However, with the introduction of the new 82mm bore used in the 2003 800 SX-R, I would expect to see a few head new designs that might make building a big bore 750 a little easier.

              If you decide to build an 840, at the moment, I think your best bet is to use a one piece head, and have a competent 2-stroke shop design and remachine the chambers to suit your big bore cylinder. It's possible that an OEM 750 SXi head has enough material to allow remachining to support your desired bore size.

              With a big bore resleeved cylinder, you can probably get away without a stud girdle if you keep your max RPM and compression ratio within reasonable pump gas limits.

              Good Luck!

              Dan

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