KDX250 Cylinder damaged

Discussion in 'Motorbike Technical Discussion' started by Greg, Sep 28, 2005.

  1. Greg

    Greg Guest

    I Just bought a siezed 92 kdx250 and after stripping it down i've found
    it was due to a broken up crank bearing. One of the transfer ports is
    quite badly damaged (large chip out it). Does this mean look for a new
    barrel or whats my options.

    Thanks in advance
    Greg, Sep 28, 2005
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  2. I suppose that cylinder has an electroplated bore instead of a steel
    sleeve, since you mentioned a chip out of the transfer port and I only
    see ONE size piston and rings on www.partsfish.com.

    You can go there and register and look to see what a new barrel would
    cost, or look for a good used one on eBay, or google for shops that
    specialize in repairing electoplated cylinders, if that's what you

    If you do have the electroplated cylinder you can NEVER use chrome
    plated rings, you have to use the softer cast iron rings. I found that
    out the hard way many years ago when Yamaha was the first company to
    use electroplated cylinders on their TD-1 production road racers...
    krusty kritter, Sep 29, 2005
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  3. Greg

    Greg Guest

    The cylinder uses a plated bore, looks like its gonna cost me.
    Greg, Sep 29, 2005
  4. I'd like to see a picture of the damaged area to guesstimate whether
    you could use the cylinder as-is. You may not realize how piston rings

    Rings don't seal the hot gasses in the cylinder because of radial
    spring tension. They seal the hot gasses in because some of the gasses
    get into the groove behind the ring and push it outwards.

    When the piston of your 2-stroke gets below the top edge of the exhaust
    port on the downstroke, all of the pressure in the cylinder is suddenly
    relieved and there is nothing left but the weak radial spring tension
    of the rings to push them outward.

    The ring tend to bulge out into the exhaust port, which is fairly wide
    if it doesn't have a metal bridge between two halves of the same
    exhaust port.

    The rings bulging into the port is what makes the horrible metallic
    clashing sound in an air-cooled 2-stroke engine, water-cooled cylinders
    muffle the clashing sound.

    There are wire pins in the piston ring grooves to keep the rings from
    rotating and having one of the ring ends SNAG in the open exhaust port.
    A snagged piston ring will break, maybe the engine will seize...

    And the edges of the ports will have a slight chamfer to guide the
    piston rings back toward their grooves in the piston.

    But the transfer ports are a lot smaller and narrower and they will
    usually have metal "bridges" between them to keep the rings from
    bulging into those ports. As I said above, the only pressure of the
    piston ring against the cylinder wall and port will be radial spring
    tension, the ring isn't pushing hard at that point.

    Once the chrome plating has begun to be peeled away, more of it could
    come off, disastrously, causing another piston seizure. Or, the damaged
    area could conceivably cause piston ring snagging and subsequent

    Or, maybe nothing further would happen, due to the lack of piston ring
    pressure in that area.
    krusty kritter, Sep 29, 2005
  5. Greg

    Greg Guest

    Cheers for the replies, I'll try and get a pic of the barrel. Is it
    possible to get the cylinder bored and a sleeve fitted perhaps.

    Greg, Sep 29, 2005
  6. It would be fairly simple to bore out a 4-stroke cylinder and install a
    plain sleeve, but how do you get a 2-stroke's intake, exhaust, and
    expecially the transfer port holes with their critical angles cut in
    the right places after pressing the sleeve in? It sounds like somebody
    would have to be sleeving a lot of KDX's to make it worthwhile to
    machine a whole bunch of pre-ported sleeves.

    But, what do I know? Maybe there is a specialist out there who's doing
    exactly that. What does google have to say?
    krusty kritter, Sep 29, 2005
  7. Greg

    Kim Neubert Guest

    Try here http://www.ericgorr.com/

    If he can't fix it, he knows who can.
    Kim Neubert, Sep 29, 2005
  8. Greg

    IRONDOG Guest

    Repair sleeves for ported two cycle engines have been readily available
    for over 30 years- L.A.Sleeve
    and other companies make them and they are available from any
    motorcycle dealer. The ports
    aren't exact out of the box but a good installer will match them with a
    90 degree cable driven porting
    tool such as is available from CC Specialties. Yes, transfer port
    angles are very critical and there are
    installers out there who don't take the time to properly match them.
    This and poor sleeve to cylinder
    fit have contributed to lots of cobbled up sleeve jobs over the years
    and have turned lots of people's
    opinions against sleeves. A properly installed sleeve will perform just
    as well as a replated cylinder
    and there are oversize pistons available for many bikes that came with a
    plated bore.
    Krusty, I'm surprised that you didn't know the 2 cycle sleeves were
    IRONDOG, Sep 30, 2005
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