Cracked my cylinder at base of jug and broke piston ring!!!

Discussion in '2-Stroke Engines' started by Motocruiser, Mar 15, 2011.

  1. Motocruiser

    Motocruiser Member

    So I took the jug off my 66cc skyhawk today to measure ports and do some polishing. When I tried to put the jug back on the piston was giving me trouble. I was able to get both rings compressed and get the piston in just past the rings but that was all. I decided it needed some persuasion so i took a rubber mallet to the top of the jug and crack goes the exposed cylinder wall and a ring! Whats the best way to go about doing this? Does anyone know the piston ring size? 46mm-1mm, 46mm-1.5mm, 47mm-1mm, 47mm-1.5mm etc...?

  2. spad4me

    spad4me Member

    The best way is to order from the guy you bought it from.
    It sounds like you had the tiny alignment pin on the side of the ring groove misaligned with the ring gap.
  3. Motocruiser

    Motocruiser Member

    I appreciate the reply. I was curious of the size so I can order some stronger rings.
  4. retromike3

    retromike3 Member

    load of the rings

    I think that it is not the strength of your rings but were they are placed. I have taken apart these things before and it is almost impossible to get them to go if you don't line up the ring gaps with the little metal pins that are in the pistons groves. If you can find the right place to put them you can do it with your fingers. took me a few tries but eventually I got it so I could do it without to much grief. The question is if you get a harder ring will it take off the very thin chrome plating thats on the cylinder wall?

  5. Dave C

    Dave C Member

    I snapped a ring without hammering on the piston. They are cast iron, needed with the hard chrome finish of the cylinders.

    You need a new cylinder, piston rings and gaskets. If you scored up the piston you should replace it, too. Maybe do that anyway, they are cheap...

    I figured the problem is a poor finish of the forcing cone on the bottom of the jug, mine was rougher than a cob. I sanded it smooth with 220 wet/dry sandpaper and used white lithium grease to assemble. Get the rings in their proper place with the pins, wiggle the cylinder front to back, it doesn't help much from side to side. I put myself at an angle where I could see what was happening with the rings, wiggled the cylinder down over the piston with no trouble.:helmet:
  6. retromike3

    retromike3 Member

    ruff to start with

    I read somewhere that they leave the sides of the piston walls ruff so that the rings and the cylinder will wear together making everything smooth in the end. That is why the brake in is so critical for those cheep two cycle motors and why they get so hot to begin with because you are machining while you ride.

    you are not "braking" the motor your polishing it in.

  7. Motocruiser

    Motocruiser Member

    @DaveC: I contemplated white lithium on the needle bearing on the wrist pin, but wasn't sure of it's ability to hold up to the heat. What has the outcome been for you?
  8. Dave C

    Dave C Member

    Lithium is intended as a assembly lube. As to the bearing the lube will protect as well as anything, it only needs to be there long enough for the normal oil lube to work it's way in and the heat helps that process.

    In reality any grease used in assembly will be gone in seconds but it delivers the protection needed during the inital startup. I wouldn't use axel grease to assemble but MolyDiSulfide or Lithium greases ment for assembly are fine to use.
  9. GearNut

    GearNut Active Member

    Axle grease will not dissolve into the oil properly as the engine is ran. Eventually the grease will break down and loose it's lubricating properties and yet still prevent the oil from getting into the bearing; then the bearing is toast.
    Assembly lube will mix with oil and the bearing will be oiled properly for it's lifetime.
    For the sake of your engine, please use the right product for the right job.
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