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Home Forums Shifter Karts When to open bottom end?

This topic contains 3 replies, has 2 voices, and was last updated by  William Martin 2 years, 5 months ago.

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  • #57018

    Jimmy Callaway

    Hello all,

    I got a stock cr125 shifter a few months ago and am looking for some engine advice. I’ve never run a 2 stroke so I’m experiencing a bit of a steep learning curve. A few weeks back I broke a ring from running too lean. The weather cooled off a bit and I didn’t correct the jetting. Did I mention I’m new to this? Anyway, broke off a section of ring. Pressumably got it hot, bottomed out the ring gap and caught the ring on the edge of the exhaust port. This slotted the cylinder pretty nicely and tore up the piston. Rebuilt the top end with a replacement cylinder and new piston. I also fattened up the mixture. The first time running it after the rebuild I killed the engine again! It ran fine (and cool) but when trying to restart it it was extremely tough to turn over and it wasn’t even trying to fire up.

    <span style=”line-height: 1.5;”>This time the piston face and head were dimpled and burred as though the engine ingested metal. Also there is a ring sized slot in the piston and cylinder at the exhaust port. The new ring was all there so the new ring didn’t fail.</span>


    <span style=”line-height: 1.5;”>It appears there were some left overs from the first blow up in the bottom end that came up the transfer port and caused trouble.</span>


    My question is:

    1) should you always split the case when you break a ring to make sure everything is out of the engine? Or are there other ways to clean bits out of the bottom end with just the cylinder off?

    2) is there any way to tell if the rod bearing is damaged from chewing up the metal or getting shavings in the rollers?




  • #57041

    William Martin

    Well, at this point your only option is a complete tear down. There is no telling what may have been chewed upon in the bottom end. Maybe it would have been possible to fish around with a magnet, turn engine upside down & flush with spray cleaner, but it isn’t reliable. Always taking a big chance when not opening up cases after a failure that left broke something in the cylinder. Guess why the cranks or crank bearings are made to slip fit, as opposed to the way Honda’s come from the factory…


    Good luck,


  • #57060

    Jimmy Callaway

    Yes, the next move I’d definitely splitting the case. And yes, crank bearings would be too easy to pass up. What I’m trying to figure out is whether I need to replace the crank and rod assembly. Are the rod needle bearings very susceptible to debris damage as compared to a conventional babbitt rod bearing (that is very sensitive to debris).

    When bottom ends give up what usually happens? Are we talking broken crank/rod (and killing the case) or a nondestructive seizing of the rod bearing?

    Just trying to get a feel for what I’m up against cost wise and risk wise.




  • #57132

    William Martin

    Unfortunately when you have metal bits flying around, there is no “usual suspect”, nearly anything can happen. Having said that, the lower rod bearing is something to be replaced “just because you can”. The cage will eventually fracture on it’s own, I sort of doubt if it will swallow metal bits from outside of the bearing, since it is spinning around trying to throw stuff away from itself.

    If you had a real rod failure, there would probably be a hole in the case somewhere…although the big end of the rod will start eating at the cases, right where it gets both halves at once, when there is enough rod looseness. That usually doesn’t happen if you replace the big end bearing at a reasonable schedule. Any trace of bluing from heat and it’s too late, you need a new rod and crank pin both.

    So the moral of this long-winded story is that tear down and close inspection is critical to keeping these little monsters alive. Clean stuff like you mean to bake on it, so you can really see the condition of the metal surface.




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