Piston Seized today at Idle! Because of loose Head bolt?

Discussion in '2-Stroke Engines' started by Fletch, Apr 10, 2011.

  1. Fletch

    Fletch Member

    I rode 30 miles flawlessly today. I brought some premix fuel with me and refilled my tank. Took off and a couple miles later noticed I wasn't idling. Pulled over to check out carb, turned up idle screw and as it's idling I hear "clink" and it stops! I push the bike with the plug out but won't turn over or budge.

    I get the bike home later and go to take off the head. One of the bolts is completely loose and unscrews wit the nut. I get the head off and there are piston contact marks on one side of the head. The piston is seized up and you can see a tiny gap on one side of the cylinder between the piston.

    Did my piston seize because of the loose head bolt?

    My theory is that the loose bolt allowed the cylinder block to become angled or crooked which caused the piston to seize up. Either that or the head was loose on one side causing the combustion chamber to be angled and the force of combustion to act unevenly on the piston. I have no idea...but there was no oil leaking from the head at all.

    I haven't taken the exhaust and intake manifold off yet to look at the rings, and I don't know for sure that it isn't the crank that seized, but I'm guessing not because of the metal gashes in the head from piston contact somehow? Or maybe the piston pushed some metal from the edge of a port up and into the head?

    I sprayed some penetrating oil around the piston and tomorrow I will try and get the block off. Any suggestions on what to do if the block won't slide off the piston? Should I tap the piston with a hammer?


  2. ibdennyak

    ibdennyak Guest

    If I had to guess, (and that's what it is) I would guess that your rings hooked a port. Possibly because the little positioning pins were missing in the piston. I would get a short piece of broom handle and pound the piston out. Sounds like you don't have anything to lose.
  3. Fletch

    Fletch Member

    Thanks... The positioning pins are there on the piston. I didn't mention that I have the piston skirt cut on the intake side, and recently I have been overheating and seizing while riding temporarily. I wait a minute and start up again. I was also running a CNS carb too lean I believe and my oil ratio was 60:1, but I'm using Amsoil Saber 100:1 oil, so I figured 60:1 is more like 30:1 of a 50:1 oil (if that makes sense). Plus 60:1 was recommended to me by someone here. So there could have been pre-existing cylinder/piston damage is what I'm getting at, although up until it seized up it was running as good as ever. I haven't tried pulling up on the cylinder block yet, but I did hit the piston head with a hammer and it didn't budge (that I could notice). In retrospect that was stupid before looking in the ports to see if the rings are caught. I suspect you're right on that... I'm hoping you're right on that! ;) I don't want to replace the bottom and top end.
  4. ibdennyak

    ibdennyak Guest

    Uh oh, lean and hot is a bad thing. Keep us posted.
  5. Pablo

    Pablo Motored Bikes Sponsor

    Amazing. Surprised you trusted it for an extended ride!

    First of all 60:1 with Saber is pretty oily. So - it's not a lube issue.

    Not sure how lean you air/fuel was, but that probably didn't help. Just the combo with a loose head sounds deadly enough, but I'm also thinking the usual ring pin failure.
  6. motorpsycho

    motorpsycho Active Member

    I would just buy a whole new motor...they're only about $80.00.
  7. Fletch

    Fletch Member

    Ok so I took off the exhaust and intake manifold. The rings weren't caught in the ports like I expected. I sprayed more penetrating oil and smacked the piston with a hammer on a piece of wood. It did move, and it is down to about where half the hole where the pin goes through the piston/clips are. The rings are completely compressed so they didn't move out of place.

    I haven't taken the engine off the bike yet. When I do I will try and knock the piston all the way out of the cylinder body because I will have a straight shot at it (bike bar won't be in the way). I hope I can get it out because the crank is moving, so I don't need a new bottom end. Any more ideas if I can't hammer it loose? I was wondering if I would have to cut through the cylinder block? If I can get it lower to where I can slide the pin out of the piston I can detach it from the crank shaft.
  8. Fletch

    Fletch Member

    Hey Pablo,

    I really think that my timing was too advanced having the skirt cut and the expansion chamber which has the same effect as closing the exhaust port sooner. I think this is why it was running so hot. I had the CNS on for just a little while, and went on a long ride because I got the part I needed for my Mikuni and put it back on. It was running beautifully. Climbing the 9 grade hill at 25mph, and heading uphill the entire ride. No temporary seizing or bogging...no smoke from the expansion header... I thought I was in the clear! ;) I forgot to mention that the seizure happened almost right after I refilled the tank with 40:1 Saber when I had been running 60:1. Maybe that just made it too lean to take the heat regardless of the added lube. It was really coincidental how that happened. Another thing I should mention is that when it had been seizing up recently and smoking from the exhaust header, I had sprayed BOTH sides of the exhaust wrap wit the DEI high temp silicone. I don't think you're supposed to do that...just the outside of the wrap? This may have been attributing to the smoking header and overheating too?
    Last edited: Apr 11, 2011
  9. keatonx

    keatonx Member

    Assuming the exhaust gasket isn't leaking, a hot exhaust+engine can be caused by an abnormally retarded(late) ignition timing. this could make it smoke. Retarded ignition can also cause excessive cylinder wear (which may be the "contact marks" you see, but since i cant see the cyl i dunno). i doubt this would cause the seizure.

    my weed eater bike had a smoking, overly hot muffler at one point. i was climbing a massive hill when one of my homemade mounts' bolt (which was a bolt going through the engine casting, right next to the flywheel) came loose and hit the flywheel. All i saw was the engine shake like mad and i heard the most horrible screatching noise ive ever heard in my life. that partially sheared the flywheel key, retarding the ignition timing a bit. not knowing, i just rode it like that. it ran super hot, didnt rev too high, and the muffler smoked like mad from the heat (was so hot that my leg just barely toughed it once, and it instantly melted my skin so bad that now i have a massive scar across my calf). eventually the damaged (wobbled 1/2 an inch, my homemade trans was so worn that it allowed this kind of movement!) and completely re-engineered pocket bike clutch caused the crankshaft to snap. and thats when i noticed the sheared timing key lol
  10. Fabian

    Fabian Well-Known Member

    60:1 :ee2k:

    Doesn't make any sense at all. 60:1 is 60:1 no matter which way you try and mess with the figures. 2-stroke oil does more than just lubricate; it is in part responsible for heat extraction and keeping any hot spots from getting too hot and distorting the bore. When running such a lean oil/fuel ratio on a cylinder and piston with poor metallurgical stability, the result is never going to be good for engine life.

    60 :1 sounds like a self created problem which could have been easily avoided.
  11. zwebx

    zwebx Member

    this is what your piston will look like on the exhaust side: [​IMG]
    60 to 1 is like.... how do i put it..... from the bores perspective it would feel like dropping the soap in prison the first time

    anyway... the reason it broke is because 60:1 is not enough oil... dont skimp on oil next time run somthing like 30:1 i personally run 25:1 but thats prolly because thats what i run in my rm125
  12. jaguar

    jaguar Well-Known Member

    I agree. I'd never run the oil any leaner than 40:1
    But the fact that you ran it at 40:1 and it seized means it's a jetting problem. Do you know how to correctly jet a carb?
    Keep increasing the main jet size till top speed starts to lessen. Use micro drill bits to enlarge the main jet size.
    Float height being too low can make it run lean. See my page at http://www.dragonfly75.com/motorbike/jetting.html
  13. crassius

    crassius Well-Known Member

    to get the piston out, try taking out all the barrel studs and turning the barrel

    had a guy seize up one time after mixing fuel in a can, then grabbed the wrong fuel can to fill the tank
  14. Tanstaafl

    Tanstaafl Member

    I am not that familiar with the small 2 stroke motors, but have had very extensive experience with large ones like Clarks, IR's and Cooper Bessemer 12 cyl 14"x14" (bore/stroke) engines powering integral gas compressors.

    Piston seizures were most commonly caused by not fitting piston rings with sufficient end gap for expansion when reaching operating temperatures.

    Does anyone check piston ring end gap on these bike drivers?
    Last edited by a moderator: Dec 18, 2015
  15. Fabian

    Fabian Well-Known Member

    I'm seeing all of the warning signs of a major f*&kup; not just the 60:1 oil fuel ratio, but the disassembly process as well.

    I think your best option is to purchase a replacement engine because the rebuild process is going to be a complete catastrophe and that's just engine disassembly, where it's hard to f*&k things up any more than they are, as the engine is already trashed; burnt out from the inside and almost cooked to perfection all the way through; though just a little more cooking would have had the engine turn a nice golden crispy brown!
  16. jaguar

    jaguar Well-Known Member

    Ring end gap on these engines is excessive, so that's not the problem.
  17. Pablo

    Pablo Motored Bikes Sponsor

    With the correct oil these engines can run all day, full throttle at 50:1. Not all oils are the same. If someone tells you this, they may not know what they are talking about. If you mix Amsoil Saber richer than 50:1 you will spluge oil out every orifice. Talk to people who have done this. They will tell you. Jaguar knows what he is talking about - you need to set your jet to match your oil ratio. If you run too oily, you can be in danger of running fuel lean.

    The usual seizure of these engines is the ring gap roll pin coming out............are there actual pictures from 2011 of this engine?
  18. Fabian

    Fabian Well-Known Member

    This above sentence is comprised of two parts and is correct, but the sentence needs to be reversed, because you can run an oil/fuel ratio, right up to the very ragged edge; walking a fine line between squeezing out the very last drop of maximum performance and that sickly sound of instantaneous engine seizure, so long as the carburettor is correctly jetted and the exhaust gas temperature is within acceptable limits, though engine life may be reduced when squeezing out that one last extra drop of performance with a pair of vice grips:

    [sic retrorsum] If you run too oily, you can be in danger of running fuel lean. You need to set your jet to match your oil ratio.