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

[Fletch]

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?

Thanks

 

[ibdennyak]

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.

 

[Fletch]

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.

 

[ibdennyak]

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

 

[Pablo]

I have the piston skirt cut on the intake side, and recently I have been overheating and seizing while riding temporarily....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,

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.

 

[motorpsycho]

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

 

[Fletch]

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.

 

[Fletch]

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.

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?

 

[keatonx]

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

 

[Fabian]

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).

60:1

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.