RM125 03 RM 125 engine failure. HELP!

Hey guys, I am very new here, but am in need of some advice. My bike ate a brand new piston I put in it after 5 minutes of riding (wiesco). The original reason for replacing the piston and honing the sleeved cylinder was that the bike overheated due to a broken coolant line on the trail. Mind you, I did the (failed) rebuild about 2 1/2 years ago. The reason for this is I joined the military shortly after my bike died.
So after I rebuilt the top end and resealed everything, I started it up with no apparent issues. I let it idle for 5 minutes max, then I took it out on the road to test. It ran good unless I went WOT; this caused it to completely bog. I wasn't surprised however because I had thrown all new jets in and I hadn't figured out the perfect mixture ratio yet.
About 5 minutes down the road I started to lay into it a bit more and more and then failure. The bike died and wouldn't start back up. The piston was not seized and was completely free. I am now at the point where I am finally able to take a look inside the engine.
The sleeve had a good amount of scoring and was beyond the point of rehoning. The piston is gauged up and has a bunch of scoring. The ring is locked in place at one specific spot and there is a half inch wide and a millimeter deep recession above this same spot.
Now my thinking is, my younger inexperienced self threw the engine together and didn't torque the head and jug down and there was too much room for the piston to catch on a lip which sent the piston pieces around the cylinder causing the damage. However, I'm not 100% sure and would like as many knowledgeable opinions as possible.
I just purchased a brand new oem jug and weisco piston. So my questions to you guys are:
What do you think caused this?,
If I install a new jug and piston will this failure reoccur?,
What are the next steps to getting this bike on the trail again?
Thanks guys! If this is not enough information, let me know and I will try to provide more
 

SRAD97750

Moderator
Staff member
...I did the (failed) rebuild about 2 1/2 years ago.... I started it up with no apparent issues. I let it idle for 5 minutes max, then I took it out on the road to test. It ran good unless I went WOT... I hadn't figured out the perfect mixture ratio yet.
This break-in procedure doesn't include the proper heat cycles the components need to come to temperature together, but certainly is not the reason for this failure.
The bike died and wouldn't start back up. The piston was not seized and was completely free.
I suspect 100% a jetting/carburetion issue. If the engine didn't come to an abrupt stop, then it stopped getting Air/Fuel/Spark.
The sleeve had a good amount of scoring and was beyond the point of rehoning. The piston is gauged up and has a bunch of scoring. The ring is locked in place at one specific spot and there is a half inch wide and a millimeter deep recession above this same spot.
This can only be from two sources.
Exterior contamination - Dusty or improperly lubricated components, no air filter in poor air quality, or something hidden in the engine during rebuild that came to the show later.
Internal component damage - Heavy detonation can cause piston chunks or ring chunks to score the cylinder.

How clean an engine needs to be during assembly is a learned thing. I have had engines do the same, but full seizure. I think you got away lucky and should try again.
Be sure everything is crazy clean prior to assembly, a very light oil application is recommended on the cylinder, piston races, pin, and bearing. -BIG DAN:thumb:
 
Im going to say this:
Try a few heat cycles before you beat it into the ground. I suspect poor jetting, float height, fuel.

Run 32:1-40:1 (your choice) mix with premium pump fuel, set your float correctly, then find your jetting that works best.

I suspect detonation and lean, and it ate your piston for lunch.

Your new stuff better be spot on close off the bat, or you will be buying again shortly.
 
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One good reason to stay away from ads that begin with “0 hours on new top end.”
Seriously though, is the damage on the sides of the piston only? Being a fresh lump it should be a dream to diagnose.
+1 on the ring gap check
Hone sleeve after overheat? CI sleeve or alum?
 
It's too bad I cannot upload pictures for you guys. The wear was most focused where the piston would contact the exhaust bridge, so thats really pushing towards the bridge getting overheated and expanding. Not to mention I didn't relieve the it either. I don't know anything about the sleeve, unfortunately.

Thanks for the advice and know-how so far guys.
 
You can test the sleeve with a magnet. If it sticks really well then it’s ferrous. They will snag a hot forged piston if you’re not careful. Ah well no matter now,
you have a new jug coming so best give attention to details on this go round. Manual and torque wrench. Or two
 
It's too bad I cannot upload pictures for you guys. The wear was most focused where the piston would contact the exhaust bridge, so thats really pushing towards the bridge getting overheated and expanding. Not to mention I didn't relieve the it either. I don't know anything about the sleeve, unfortunately.

Thanks for the advice and know-how so far guys.


Sounds like the piston didn’t get drilled at the exhaust bridge.



:devil:
 
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