Small end bearing cracked

[SULLY]

Just in the middle of doing the top end on my son's RM125 O3, and when I removed the small end a couple of the needle rollers fell out of the cage. On closer inspection there are several cracks around the outer edge of the bearing (allowing the needles room to fall out).

It hasn't been changed since last year (complete Vertex piston kit), but hasn't had many hours use. All other things look ok, plug, burn pattern, big end play etc...all Ok.

I've not seen this before, any tips on what may have caused it?

 

[darringer]

My guess is the piston and ring(s) were worn beyond spec and rocking in the cylinder. Did you mic the cylinder, piston and ring(s) to measure actual wear?

 

[SULLY]

Hi Darringer....no mic available sorry, got a good (analogue-Miyoto) verneer caliper though!!

No I didn't mic the bore last time, I just fitted a new piston kit (piston, wrist pin, circlips) and a new SE bearing. The bike has had little use from new, and bore looks fine, although I appreciate it may have worn and the wear not be visible.

 

[darringer]

The bore is probably fine. The piston is what wears the fastest. Look for shiny worn spots on the skirt. There must have been some extra movement of the piston to crack the bearing cage. Unless possibly overheating from lack of lubrication,or even detonation. Although there could be other signs of that, like a burnt spot on the inside of the piston, under the crown.

 

[SULLY]

Yeah checked for hot spot under the piston- nothing, all clean. Vertex pistons come with a matt gray to black coating on the side walls, this is hardly worn off the old piston (I'm just changing it cos it's about time- it really hasn't done much work)
Plug looked about right, light tan deposits evenly coating the insulator. Also piston crown has only a VERY light coat of coke over 60%, the rest is still shiny

I guess it's possible I just had a duff SE bearing? overhardened or something perhaps?

 

[2-Strokes 4-ever]

Possibly a worn pin? We always do the pin and bearing when a fingernail run down the length of the pin can detect any imperfection.......(usually at every other piston intervals a pin and bearing are due, based on our experience)

 

[SULLY]

I'd done a complete piston kit last time, pin , bearing etc...