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Old 02-19-2012, 10:16 PM
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biknut biknut is offline
Motorized Bicycle Elite Member
Join Date: Sep 2010
Location: Dallas
Posts: 6,280
Default My small end bearing failed

Has anybody had this happen. I was riding along and all of a sudden my motor started making a sound like the piston was hitting the head.

The motor is my new BGF 80 with a MM gen 2 head and SBP pipe. It had about 165 miles on it.

Tonight when I took the cylinder off, I found that the cylinder and piston look great, but the head, and piston top, have little beat marks from some kind of metallic object. First I looked closely at the piston skirt, and rings, and also the cylinder wall. All that stuff look brand new.

Then when I took the small end bearing out of the rod, one of the needle rollers fell out of the retainer. I tried to put in back in, but it fell right out again. A closer inspection revealed that this roller was slightly shorter than the others.

Then it all made since to me. A piece of the roller broke off and must have fallen into the crankcase, where it was sucked up the transfer port and onto the piston top.

I can tell by looking at the marks on the piston top, and combustion chamber, that they're obviously the exact shape of the missing piece of roller made by it being crushed between the piston and head.

At the time this happened I was in the middle playing with my new MM adjustable rotor. Initially I assumed I must have seized the piston or something by advancing the timing too far, but I've now ruled that out as the cause.

This looks like a clear cut case of the China Syndrome

Does anyone know where I can find better small end bearings?
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