When rods fail...

Lutz

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Oct 3, 2001
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#1
I was wondering:

When connecting rod failures occur, where does it usually happen? :think: I'm under the impression that it is usually the big end of the rod that fails, but I was wondering how frequently failures are seen at the small end, or somewhere in the middle for that matter.

The main reason I ask is that my 1995 KDX bottom end has about a zillion miles on it, but last time I had it apart, the big end side clearance was well within factory specs, and I couldn't feel any radial play. However, I could 'twist' the piston in the bore when the engine was assembled with the head off. The twist was a pretty significant amount, but I 'think' this is relatively normal with needle bearings.

So, does the small end of the rod ever wear out before the big end, or has anyone seen a small end failure of the rod?

Thanks,
Lutz
 

Rich Rohrich

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#2
Rods fail top, bottom and midway depending on the stresses and root cause of the failure. Big end bearings are pretty common failures on two-strokes because they tend to be under lubricated and left in beyond their usable service life. Small end bearings also fail but they are cheap and easy to replace so most guys tend to change them regularly.