Blown Motor Concerns

4 replies to this topic
  • sirhk

Posted March 24, 2002 - 04:13 PM


Well, it's been a while since I've been on here but I know I can find answers here. My top end went on me two weeks ago. Pulled it apart and the piston had almost completely cracked in half. replaced the cylinder, piston, rings, and gaskets, and $422 later it's running again. While working on it I did notice some things that REALLY concern me and I wanted some feed back and opinions on them.
1st. On the rod where the wrist pin runs there was some very slight scoring. Probably only a couple .001's deep but just enough to feel. Is this going to be REAL expenxsive in the future? Anyone else notice this on theirs during rebuilds. It looks like a lack of oil to the wrist pin where it runs in the rod.

2nd on the crankshaft itself right where the rod connects both sides of the crank are colored from heat (tinted blue). The crank is blueish for probably 1" in each direction in diameter from the rod. Anyone else seen this?

Any bets or opinions on if my bottom end is going to self destruct too? Should I have bit the bullet and done the bottom end also?

BTW I've been running Mobile 1 15/50 for the past 1.5 years and change it religiously at least every third ride but usually every other ride.

  • YZ400Court

Posted March 24, 2002 - 05:04 PM


The discoloring of the crank is from the factory. They must heat it when outting it together. The marks in the rod...I'd have replaced it, but it's too late now. Ride it like you stole it and keep using that good quality oil, it will probablly run for a long time.

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  • SFO

Posted March 25, 2002 - 05:52 AM


My small end was scored after 3months of riding.
I am on my third top end and anticipate replacing both the rod and camchain on my next freshen up.

  • GPS Dual Sport

Posted March 27, 2002 - 05:56 AM


People smarter than me has said the early YZ and WR had problems with wrist pin scoring. The later models have a larger wrist pin to fix the problem.
The gun metal blue you see on the crank is from inductive hardening process in the crank pin area. This is normal manufacturing process and nothing to worry about.

  • sirhk

Posted March 28, 2002 - 04:36 PM


Well that makes me feel a little better!! Yeah, I know that I probably should have done the rod but damn, splitting the case is a little beyond my talents. I'll probably ride it the rest of this season and maybe take a day to pull it apart this summer since it was pretty easy and inspect the rod again.

Thanks for the info guys!!


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