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1994 Yz 250 Auptopsy - Bearing Failure

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Hopefully the community or someone from the community will learn from this.

 

I bought this bike last year. It had sat for 10 years in a barn as the rider could no longer ride it. When it was put away, it had a fresh top end and I imagine over the course of the 10 years it was started up a few ttimes. I do not know how frequently, just speculation.

 

I had a big end crankshaft bearing failure. I have taken several pictures of the tear down process so that I can document it for myself and everyone loves pictures :)

 

2013-08-23 19.51.11.jpg

 

Pictures of the piston:2013-08-24 08.20.51.jpg2013-08-24 08.21.51.jpg2013-08-24 08.22.01.jpg

 

Picture of the Head

2013-08-23 20.35.30.jpg

 

Cylinder

 

2013-08-24 11.58.28.jpg

 

2013-08-24 11.59.00.jpg

 

2013-08-24 11.59.23.jpg

 

 

Bottom End

2013-08-24 11.41.55.jpg

 

2013-08-24 11.42.39.jpg

 

2013-08-24 15.14.07.jpg

 

Oil Seal Failure

 

2013-08-24 15.14.12.jpg

 

2013-08-24 15.14.18.jpg

 

2013-08-24 11.59.48.jpg

2013-08-24 15.28.15.jpg

Edited by BreakNeckYZ

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Here is the interesting part...

 

 

The Bearing

 

2013-08-24 15.28.15.jpg

 

2013-08-24 15.28.24.jpg

 

2013-09-05 19.16.00.jpg

 

2013-09-05 19.16.06.jpg

 

After pulling the bearing out I decided to pop it apart and further investegate the rollers and races. I found that BOTH bearings had suffered corrosion on the inboard race. Bothe areas (on each bearing) are consistent with corrosion that would occour on a load bearing side that is sitting.

 

2013-09-07 19.18.27.jpg

 

So if you are looking at a bike that has been sitting for a while... you may want to consider the "unseen"

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image.jpg

Picture of the crank.. No plastic stuffers. Did they use plastic in the older cranks? image.jpg

image.jpg

image.jpg

While cleaning the cases I noticed 2 things. The frame was painted while the motor was still on the bike and someone marked the bottom of the case.. I'm guessing the motor has been apart before.

Picture of the gears. Look good to me.

image.jpg

Anything look out of norm?

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The crank oil seal IMO

For sure... I was asking if anything looked out of the norm on the last pics. I have not seen a crank that looks like that in any other pics on here.

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Santa came today and dropped of parts. New bearings, seals, gasket set... The difference between the old and new bearings is eye opening for sure.

Bearings are in the freezer.... Bottom end will go back together soon.

Talking pictures along the way and I will post them.

Edited by BreakNeckYZ
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I have a 94 IN THE BACKYARD THAT THE PISTON BROKE and it seized,one day ill rebuild it. Im sub'd. Clean bike btw!

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Bearings went in today! Heated case halves up in the oven for 45 min @ 250 degrees. Put bearings in the chest freezer overnight.

A quick piece of advice... Move fast! Hopefully tomorrow I will put the case back together. Pictures to follow.

Edited by BreakNeckYZ

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When you first said lower crank bearing and the bike has been sitting for ten years I knew right away what happened. It was caused by one rust spec on the crank bearing. From looking at the crank with that light surface rust it has has I knew right away what happened as it has happened to my self. I will bet it was caused from condensation from the engine sweating during the winter or a long humid stretch. If a 2T is to be stored over the summer or winter. Your best bet is to Fog the engines as it will keep the bearings coated with a rust inhibitor so this does not happen. Its hard to believe one little spec of rust can take out a bearing but it will. That little spec turns in to a pit and that is the start of the end. Also think about how fast that little bearing is going. Put a defect in any internal part at speed and it will fail.

 

Now as far as your cylinder goes. Unless there is deep grooves from the transfer ports up I would just hone it out providing the cylinder bore is not out of round or over tolerances.

Edited by Zrt1200

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Powerseal has the cylinder right now. It is getting re-plated and bored to match a weisco pro-lite piston. Cylinder should be back next week. I took the crank to a pro-shop and had it looked over. Going to be riding soon.. :)

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The corrosion you see on that bearing was probably not corrosion due to moisture.  One of the primary end of life failure modes for bearings is spalling.  This is where the surface of the bearing becomes so brittle after enduring many fatigue cycles that it begins to fracture and flake away.  Here is an article on bearing failure.  Spalling damage is showin in the first pictures.

 

http://www.nrel.gov/wind/pdfs/day2_sessioniii_4_timken_evans.pdf

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The corrosion you see on that bearing was probably not corrosion due to moisture.  One of the primary end of life failure modes for bearings is spalling.  This is where the surface of the bearing becomes so brittle after enduring many fatigue cycles that it begins to fracture and flake away.  Here is an article on bearing failure.  Spalling damage is showin in the first pictures.

 

http://www.nrel.gov/wind/pdfs/day2_sessioniii_4_timken_evans.pdf

That does not explain spauling on the inner race in the same area on both bearings over the same exact span (two ball bearings on the top load side). the wear does not seem consistent with the spauling, not corrosion theory. BTW Awesome info :)

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Bearings went in today! Heated case halves up in the oven for 45 min @ 250 degrees. Put bearings in the chest freezer overnight.A quick piece of advice... Move fast! Hopefully tomorrow I will put the case back together. Pictures to follow.

btw, if you take a blowtorch to the cases, it expands more and it heats up quicker.

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2-stroke engines only load the crank in one direction so the bearing wear will be concentrated on one side of the bearing.

The static load (weight of crank) forces contact on two bearings when not in motion. This contact patch is the only place on both bearings where any defect can be noticed. Btw, this was my guess, verified through a tech specialist I know that deals with mechanical loads, bearings and spinning things :)

The oil used in this motor was Klotz synthetic. Like I mentioned it sat for 10 years.

Either way, they were in bad shape and I am lucky they lasted as long as they did and the failure wasn't worse.. The crank survived :) the cases had minimal damage. Could have been worse.

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