Mystery Pieces in Engine...PICS

20 replies to this topic
  • Sasquatch x7

Posted March 05, 2012 - 10:35 AM


Before winter, I bought a 2001 YZ426 with some engine problems for cheap. We just split the cases, and discovered a few broken bits floating around inside. Among these pieces was one of the circlips that goes between either camshaft and the cap (previous owner installed an auto-decomp cam and must have dropped it in when doing the swap as I now have three of them), and two more objects that we can't determine what they are or where they go. They might not even belong in there at all, since we cant find what they broke off of. Here are some pictures:

Posted Image

Posted Image

Posted Image

The first piece was found wearing on the crank, and the second piece is broken on the top, bottom, and left side.

Edit: The first piece is about an inch long for reference

Any ideas?

Edited by Sasquatch x7, March 05, 2012 - 10:45 AM.

  • grayracer513

Posted March 05, 2012 - 11:41 AM


The third piece looks like a piece of a timing chain. Don't recognize the others.

  • Sasquatch x7

Posted March 05, 2012 - 11:51 AM


The first two pics are of the same piece, front and back. The piece in the last pic is about the size of a quarter, maybe a little smaller. I think a piece of an actual timing chain is out of the question since it is so big. Its still possible that they are foreign objects that don't belong.

  • CO_Rider

Posted March 05, 2012 - 01:18 PM


Could it be a peice of a bearing retainer clip? Not sure if the 426 has them or not, but could be an idea.

  • Birdy426

Posted March 05, 2012 - 09:24 PM


Pieces of the aft cam chain guide?

  • yz450fcranker

Posted March 07, 2012 - 07:33 AM


first piece might be a piece of crank the second piece might be something next to the crank

  • kx450f63

Posted March 07, 2012 - 07:42 AM


Give us more pictures, different angles. Have you inspected the transmission? I think it is unlikely these pieces are not meant to be in there somewhere. Clean and inspect every piece, you will find your broken parts.

  • Sasquatch x7

Posted March 07, 2012 - 03:47 PM


I don't have access to the motor right now since it's at my dad's garage, but the first piece being a chunk of the crank sounds reasonable since that's where it was found...just haven't pinpointed where. Here's the only other picture I have:

Posted Image

We haven't taken the transmission apart yet, but all the gears seem to be in good condition. That's our next step.

I'll get a picture of what happened to the crank when I can.

  • grayracer513

Posted March 07, 2012 - 07:27 PM


Take a very close look at the big end bearing. I'm beginning to wonder whether that might be a part of the rod bearing roller cage, although it looks a little large for that, and oddly shaped. Maybe a side shim... can't tell.

  • chris856

Posted March 07, 2012 - 07:57 PM


Could that be a piece of the case? I'm not sure how to explain it but dosent the inside have a webbing kind of look? Maybe a piece of that broke off.

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  • niles

Posted March 08, 2012 - 12:31 AM


To me it looks like a chunk of the weight from the balancer gear.

  • kx450f63

Posted March 08, 2012 - 05:48 AM


You will need to inspect the entire package. You will be surpised to find what the engine and transmission can do to pieces just floating around in there. I broke a few fiber clutch plates and before I had any indication of a clutch problem, the pieces were balled up inside the oil pump pick up in the middle of the cases. I'm not a Yamaha guy but the piece that is shaped like a triangle with the partial holes stampted in it looks like a bearing retainer of some sort. The internals of the engine will bend, distort, and reshape parts to an unreconizable shape. You need to find where they came from to make sense of it.

  • Sasquatch x7

Posted March 09, 2012 - 07:54 PM


Here's what happened to the crank...

Posted Image

To me it looks like someone took a die grinder to that pin. I'm not sure how flush it's supposed to be with the face of the crank; it could come over ~1/32" towards the other side.

With the surface of the crank being scarred and minor scoring on either side, I'm assuming it's best to replace the whole assembly?

  • kx450f63

Posted March 12, 2012 - 04:43 AM


Still no idea where the pieces came from? Bearing cage pieces?

  • Sasquatch x7

Posted March 12, 2012 - 06:16 AM


Still no idea...

Nothing is missing from the crank (other than what's pictured above), all the bearings and bearing retainers are good, and I've inspected every piece that we've taken out so far. The only thing still left is the transmission.

The only damage I can find is the crank pin and the oil pump idler gear. The shaft that the gear was on fell inside the case while the bike was running and the gear fell on the outside. A couple pieces of each are chipped off but they're all small and accounted for.

  • bonehead7101

Posted March 12, 2012 - 08:39 PM


To me it looks like a chunk of the weight from the balancer gear.

i agree, looked at the fiche and only 2 things i could think was balancer gear weight, or maybe stator plate,

look at #10


  • kx450f63

Posted March 13, 2012 - 03:09 AM


i agree, looked at the fiche and only 2 things i could think was balancer gear weight, or maybe stator plate,

look at #10


I think you've got it. As soon as we get confirmation from the OP he will tell you what you win. Good job.

  • Sasquatch x7

Posted March 13, 2012 - 12:31 PM


That's a good guess...but here are parts 10, 11, 12, 15, and 16 on the above diagram:

Posted Image

Not quite a match as you can see. It doesnt fit behind the weight either. The mystery chunk is about 3/32" thick, which seems a little thin to be part of any weight.

  • kx450f63

Posted March 14, 2012 - 04:25 AM


Have you inspected everything on the clutch side? You obviously have the cases apart, any news about the transmission?

Could they be pieces from the shift de-tent assembly?

  • Sasquatch x7

Posted March 14, 2012 - 10:24 AM


The previous owner took the clutch side cover off and disassembled the engine to the point where he could find the problem. As far as I can tell, he only took the clutch out.


As he described it, a gear (part 11) on the above diagram somehow fell off the shaft (part 12), and the shaft landed inside the cases. We got the shaft out in two pieces before we split the cases.

The only thing still assembled is the shift drum/forks and the transmission. I haven't had much time to work on it lately, but will hopefully have a chance by the end of the week.

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