[Nolifesigns]

The most likely reason would be rod-bearing play. The reason I say that is the missing material at the top is typically from the piston rocking excessively at full extension. The guide can go from excessive timing chain wear or tensioner not keeping pressure correctly. Lack of proper oil or contaminates could or did contribute to this as well, especia;;y since 06 was originally missing the oil squirter.

 

[Nolifesigns]

Definitely meant the bearing in the connecting rod on the crank side. If you dont have the oil mod I would not skip on doing that. If it was mine I would likely just do the top end, replace the cylinder, guide, timing chain, tensor, and do the oil mod if it doesn't have it. There is a few cheap kit you can buy without issues...just get an OEM or decent aftermarket piston since that's what is not good in those cheap kits. Then I would keep an eye on it and re-inspect it after break-in. If you hear chatter, plan on eventually doing a crank with bearings.

 

[Nolifesigns]

I wouldn't skip out on the oil mod as it is more than likely what caused the wear you see, especially on the track as it is under load where the issue exists. You can buy pre-done case sides for cheap on eBay once in a while or do it yourself if you are able to.. it's not too hard to do and way cheaper by far (seen parts for like $20 USD). Those are big chunks, defenilty see where that came from. Might not be a bad idea to repeatedly flush the full engine with diesel or a thin oil and then clean out the oil tank and lines really well.

 

[Nolifesigns]

That was the type of bad I was guessing. It's usually exactly how it feels when you turn it like you did. I have a cool link to videos that will make splitting the cases pretty painless and if you do it right you won't need to take out the tranny. The only bearings that usually go bad are what you had happen and the main crank bearings themselves. it's rare any both others go out.

 

[Nolifesigns]

Personally, I wouldn't use a used crank because it's the rod bearings that pretend to be good. You are already tearing it down so why chance that part. Also, it needs to be an 04 WR450F and if I recall correctly you will need to reject it because you are going to be bumping up your compression some.

 

[Nolifesigns]

That does look different than what I have seen but the oil galley should still be in the same place and you likely won't need to do anything other than drill the hole.


The image below is where it needs to be placed so as long as you can line it up it should still be good.

 

[Nolifesigns]

That sucks. I would still keep an eye out for a used one myself. It really is worth doing.