Dropped valve on 06 YZ450

I dropped a valve (at idle) on my 06 YZ450, and I have a few questions:


1. How worried should I be about damage to the bottom end? I feel like I should be okay because it happened at idle.


2. If the bottom end were to fail (I've never had this happen before) what are the chances that it will cause damage to my brand new top end if I take my chances with question 1?


3. When i initially took it apart. The shim and retainer for the valve which dropped were not there. I've pulled the left side cover and fished around with a pen magnet, and found one piece of metal which may account for about 10% of what disappeared. I found a good deal of metal flakes in the oil filter, but that probably only accounts for another 1%. I drained the oil into a common drain pan, I may fish around in there with the magnet. Anywhere else i should look? Any tips for this situation?


Any help is appreciated.






This is what I do in these situations. If the valve ever contacts the piston, I replace the piston, wrist pin and rings without question. There is likely no damage to the bottom end from the dropped valve but it doesn't hurt to spec out the crank. If the bottom end decided to fail I am 80% confident you head will be fine.

When I comes to debris in the motor/trans I wouldn't risk it. I would split the cases and inspect and clean every port, part, nook and cranny. That stuff finds a way back through the system and causes more damage. Just not worth the risk in my opinion.

On the other hand, if the crank seizes at speed, it can easily make a $3000 lunch box out of the entire engine.  If it dropped the valve at idle, you should be OK, but I would carefully inspect the crank for axle runout and rod play per the manual.  The typical damage to a crank in such cases can be that the crank gets knocked out of true, the rod gets bent or twisted, or the crank pin gets "stamped" with dents from the rollers in the bearing. 

Sounds like the general consensus is to suck it up and find all foreign material and inspect the crank, which means I will have to split the cases. Never done that before, should be interesting :)


Thanks for the advice guys.

Okay, So cases are split, found a lot more metal inside, I'm glad i took it apart. All the bearings seem fine, do they need to be replaced? If they fail will they also destroy my top end? I know it seems silly to ask, but i'd rather spend the cash on a WR 4th and 5th, and I'm not planning on rebuilding the crank (so the connecting rod bearing will be original). What do you guys think?


What are the main culprits for the bottom end to fail at speed? It seems like this would be the case where my top end would get destroyed....





One more question. This shift fork (shift fork c) has clear signs of wear, but how much is too much? The manual doesn't have any measurable values listed..


The fork is bent.  Contact at the circles with the wear pattern as indicated shows that.  Carefully examine the wear at the locking lugs of the gear the fork controlled.


The culprit in bottom most end failures is the big end bearing, which can seize due to a cracked cage, or due to progression of damage suffered when the piston impacted the valve against the head.  Depending on how it fails, it could just stop the engine, or it could take out the entire engine.  


Thank you for the edited picture Gray, wasn't expecting that level of detail when asking the internet, and I bet someone else benefits from the explanation. I don't post much here but have found a lot of your posts helpful. Glad you ride the same bike I do!


I took my crank into work today and measured the runout (measurement "a" in the service manual), measured value was .0038", manual says the limit is .002". Which leads to my next question (which may just be wishful thinking), is that measurement the total travel of a dial indicator positioned on the bearing surface? Or is it from the centerline of the crank? (which would half my result and put me in spec).


To measure, I put the entire crank on a lathe, verified that there was zero travel at both ends, then measured on the bearing surface on each side and found 0.0038" of movement on both sides. Seems to me like the crank is bent and needs to be replaced (unless I'm interpreting the measurement wrong).


I took a photo of the crank set up in the lathe before i got the dial indicator. Also included a picture of the "a" measurement included in the manual for easy reference.


I'm afraid this just got a lot more expensive....



You should be using V blocks to check the crank, not a lathe

Why v blocks? Live centers on a lathe are true centers?

The centers should be OK, but V-blocks, particularly those with bearings for the shaft to sit on, are more accurate because they support the crank in the same way it is supported in the main bearings.  For one thing, the center on the magneto side is subject to being abused by pulling the flywheel, etc.


And yes, the spec is .002" total run out, but it's exceptionally unlikely that the crank is bent.  The crank is assembled from three pieces pressed together, and it's just a matter of realigning the left and right halves on the crank pin.  A shop that rebuilds cranks can take care of that.  

Are you recommending sliding Crank bearings on the crankshaft, supporting the crank shaft wtih V-blocks holding the bearings, and then measuring the run out? Is there enough "real estate" on the surface that mates with the bearing to perform that measurement?


fixturing it this way makes sense to me, it simulates how the crank is supported in the installed condition. I guess the way I measured it (included a pic with some notation) will give me worst case results. I just saw the arrows on the service manual and figured it should be supported on either end.


Needless to say I have to take the crank to someone to look at it, I already sent my head into engine dynamics in Petaluma, so my crank will go there too.


If you are using "V blocks" that are equipped with two bearings at the top of each for a shaft to lay in, the original mains are not needed.  If they are just a steel, aluminum, or even hardwood stand with a V cut in it, the best results would come by using bearings on the crank.


But in either case, the measurements taken while using V blocks rather than centers would have to be taken from near the ends of the shafts.  Positioning the dial close to the main bearing won't show the misalignment.  Either way is valid, but there is the caveat regarding a damaged center.


If you imagine a line perpendicular to the shaft axis that passes through the centers of both crankshaft and crank pin bore on each half of the crank, what you are looking for is for those two lines to be absolutely parallel to each other, which would cause both crankshaft axles to run on the same center line.

Something like that.  Hopefully cheaper.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now