Starting my top end come the questions!

I have a 2006 YZ250F. I've decided to tackle my first 4 stroke top end (I have experience with 2 strokes). After about a week of research, I've decided to dive in, take my time, follow my manual and online resources (TT), and ask questions along the way (hope you guys don't mind...).

I tore down the bike to the point where I have the least confidence, and seems to be the most confusing: the valves / timing / head. Most of my questions are more along the lines of someone verifying my understanding of how things work. Here are my questions:

1. When I turn the crank and, and look through the “viewer hole”, it almost looks like there are 2 different timing marks. One looks like an “H” and one looks like an “I”. I’m assuming the “I” is the TDC? Is the "H" a timing mark?

2. I’m wondering why the motor has to be at TDC to disassemble the cams? Is it because at TDC there is no pressure on the cam lobes?

3. The manual talks about loosening the cam cap bolts in a “criss-cross” pattern, or you can cause damage. Is there an exact science to this? I’m thinking that there are 6 bolts on one of the cam caps. I guess I’m looking for a little more guidance in this area. Is there a right and wrong way to remove the caps?

4. Here is my understanding of timing (correct me if I’m wrong): When the piston is at TDC, your flywheel timing mark should be aligned (looking through the viewer hole), along with the punch mark on each cam sprocket should be “flush” and align with the top of the head (each punch mark on the outside edge of the head. Am I correct that the “E” and the “I” stamped on the sprocket have nothing to do with the actual timing alignment? I’m just looking at the little “punch mark” in each sprocket, correct?

5. When I took out my crankshaft cap and viewer cap, there was a little moisture in there. Is that ok? It looked like there was some dampness on the inside of the cap, and a drop of something that looked a little “milky”. Is that ok, or is there a different issue?

6. Just to verify, I have to remove the cams to remove the head and cam chain, correct?

7. Before I tear it down, I want to have a firm understanding a valve clearance. Here is my understanding: When the piston is at TDC, there should be a clearance of between .10 mm and .15 mm between the cam lobe and the “pad”. If the clearance is less than .10mm or greater than .15mm, I need to change the pads. Am I correct in my understanding? Again, this is a bone stock 2006 250F (low hours), that has never had a valve adjustment.

I'd like to thank everyone for there help! I've gotten a lot of great info from you guys!

1. Yes go off the I mark. The H is ignition timing

2. It doesn't really hve t be at TDC for tear down but it does make it easier on re-assembly

3. Criss cross pattern assures that they don't get bound up. It's more important to install it in a criss cross pattern and tighten the bolts in increments.

4. Yes go off of the punch marks on the cam sprockets. They should be lined up with the top of the head. If one cam is a c--t hair off, don't worry about it too much but you sould replace the timing chain

5. Could be condensation or a bad head gasket depending n how much moisture is in there. Inspect your head gasket when you get it off and inspect the head and the top of your cylinder for flatness.

6. Yes

7. Under the buckets are shims. Make sure you keep all the buckets and shims in order on the correct valve and put it together the way it came apart. Check the clearance after you put it back together.

Have fun and I hope I was of some help


Thank you! you are a tremendous help.

I'm fairly new to this, and just want to verify your terminology...when you say "under the buckets are shims". The term "buckets" is referring to "cam lobes", correct?


What Swede said is all correct. The buckets (sometimes called lifters) are the metal "cups" that the cam lobes hit against. They ride in bores in the head and sit directly on top of the shims (or the little pads you change to adjust the clearance). The shims are just spacers that you can get in different thicknesses to adjust how much clearance there is between the cam lobe and the bucket at TDC. If the clearance is too small, you switch to a thinner shim to get the clearance right.

Just to verify, the clearance I'm measuring is between the cam lobe and the bucket, correct?


Just to verify, the clearance I'm measuring is between the cam lobe and the bucket, correct?


Yes that is correct.

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