Check My Math....Valve Shims

So my bike has been a bit hard starting and lacking some power. Decided to check the valves last night. The exhaust valves were in spec. The intake valves are way out. It is a 2008 YZ450 with approx 150 hours on it with no motor work at all. I race harescrambles so it is mostly low rev use


Intake Valves Spec- .10 to .15 MM


Clutch side intake valve- greater then .508 MM- That was the largest feeler gauge in my set. It was close to being a tight fit though.

Stock Shim- 1.81 MM

New Shim- 2.2 MM


Center Intake valve- .229 MM

Stock Shim- 1.81 MM

New Shim- 1.95 MM


Cam Side Intake Valve- greater then .508 MM- That was the largest feeler gauge in my set. It was close to being a tight fit though.

Stock Shim- 1.78 MM

New Shim- 2.2 MM


I am new to the inside of a dirt bike motor. My friend helped me out for my first time. He has done this a number of times. We both agreed that putting a post up would be a good idea to check our work. He was kind of worried because the intake valves were so loose. While it is apart we are going to put a new timing chain in it. I have to reuse the valve cover gasket as a new one wasn't in stock at the cycle shop. I figure it should be fine but if it leaks I will get a new one.


Any suggestions, concerns and does the valves being that loose sound normal?


Thanks for the help guys

With the intake valves being 5 times to suggested clearance they need replaced ASAP. They need replaced before the valve face breaks from the stem and destroys the seat and piston.

Normally you can shim them one time if they are just out of spec. Shim then once, when they are out again replace. Once that coating wears off the Ti valve they wear very quickly.

I'd get 3 new intake valves and replace the exhaust while your in there. Get new valve stem seals and id get new valve springs- only $7 a piece.

It's a must to get the valve seats machined for a good seal and long life. Doesn't cost much either. Usually $20 per seat- $100 total max.

Also not a bad idea to replace the piston based on the hours.

Your math is OK, but you have a problem:  Normally, valves don't get loose, they get tight, so there's something abnormal going on already.  The two most likely probabilities are damaged valve stem tips (hard "cap" broken, mushroomed, or missing) and a partially seized camshaft that has damaged the bore in the head/cam cap.


Time to have a look.

Sorry I wasn't back to the post sooner, we had a huge snow storm here and 1/2 of the roof that our dirt bike shop is in collapsed. Luckily it wasn't over our section. Right now we have no power at the shop until I reroute some of the main electrical feeds, basically, a total nightmare, anyway.


Because they were so loose we checked the cam for any play, it was tight in it's place. The valve caps look perfect, the valves look perfect also. No wear or damage at all? Could the valves have been set that loose from the factory?


If there is no wear on anything should I still replace? My buddy said the same thing when we took it apart, he has never seen valves get loose nor be that loose but everything looks perfect inside?


Thanks for the help guys, we both appreciate it!

Shim it and run it, but check the clearances again after the next ride or two.

That's what we were thinking Grey. Thanks allot for your help! I will let you know how we make out.

Going to get it put back together this week but I just bought a 2013 KTM XCW. I will get the YZ out and ride it some when it warms up. If it comes back out of adjustment I will let you guys know. Since it will be a back up bike. I will probably just rebuild the entire motor over the summer anyway. Going to use the KTM for Enduros and harescrambles and turn the YZ into my MotoX bike. Thanks again for the help.

Got the valves all done, fresh oil. Fired it up and it was back firing bad and would not idle, leaking coolant out of the water pump too, LOL. All at once!


new pilot jet(I know I can clean them but they are cheap so I just replaced it)

Drained all fuel and put fresh in

Replaced water pump seal


Put her all back together and headed to a moto track. Fired up first kick out of the trailer. Running great, starting easy and I even started it in gear. Only got three 10 minute motos in at the track. Going to ride her again next weekend and then recheck the valves but for now she is running great!

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