Thinking about a rebuild and some machine work.

I developed a problem with my 03 YZ450f this weekend, Its leaking water into the oil and blowing oil and water,prolly out of the valve cover breather. Ibelieve the problem to be a blown head gasket and hopefully no more.

Here is the question.... I was planning a rebuild this winter now maybe sooner. Im thinking to replace valves with stainless valves and have the valve seats cut, repalce timing chain and tension slider and replace the piston.

Does this sound like enough of a rebuild? I have only owned this bike for a year and Im unsure of its exact history. Im sure the last owner didnt get into the motor. Im just looking for advice on what all needs to be covered to make the bike solid and reliable. Its been great until now. I have shimmed the valves once and the middle is using a .180 shim to get it into middle spec.

Your thoughts?

The 3 possible points at which water can normally enter the oil in a YZ450 are:

  1. Head gasket
  2. The water passage between the crankcase and cylinder
  3. The water passage between the crankcase and the right crankcase cover

This assumes that no parts are cracked or porous and that some knuckle head didn't glue the water pump leak port shut.

The fact that the center intake has a 180 in it now doesn't tell me anything because there is no mention of the size the shim that was in it originally, or what the clearance was then, what it was set to, and what it is now. On the whole, it may be a good idea to go ahead and replace the valves, but it also may not be necessary even yet.

There is no advantage that I can see to using stainless steel valves. If the seats are refinished correctly (and that is a very important qualifier), the OEM Ti valves will last as long or longer than any SS valve I can think of. Know that in order to run SS valves, you need to upgrade the springs because the SS valves are 160-170% the weight of Ti, and the springs need to be stronger to prevent valve float at high speeds. Kibblewhite valves are tempting because they are a good product and the valves are significantly less expensive than OEM Yamaha, but their spring kit uses titanium retainers to reduce the valve train weight to near that of the stock components, and goes for more than $200 by itself, somewhat washing out the savings. If you can find another source of suitable springs, then you might save some money.

You should check the condition of the rod bearing carefully. It was a weak point on the '03. Not really weak compared to other bikes, but weak compared to other YZF's, and a known source of trouble. The problem was that the roller cage would fail and the rollers would skew themselves over the crank pin and lock up. Potentially catastrophic. Check the manual for the details on measuring the clearance.

Trans bearings would be a good idea, as would any locking trans gears that show wear at the lugs, and a careful inspection of the mains.

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