YZ450F -03 top end (valves and piston) done

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  • Anssi

Posted July 24, 2005 - 10:45 AM


Hi, just wanted to share.

I had about 140 hours on my -03 YZ450F, and from all the scary stories about four-strokes, and thinking I will keep this bike for about another 140 hours, I decided to do a top end replacement. Ordered the valves, springs, valve seals, cam chain, piston and rings(plus other stuff, will explain) from oem.thumpertalk.com.

I had adjusted the valves and replaced the cylinder and piston rings before, so everything was familiar (and the valves and ring end gaps were still in spec) until I had to remove the cam chain. Popped the fly-wheel (I had replaced that with a heavier one earlier and had the puller) and then the stator plate or whatever that is behind the flywheel and around the crankshaft.

After that I could just take the cam chain off. It didn't seem stretched compared to the new one(and moved freely), and the only visible wear through this whole operation was with the sprocket on the crankshaft. It had marks from the chain that you could see and barely feel, but looking from the side the teeth had not deformed at all so I think the crank will be fine for the rest of my riding.

I also replaced the plastic slides or whatever the chain slides on, but the old ones looked like new, as did the cam chain adjuster. I had ordered the parts so no use not using them.

Replaced the valves, no drama there. Springs still in spec as well, replaced them as well as the seals and also the washers on the top and the bottom of the springs. Replacing the washers was a complete waste of time. In a stroke of luck, all of my valves were still in spec (albeit barely) and were still in spec after 2 hours of break-in.

Putting the thing back together was no problem, but I did look at the manual closer, and did realise that my cams had been off by one tooth (oops). There are supposed to be 7 links between the "other" marks on the camshaft sprockets, but I only had 6.5 because of how the picture in the manual looks (it is taken looking at the camshafts in a downward angle, so the other dots look like they are above the top of the head, when in fact they are a few mm lower).

There was a bunch of carbon build-up on my head and a bit on the valves and piston. Might have been the bad timing or just crap jetting and slow riding.

I also replaced the needle jet in my carb (the one the main jet screws into). This was a problem the year before when I hadn't ridden my bike for months and I suspect the alcohol we use in the fuel to make our carbs not freeze had something to do with it. My bike wasn´t running great, so I just did this as a precautionary measure.

After all this the bike runs noticeably smoother than it did before. Between the new piston and rings, valves(probably little effect), the valve timing(probably the biggest effect), I'm not sure what had the biggest influence in this, but I don´t too much care either. I expect to adjust the valves once, replace the piston rings once and then just ride until I get rid of this bike.

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