Valve Clearance YZ450 Cam Mod

Installed the new cam on my 98 WR400, measured up valve clearance and I had a whopping .90mm :) clearance on the left valve and approx .45mm on the right. Measured cold.

I didn't measure the clearance with the original cam before I removed it..Doh. I presume I am getting a 'loose' valve? Is it possible I have a 'sticky' valve and the clearance is not as big as I may think, just I am measruring it in the stuck position.

Should I be worried or should I just slam the correct shim in there (from 175 to 235).

I checked the tolerance about 1 yr ago and it was well within the norm. Now the clearance difference seems huge to me.

Cam shaft cover bolts torqued down to correct torque (10nM)

One last thought, I have been getting intermittent performance wobbles at constant speed. I'll be humming along and then the motor starts to sputter, but still works...mostly. That lasts a couple of minutes then clears up for a while and then maybe next time I ride the same thing.

The more I write the more I think a top end service is in order. What do y'all think ?

Mechanically I can do the CAM shaft switch without referring to the manual. Is a top end teardown much more difficult? Do I need any special tools? How long does it take ?

It would pay to look at the valves... you are halfway to doing it anyway... not that hard a job...

Most of the cam converts have been seeing .2mm to 0.3mm increases in the clearances with the 450 cam... but.9 is a lot... and I would wonder if it is not worth putting the stock cam back in and seeing what you get... then you have a baseline to work from...

The only other tool you will need is a suitable valve spring compressor

Good luck...


Is a top end teardown much more difficult? Do I need any special tools? How long does it take ?

It is NOT hard to do Simon. The only issue I had was when I developed a loop in my cam chain UNDER the crankshaft sprocket. I had no idea it was there, and looking down into the hole, couldn't see it either.

It is NOT a matter of just turning the crank.

>> You MUST pull out the removable cam chain guide/pad that is on the forward end of your engine. Then turn the crankshaft clockwise to roll that loop out. Then re-insert the guide.

It took me two days to figure this out 2 years ago.

As for time, if you have your replacement piston, rings, wrist pin, c-clips, AND GASKETS!, add another hour MAYBE.

You are still using the original (non big bored) cylinder, correct?

NH Kevin, David thanks for the tips, I'll be sure to have a look at the top end a little closer. So far not gone for the rebore and bigger piston.

Hey fellas,

I am just completing a top end on job on my bike. I would highly suggest buying a aftermarket gaskets and piston/rings sets than OEM. MUCH MORE INEXPENSIVE to go aftermarket and the quality is just as good.... Anyway it was a pretty easy job.

I bought the bike used last year. I never checked the valve clearances due to a service reciept of a tune up by the guy that I bought the bike from. The clearances were about .010 to tight. OUCH!! I immediatly stripped the head and cylinder off to find NOTHING really wrong except excess carbon build up around the valves seats. The carbon is from running the bike too rich. Well good luck on the top end.


That is interesting. I am doing the cam swap right now. My shims just came in yesterday. I had to go @.2mm bigger... or so I thought. I found both my exhaust and all three intake valves were at the extremely tight end of the clearance range. I mistakenly(?) thought that this was how it had been set from the factory, since cam lobe wear, lifter wear, and stem tip wear would only increase the clearances.

It was pointed out to me in a post last week that I could be seeing some valve seat/face wear which would decrease the clearances if the valves had sunk into the head a little. You did not find this to be the case on yours? I had overlooked that possibility and had chalked the tight clearances up to good oil and no wear. So I reordered shims for the new cam that I hope will loosen them up a bit and put me more in the middle-to-loose end of the tolerance range. I will know later today when I install and remeasure. I will also remeasure in a month or so.

My bike has always had rather sooty plugs. I wonder how much carbon buildup it could be causing and what effect that may have. I am following yamaha.dudes advice and installing a stock yz needle to go with the cam. Also doing the jamesnow and p-38 mods. I would like to see a better plug color, but will have to wait and see what effect these mods along with the Big Gun exhaust will have. I just hope I'm not tossing too many variables into the equation at once.

Can anyone suggest a part number for an on-line store for a valve spring compressor. They have jack here in Switzerland for motorbikes.

Missile, not so long ago, someone posted about a real effective homemade spring compressor they made themselves. I don't recall the specifics, unfortunately, but it was made with readily available materials.

Found it: title is "cheap homemade valve-spring compressor tool"

Do a search under "valve spring compressor". Sorry, I don't know how to link you straight there. :)

Easy to link to a post... do the search, then when you have the post in your browser, just select the URL in the address bar, press CTRL+C (to copy the URL address) and then paste (CTRL + V) it into your message... and post it as normal...



Guys, yet again TT comes to the recsue! I need to buy a couple of the sockets and I DO have a wife so all is well :)

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