valve question.

this morning I began disassembling my bike to adjust/inspect the valves. I inspected the valves on my '99 yz400f in late summer '01 and they were all within spec. I bought the bike used a year and a half ago and i'm not sure what work has been done by prior owners. We'll back to my question. Upon inspection of the valves I found my valves out of spec.

Exhuast: Intake:

L. R. L. M. R.

.203 .203 .127 .127 .127

All of this measurements were taken with metric feeler guages. According to my book spec for my bike is Exhuast:.25- .30mm Intake: .15-.20mm cold. It was my belief that the valves would loosen after time not get tighter. I have double and even triple checked my numbers, and there correct. If anybody can give me their experience with their valves falling out of spec. let me know if they get tighter. Thank you

Funny, I posted the same basic question as yours the first time I checked my clearances. It's a little confusing at first, but as I understand it, the clearances get "tighter" because of the wear on the valve and valve seat. As the valve/valve seat wears, the valve stem moves closer to the rocker assembly. When the clearance becomes too tight, performance suffers because of valve overlap, i.e., one or more of the valves not being fully closed when it is supposed to be and also beginning to open too soon. So most of the time, you will be installing a thinner pad. Some people buy them, some just grind the existing pad down to the required measurement.

The shims-under-bucket design leads to tightening of the valve lash as time goes by due to the valves driving deeper into the head(seats). When the engine is totally cold, the valves actually make fairly strong contact with the seats when first started and, over time, the lash decreases gradually until the lash is tighter than spec. As the engine reaches normal operating temperature, metal expands and the lash nears "0". There still needs to be a little pressure exerted on the seat to seal the combustion chamber. If you've noticed the greater lash specs for the exhaust , it's due to the higher heat and greater expansion of the metal. Typically, if you aren't a revver, you will find that once the valves are adjusted the first time, they remain in spec for quite a while.

By the way, the 400/426 does NOT have a rocker assembly. The cams are positioned above the valves and act directly upon the buckets.

[ April 04, 2002: Message edited by: Boit ]

thank you for your replies they helped greatly. I've been to both local dealerships and they both told me on the phone that they have the shims I need in stock. When I get there they both realized the '98 and'99 have different shim outside diameter(9.5mm). They told me the '00 and up have a smaller outside diameter, and they have plenty of shims for '00 and up. I had to special order the shims, but they agreed to exchange the new shims for my old shims. :)

Bad news is that I will not have my parts until tuesday :D

Hey #302

Here`s a little tip whilst you`ve got the time.Bolt the camshafts back in with no valves and see if they turn freely.I did this following a tip from an ex Yam workshop manager to our 426, we found we could hardly move the camshafts.The matter is dicussed in some depth on dirtrider net under "426 camshaft tip or maybe not"

I will try it, and I'll let you know what the results were.

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