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What valve shims after installing new valves?

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I'm getting ready to put new valves in my 08 WR250F. The question is, what shims will I need when the new valves go in? It had new seats cut about 20 hrs ago. I'm guessing that the new valves will pull the clearances out about as much as they were tight to begin with. For example, if a valve has a 1.64 shim in it and measures .07mm now, assuming it measured .15 when new, with a new valve it will measure .07+(0.15 - .07) = .15. If that's true, I won't need to get new shims. I realize that the right answer is to wait until I get the valves in the head, measure them and then order the shims I need. Only problem with that is a big race is coming up and I won't have much time to order shims once the valves get here so I need to order them now. I thought some of the experienced engine builders out there might have some rule of thumb for the shim differences after putting new valves in.

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You can't really go off of that because each valve will be a little different due to machining tolerances. You should really get your seats cut again for the new valves. The only way to get the shims right is to put in the new valves, stick in shims that you know are too small, measure the clearance with those shims, then calculate the shims needed from there.

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Thanks for the feedback guys. It's interesting that everyone says to re-cut the seats. Williams Motowerx cut the seats in August, 2011 and I don't have an hour meeter but I ride maybe three times a month for an hour and raced once a month for an hour and a half so it has to have at least 20 hours on the seats with the old valves. Back in August, he didn't recommend new valves, but did the dimples on my old ones and cut the seats. He said I wouldn't need to have the seats cut again now with the new valves.

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Buy an entire box of all shims off eBay for 80 dollars? Cheap that.. I've gotta pay over 200 $ for the same thing in the Uk. You're always gonna need shims.. :smirk:

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