Originally Posted by
Trey
One thing to keep in mind is that SR20 heads are one of the industries tightest tolerance heads made. The shims that ride on the valve tips come in thicknesses that vary in ".001. That means that the acceptable range is just ".0005. This is important once you realize that the cam pushes on the rocker arm and the arm pushes on 2 valves. If the valves are at different heights, then the rocker is at an angle, which wears on the cam/ruins it/causes extra friction. This might not even be the problem but it is a flaw in the design of the SR20. If this ends up being why the car is starting to break down then pm me and we will take it one step further and give you a shim height that is just ".0002 or less. It is much better for the motor.
Why would the tolerance be half the shim height increment? It's impossible to get within the tolerance then, because unless there is another area for adjustment than it isn't able to be achieved. The closest you would be able to get is 0.001 or less, but without any other adjustment that's the best you can get. So all the valves would be somewhere between 0.000 and 0.001, with no ability to match them closer than that without sub-0.001 shims available.
Is the tolerance you're talking about measured at the cam, meaning that the rocker ratio must be exactly 2:1? I've never played with SR20 shims before, I'm confused..
*EDIT*
I found the answer. It's 0.14 to 0.17 mm clearance at the cam, which is 0.00056 to 0.00067 inches. So that makes sense now, because the rocker ratio is listed as 1.43:1, so a 0.001 shim can give a change of 0.00069, so you have to be better than that because you can get somewhere less than 0.001 (at the valve) with the shims.
So the 0.0005" clearance is a completely arbitrary number and has nothing to do with the shim height, because the rocker ratio isn't 2:1.
I understand now.