Valve Spring Installed Height Tolerance
 

Is there an IH tolerance or should they each be installed exactly right on the recommended IH?

"An installed height of +/- 0.020 inches is acceptable."

https://www.motortrend.com/how-to/16...spring-height/

Once you have determined the shortest installed height, it will be necessary to use shims to obtain this height (±.020” is acceptable) on the remaining valves.

https://www.compcams.com/valve-springs-101

I've seen one source that stated +/_ 0.010 but that was not the norm




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Other then having the needed minimum retainer to seal, or retainer to guide clearance the installed height is all about the spring.

The installed height needs to provide the minimum seat pressure as stated on the cam card while NOT having the installed height such that the spring is at coil bind.

On most duel spring set ups I have used for a non roller hydro cams in motors that can buzz to 6000 rpm, the use of a .030” shim will add 10 psi to the seat pressure.

When setting up the installed height with new springs I almost aways add a .030” shim just to compensate pressure loss once the spring breaks in.

If it’s for a small valve head 1.96”/ 1.66” and I already have the 125 seat pressure I like to start off with and if I know the motor will not need to go above 5800 then I might leave out the .030” shim.
This applies to a true duel spring set up.
If due to the customers choice I am using a set up that is only a outer spring and a flat wire damper, the. I do add the .030” shim.

If you want to be obsessive, it's the spring forces you want to equalize, not necessarily the installed heights... so installed heights end up being whatever is needed for each spring.

And then the question whether equalizing pressure(the theory behind equal IHs) is will the motor really notice a difference of 5 or 10 lbs on the seat?

You can only be as close as the smallest shim, which is .015". You could try moving valves around and changing retainer and keepers, but at the end of the day, .015" to .020" is good enough.

If you are really worried about .015" difference at zero lift, check each lobe of the cam and each rocker arm for the correct lift and ratio, then see if .015" really matters.