One dimension, measured sixteen times
Installed height is the distance from the spring seat in the head to the underside of the retainer, with the valve, the locks and the retainer assembled. It is not a design figure you can look up for a head — it is what that particular seat measures after that particular valve job, and it varies seat to seat by more than people expect. A head that has had the seats cut sinks the valves, which lengthens the installed height and takes load out of every spring in it.
The datasheet figure is the other half. Spring manufacturers quote a seat load at a stated installed height, and that pairing is the specification — a load without a height means nothing at all.
Shims only go one way
A shim sits under the spring, raises the seat and shortens the installed height. So it can bring 1.900 down to a 1.850 target, and it can do nothing whatever about a seat that already measures 1.830. The page says so plainly rather than returning a negative shim, because the fix in that direction is machining the seat deeper or changing the retainer, and that is a decision for whoever is doing the head.
With 0.015 in shims, closing a 0.050 in gap takes three of them for 0.045 and lands the height at 1.855 — five thousandths off the target, which is what the granularity of the shim pack costs you. The page rounds to the nearest step rather than always going up, and the table underneath shows the rows either side so the choice is visible.
The load and the coil bind clearance are the same number seen twice
This is the part worth internalising. Every thousandth of shim raises the seat load by the spring rate times a thousandth, raises the load at full lift by exactly the same amount, and takes exactly a thousandth out of the clearance to coil bind. At 380 lb/in, a 0.015 in shim is worth 5.7 lb of seat load and costs 0.015 in of bind clearance. There is no way to buy one without paying the other.
On the defaults, the 0.045 shim lands a 128.1 lb seat load, 356.1 lb at 0.600 in of lift, and leaves 0.075 in before the coils touch. Against a 0.060 in figure that is 0.015 in in hand. Change to a spring with a lower bind height and both columns improve; shim it harder and the load column improves while the clearance column gets worse.
The straight line is an approximation
Loads on this page come from a single rate multiplied by a displacement, and real springs do not behave like that. They stiffen approaching bind, a dual spring is three parts that are not in step with each other, and a beehive changes rate through its travel deliberately. Within a shim step or two of the datasheet height the line is close enough to work with. Two hundred thousandths away from it, the datasheet load at a stated height is the honest number and the extrapolation is a guide.
What it does not settle
Whether a given seat load is right for a given cam, whether the clearance to bind is enough, whether the retainer clears the valve seal or the guide boss at full lift, and whether the spring is the right spring at all — none of that is here. Those are engine builder judgements made against the cam, the valvetrain weight and the intended engine speed, and the page reports arithmetic on numbers you supply without offering an opinion on any of them.
Questions people ask
What is valve spring installed height?
The distance from the spring seat in the cylinder head to the underside of the retainer, measured with the valve, the locks and the retainer in place. It is measured, not looked up, because it changes with the valve job — cutting seats sinks the valve and lengthens the installed height on that seat. Spring manufacturers quote their seat load at a specific installed height, and the two figures only mean something together.
Which way does a shim move the installed height?
A shim goes under the spring, raises the seat and shortens the installed height. So it closes a gap when the measurement is longer than the datasheet figure, and it does nothing at all when the measurement is already shorter — that direction needs the seat machined deeper or a different retainer, which is a machining decision rather than an arithmetic one. The page says so rather than returning a negative shim.
How much does a shim change the seat load?
The spring rate times the shim thickness. At 380 lb per inch, a 0.015 in shim adds 5.7 lb at the seat and the same 5.7 lb at full lift. It also removes exactly 0.015 in from the clearance to coil bind, and that is the trade the page puts in one table so you can read both columns from the same row.
What is coil bind and why does it show on this page?
Coil bind is the height at which the coils touch each other and the spring stops being a spring. It matters here because shimming a spring to raise its load moves the whole assembly closer to bind by the shim thickness, and full lift moves it closer still. The page prints the height at full lift, the gap to bind, and how that gap compares with whatever figure your engine builder works to. It does not tell you what the gap should be.
Why are the load numbers only approximate?
Because they come from one rate multiplied by a distance, and real springs are not linear. They stiffen as they approach bind, dual springs are several parts acting together, and beehive springs change rate through their travel on purpose. Close to the datasheet height the straight line is a good approximation. A long way from it, trust the datasheet load at a stated height instead of the extrapolation.