The formula, and the sign everybody gets wrong
On a shim-under-bucket engine the shim sits between the valve stem and the inside of the bucket. Its thickness is dead space: the thicker it is, the further the bucket is pushed toward the cam, and the smaller the gap between the bucket and the cam base circle. That gap is the clearance.
New shim = old shim + (measured clearance − target clearance)
If you measured tighter than target, the bracket is negative and the new shim is thinner. If you measured looser than target, the new shim is thicker. Say the target is 0.15 mm, you measured 0.10, and the shim in there is 1.80. Then 1.80 + (0.10 − 0.15) = 1.75, and a 1.75 shim gives you the 0.15 you wanted. The clearance was too small, so material had to come out, and the shim got thinner.
The reverse is the one that produces a wasted afternoon: measure 0.22 against a 0.15 target and the shim has to grow to 1.87, not shrink. People who reason from "the gap is too big so I need a bigger shim" happen to get this one right, and then get every tight valve wrong.
Checking backwards is quick. The clearance you end up with is:
Resulting clearance = measured clearance − (new shim − old shim)
Every millimetre added to the shim is a millimetre taken out of the clearance. It is a one-to-one trade, which is what makes the arithmetic trivial and the rounding the only real decision.
Rounding to sizes that exist
Shims come in steps — commonly 0.05 mm, sometimes 0.025 mm on finer kits — so the exact figure almost never exists. That is why the table above shows the exact shim and the nearest available one separately, and then shows what clearance the available one actually gives.
| Measured | Shim now | Exact shim for a 0.185 target | Nearest 0.05 step | Clearance you get |
|---|---|---|---|---|
| 0.10 | 1.80 | 1.715 | 1.70 | 0.20 |
| 0.12 | 1.85 | 1.785 | 1.80 | 0.17 |
| 0.22 | 1.75 | 1.785 | 1.80 | 0.17 |
| 0.19 | 1.90 | 1.905 | 1.90 | 0.19 |
Two things fall out of that table. The fourth valve does not need touching at all, because the shim already in it rounds to itself. And the second and third valves both want a 1.80, one of which is already coming out of valve two — which is the shim shuffle, and it is why the calculator matches the sizes leaving against the sizes needed before telling you what to buy.
Aiming at the middle of the range rather than at one end is the usual choice, because it leaves the most room in both directions before the next service. There is an argument for aiming loose on valves that historically tighten, and the calculator offers the choice rather than making it.
Which way clearances drift
Valves do not stay where you put them, and the direction is not random. As the valve face and the seat wear, the valve sinks further into the head, which pushes the stem tip upward toward the bucket, which closes the clearance. Exhaust valves run hotter and see more of this, which is why on many engines the exhaust side is the one that tightens over time while the intake barely moves.
A clearance that has closed to nothing is the failure mode worth knowing about, because it is silent. A valve held slightly open cannot seal against the seat, so it cannot dump its heat into the seat either, and it runs hotter and wears faster, and the process accelerates. Nothing rattles first; a tight valve is quieter than a correct one, which is exactly backwards from what people expect a problem to sound like.
What the interval is, whether it is measured in miles or hours, and whether your engine is one that tightens or one that stays put, is engine-specific and comes from the manual and from records for that model.
Measuring so the numbers mean something
A feeler gauge reading is only as good as the position of the cam. The gauge goes between the bucket and the base circle of the cam — the round part, opposite the lobe — and the engine has to be turned to the position the manual specifies, which is typically top dead centre on the compression stroke for the cylinder in question. A cam a few degrees off base circle reads tight, and reads tight consistently, which is convincing and wrong.
Temperature is a stated condition, not a detail. Most manuals specify a cold engine, defined as a stated temperature or as having stood a stated time, because aluminium and steel expand at different rates and a warm head measures differently. A reading taken warm on a specification written for cold is not comparable to anything.
Feeler gauges also lie if you let them. The gauge should have a light drag through the gap, not slide freely and not need pushing, and a bent or burred leaf reads thick. Go-no-go is more reliable than judging drag: find the leaf that goes and the next one that does not.
What this page is not
It is a calculator for shim thicknesses. It does not know your clearance specification, and the range you entered is echoed back exactly as you typed it and compared against your own numbers — it is not a verdict that an engine is in or out of specification, or that it is fit to run.
Getting at the shims on most shim-under-bucket engines means removing camshafts, and putting camshafts back means cam timing. An engine timed wrong on reassembly can put a valve into a piston on the first turn. Torque figures, sequences, timing marks, chain tensioner procedure and whether the engine is an interference design are all manual material, and the work belongs to somebody equipped to do it.
Questions people ask
Does a thicker shim increase or decrease valve clearance?
It decreases it. The shim is packing between the valve stem and the bucket, so a thicker one pushes the bucket closer to the cam and closes the gap you measure with the feeler gauge. It is a one-to-one relationship: add five hundredths of a millimetre of shim and you take five hundredths out of the clearance. The reason this trips people up is that the intuitive picture runs the other way — a bigger part sounds like it should make a bigger gap — but the gap being measured is above the bucket, not below the shim. If it helps, remember which direction the fix goes rather than the mechanism: a tight valve needs a thinner shim, a loose valve needs a thicker one.
Can I reuse the shims I take out?
Usually yes, if they are not damaged, and the shuffle is how most people avoid buying a full kit. When several valves need changing, the sizes coming out of one bucket are frequently exactly what another bucket needs, and the calculator above matches them up before telling you what is left to obtain. What to inspect before reusing one is whether it is flat and unmarked: a shim that has been running with a pitted or dished face, or that shows wear on the side that contacts the valve tip, is telling you about something else that needs attention. Also check that the marked thickness matches a micrometer, because the marking wears off and a shim from a parts drawer with a faint number on it is a guess.
Should I aim for the middle of the clearance range?
The middle is the common choice because it gives the most room in both directions before the next check, and because shim rounding means you will not land exactly where you aim anyway. There is a reasonable argument for aiming toward the loose end on valves that historically tighten with use, which on many engines means the exhaust side, since a valve that starts near the loose limit takes longer to reach the tight one. There is a counter-argument about noise and about wanting to be inside the range on the day rather than at the edge of it. The calculator offers all three targets rather than picking for you, because which end to favour depends on the engine, its history and how long until the next service, and the manual for your machine is the reference.
What happens if a valve clearance is too tight?
The valve stops sealing fully against its seat. That is a problem beyond compression, because a valve dumps most of its heat into the seat during the time it spends closed against it, and a valve that never fully closes never gets that chance. It runs hotter, the face and seat wear faster, the valve sinks further, and the clearance closes further — the process feeds itself. The unpleasant part is that it is quiet. Loose clearances tick audibly and get investigated; tight ones make a valvetrain sound smoother right up until compression starts falling off or a valve burns. That is the reason clearance checks are on a schedule rather than done when something sounds wrong.
Do I measure valve clearance hot or cold?
Whatever your service manual says, and it is a real distinction rather than a formality. Aluminium heads and steel valves expand at different rates, so a head at operating temperature has different clearances from the same head cold, and the difference is comparable to the whole specified range. Most manufacturers specify cold, and define what cold means — a stated temperature or a stated time since the engine last ran. A specification given for cold and measured after a ride is not comparable to anything, and neither is a hot specification measured cold. The other condition that matters as much is cam position: the feeler gauge goes between the bucket and the base circle of the cam, with the engine turned to the position the manual specifies, and a cam a few degrees off that reads tight every time.