# A spec that tells you not to trust half of itself

> A valve clearance table gives a hot figure and a cold figure, then instructs you to ignore the hot one. Four numbers, and the honesty is in the line underneath them.

August 19, 2026 · https://justog.club/stories/valve-clearance-table

A two-by-two grid. Rows for inlet and exhaust, columns for cold and hot, four figures in
millimetres. Underneath it, in orange: ALWAYS SET COLD.

The table gives you two ways to measure and then tells you only one of them is trustworthy.

## What is a valve clearance actually setting?

The small gap that lets a valve close fully once the engine's hot and everything has expanded.

Metal grows when it's heated. Leave no gap at all and a valve stem that's expanded from engine
heat can hold the valve open a fraction, which lets combustion pressure and heat escape past a
seat that should be sealed. The clearance is built in specifically to survive that expansion — too
tight and the valve never seats properly when hot; too loose and it clatters and the valve gear
wears faster than it should.

## Why are the exhaust figures bigger than the inlet ones?

Because the exhaust valve runs hotter and grows more.

Inlet reads 0.20mm cold, exhaust reads 0.30mm cold — half again as much clearance on the exhaust
side before the engine's even started. The exhaust valve sits in the path of burned gas leaving the
cylinder, which runs considerably hotter than the incoming fuel-air mixture on the inlet side. More
heat means more expansion, which means the gap has to start larger to end up correct once
everything's at temperature.

## So why does the table give a hot figure at all, if you're told to ignore it?

Because it's telling you what the gap becomes, even though it isn't telling you to measure there.

INLET reads 0.25mm hot against 0.20mm cold; EXHAUST reads 0.35mm hot against 0.30mm cold. Those
numbers exist to show the relationship, not to be used with feeler gauges. Getting an engine to a
known, even, fully-settled hot state — every cylinder at the same temperature, at the same
moment — is genuinely difficult to do reliably outside a lab. Cold is just room temperature, which
anybody can get an engine to by leaving it alone overnight.

## Isn't a hot reading closer to how the engine actually runs?

It is, and that's exactly why it's the wrong one to measure by.

A spec that included only a hot figure would be asking for a condition nobody can reliably
reproduce twice. Two mechanics measuring the same "hot" engine an hour apart, or on two different
cars, could easily land on different numbers — not because either measured badly, but because
"hot" isn't a fixed, repeatable state the way "cold, overnight, untouched" is. ALWAYS SET COLD is
the table admitting that the reliable number and the true-running number aren't the same one, and
choosing the reliable one on purpose.

## Does a wrong clearance actually matter, or is this precision for its own sake?

It matters in both directions, which is the reason the number is checked rather than assumed.

Too tight and a valve can't close fully once hot, burning the seat and losing compression. Too
loose and the valve gear clatters, wears faster, and the timing of when each valve opens and
shuts drifts slightly off what the engine was designed around. Neither failure is dramatic on the
day it happens — this is a slow-wear problem, which is exactly the kind that a printed table
protects against, the same way [a gap set wrong on a spark plug](/stories/spark-plug-gap-card)
causes trouble that shows up weeks later rather than immediately.

## Can real engine figures go on the shirt?

The clearances themselves are physics — nobody owns a gap measured in millimetres.

What belongs to somebody is the specific manufacturer's engine code the figures were published
against, and their own workshop manual layout. Left as generic inlet and exhaust figures, the
table stays exactly what it looks like on the page: a spec you'd actually check a feeler gauge
against, not a catalogue entry — the same distinction that keeps
[a torque sequence](/stories/mechanic-garage-shirts) usable while the manual it came from
stays somebody else's.

## Does it print well?

Yes — a two-by-two grid is about the simplest table there is.

Cream for the figures, orange for the instruction line, on slate or charcoal. The whole point of
the design is the contrast between the calm grid and the one line underneath that overrides half
of it, so keep that line legibly separate rather than folding it back into the table.

## How do I get one made?

Describe the engine at [JustOG](/) — the clearances, inlet and exhaust, and whether to show the
hot column at all. Pick a direction, drag the crop frame, see it composited on the real garment,
and it's made to order and shipped.

Designs other people have published are in the [shop](/shop).

Four numbers, and one line telling you which two of them to believe.