# A card that tells you to check what you just bought

> A spark plug gap card lists three settings against three engines, and tells the fitter to check the number rather than trust it.

August 19, 2026 · https://justog.club/stories/spark-plug-gap-card

A spark plug drawn in section, the electrode gap picked out by a single arrow marked GAP. Beside
it, a table: three engine codes, three settings — E1 at 0.7mm, E2 at 0.8mm, E3 at 0.9mm. Beneath
that, four words: CHECK BEFORE FITTING.

The plug in the box is already set to something. The card exists because "already set" and "set
correctly" aren't the same claim.

## What is this, exactly?

A reference card for setting the electrode gap on a spark plug before it goes into the engine.

Three rows, three engines, three settings — each one a tenth of a millimetre from its neighbour.
It's the kind of card that lives in a toolbox, gets consulted for a few seconds, and gets put
straight back.

## Why does a gap this small matter so much?

Because the spark has to jump across it, and the size of that jump changes how the whole cylinder
fires.

Too narrow and the spark is weak — a poor light-off, especially cold. Too wide and it can misfire
under load, or fail to jump at all under high compression. A tenth of a millimetre either way is
the difference between an engine that starts cleanly and one that doesn't, which is a lot of
consequence to load onto a gap you could close with a fingernail.

## Why does the card say to check rather than assume?

Because a new plug arrives pre-gapped from the factory, to a setting that's usually — not
inevitably — correct for the engine it's boxed for.

Boxes get mismatched. A plug meant for E1 ends up on the shelf next to E3. The gauge check takes
seconds and catches the error before it's inside the engine, which is the only place it's expensive
to find. A card that tells you to verify what you just paid for is a small piece of institutional
distrust, printed and laminated.

### Which engine code should it carry?

Whichever one the reader actually works on. A specialist who only ever touches one engine wants
their own code and their own figure, set large, rather than the full three-row table — the
specificity is the point, and a generic table is a texture.

## Can real engine codes and figures go on it?

The gap itself, and the fact that it has to be checked, are mechanical truths that belong to
nobody — the same way
[a torque figure is a fact about a fastener rather than a brand](/stories/mechanic-garage-shirts).

A manufacturer's own model name or part number is different: a mark somebody owns, licensable if a
real one is the point. E1, E2 and E3 are invented codes doing exactly the job a real one would,
without asking anybody's permission first.

## Does this work for anything other than a spark plug?

Any part that ships pre-set to a figure somebody else chose has the same card waiting in it —
tyre pressures, [valve clearances](/stories/valve-clearance-table), chain tension, the
torque on a wheel nut.

[The tyre plate makes the same argument at the door of the car](/stories/tyre-pressure-door-plate):
a number that's wrong the moment conditions change, printed somewhere it can't be missed.

## Isn't checking a brand-new part a bit paranoid?

Not to anyone who has fitted the wrong one. A plug that's gapped wrong will still thread in, still
look correct, and still run — badly, intermittently, in a way that takes longer to diagnose than
the check would have taken to perform.

The paranoia is cheaper than the diagnosis. That trade is the entire reason the card exists.

## Does it print well?

Yes — a section drawing and a numerals table are close to ideal artwork.

The gap arrow is the one place with fine detail, and it wants to stay bold rather than fine —
[detail below the weave's floor fills in](/stories/bold-shapes-survive-fabric), so the
electrode and the arrowhead both want drawing coarser than the real diagram. Black with white and
orange keeps the table legible and the arrow the obvious accent.

## How do I get one made?

Describe the card at [JustOG](/) — the engine codes, the settings, and how many rows to keep. 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 words at the bottom, and they're an instruction to distrust the box it came in.