Spark Plug Boxes.

Because they are fragile in two places, and both failures are invisible until the engine runs.

The ceramic insulator — the white body — is brittle. It chips and cracks on impact, and a hairline crack cannot be seen. A cracked insulator causes misfires once fitted.

The electrode gap is a precise pre-set measurement across a thin metal electrode. A knock in transit opens or closes it. The plug looks perfect and runs badly.

Loose in a box, plugs knock into each other — ceramic against metal electrode is the worst possible contact — and into the box walls.

What makes this expensive is when the failure is found. Not on your shelf, but by a fitter after installation. The labour to fit and remove a plug is worth many times the part, so a damaged plug costs far more than its replacement.

Individual cells hold each plug isolated and immobile. For premium and iridium plugs we would recommend moulded pulp cradles, which absorb shock better than die-cut board.