Spark Plugs on an AC Aceca-Bristol — the 10mm Question
A genuine quirk of the Bristol engine: it uses small 10mm spark plugs, and the original plugs are long out of production. The accepted solution among owners is a suitable motorcycle plug, which fits and works at least as well as the original.
Specs — confirm against your car
- Plugs: the Bristol six uses 10mm plugs (six of them). The original plugs are no longer available; owners now fit a suitable 10mm motorcycle plug. The Bristol Owners Club or your engine specialist can advise the current recommended plug and heat range for your car's state of tune — confirm before buying.
- Gap: set to the gap specified in your handbook or by your plug supplier — motorcycle-plug gaps vary by type, so don't assume a figure.
- Six plugs; this is a triple-Solex engine, so plug condition is also a useful read on carburettor balance.
Procedure
Work on a COOL engine.
Pull each plug lead by the connector (not the wire), one cylinder at a time so the leads return in the correct firing order (refer to the manual for the six-cylinder firing order).
Brush or blow debris from each plug well so nothing drops in. Undo each plug with a thin 10mm plug socket.
Read the old plugs: light tan/grey = healthy; black/sooty = rich or weak spark; oily = oil past guides/rings; white/blistered = too hot or weak. Because there are three carburettors, compare plugs in pairs to spot a carb out of balance.
Set the gap on the new plugs as specified by your handbook or plug supplier.
A smear of anti-seize on the threads, start each plug BY HAND to avoid cross-threading, then nip up gently. Refit the lead.
Start up and check it runs evenly on all six.
This guide is not a substitute for the official Bristol/AC Workshop Manual.
About the author
An independent classic-car enthusiast in North Yorkshire with decades of hands-on experience on British classics — including the AC Aceca — writing the single-model reference he always wished existed. Read the full bio.