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.
This page covers the Bristol 100D2 engine, chassis prefix BE. If your car has AC's own Light Six, prefix AE, see the AC engine page instead.
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
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.