 It passes through the wall of the combustion chamber and therefore must also seal the combustion chamber against high pressures and temperatures without deteriorating over long periods of time and extended use.Spark plugs are specified by size, either thread or nut (often referred to as "Euro"), sealing type (taper or crush washer), and spark gap.Common thread (nut) sizes in Europe are 10 mm (16 mm), 14 mm (21 mm; sometimes, 16 mm), and 18 mm (24 mm, sometimes, 21 mm).