Combustion chamber deposit interference (CCDI) is the result of physical contact between deposits on the piston top and the bottom of the cylinder head and is manifested as a loud, metallic banging sound when the engine is cold. CCDI is limited to the engines that have been designed primarily to reduce emissions, with minimal clearance – one millimeter or less – between some areas of the piston top and the cylinder head (squish areas) when the piston is at top dead centre.