The "Clearance volume" of the engine is the volume between the cylinder head and the piston when the piston is at top dead center.

The compression ratio is the ratio of total volume of cylinder (displacement volume + clearance volume) to the clearance volume.

If clearance volume of I.C engines is increased, the compression ratio will decrease.

The compression ratio in petrol engine is kept less than in Diesel engine because higher or equivalent compression ratio in petrol engines is not possible due to pre-ignition.
Pre-ignition happens when the fuel-air mixture in a spark ignition engine ignites before it is supposed to because of contact with a hot surface. Over-heated spark plugs/ exhaust valves are the main causes of pre-ignition.

The purpose of a cylinder head gasket is to prevent the combustion gases from leaking from the joint between the cylinder block and the cylinder head.

An engine block or cylinder block is the structure which contains the cylinders, and other parts, of an internal combustion engine.