a detachable timing gear cassette unit for an internal combustion engine . the cassette has side walls separate from the cylinder block of the engine , and the side walls form a housing . the housing contains the camshaft sprocket , the crankshaft sprocket , a timing chain that connects the sprockets , a tensioner to maintain a suitable tension in the timing chain and positioning ribs to position the sprockets within the housing . engine oil is circulated through the housing , in a generally downwardly direction , to lubricate the sprockets and the chain and to cool the oil . the cooling of the oil is assisted by internal ribs on the inside of the housing and by external ribs on the outside of the housing .