It may also be due to weak valve springs, insufficient tappet clearance, warpage and misalignment. Warpage occurs chiefly in the upper valve stem due to its exposure to intense heat. Oil and air are sucked past worn intake valve stems and guides into the combustion chamber, causing excessive oil consumption, forming carbon and diluting carburized fuel. Misalignment - is a product of wear, warpage, and distortion. Wear perhaps hastened by insufficient or improper lubrication will eventually create sloppy clearances and misalignment.
Distortion is caused by unequal tightening of cylinder head bolts.
Warpage of the entire engine block is not uncommon, usually due to excessive heat caused by a clogged cooling system. More frequently the valve guide warps because of the variation in temperatures over its length. The lower part of the guide is near combustion heat while the upper is cooled by water jackets. Any wear, warpage or distortion affecting the valve guides, destroys its function as an accurate bearing to keep the valve head concentric with its seat and prevents leakproof sealing. Valve Spring - Valve springs must be of a uniform length.