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valve clearance shim under bucket why is valve clearance necessary? the valve must remain sealed against the valve seat during the time that the cam lobe isn't pushing it down to ensure that the seat remains in contact with the valve a small amount of clearance is required between the backside of the camshaft and the valve lifter the clearance allows for heat expansion of the parts to expand and contract without affecting the valve sealing against the valve seat valve seat sealing is critical to engine compression and during combustion why must valve clearance be adjustable? the valve seat and valve face wear from the constant hammering action that occurs during operation this causes the valve to recess slightly into the head also the hammering action of the cam lobe on the lifter also can shorten the valve stem slightly shims or adjusters must be used so the valve clearance can be periodically adjusted back within specification clearances are on the order of thousandths fo an inch so they are obviously quite small but nonetheless necessary cam lobe cam shaft valve clearance valve lifter bucket shim interchangeable valve keepers valve springs valve stem seal valve guide port valve stem valve seat shim under bucket valvetrains are used widely on many motorcycles and almost exclusively on newer sportbikes