Valve Cutting

Another important aspect of headwork is the valve job. Believe it or not, more than 50 percent of the flow gains generated by good headwork can be found in the valve job.

dirt bike valve jobIn a cost-conscious, mass-production environment, there is simply no time or money to spend on small details like multi-angle, precision valve jobs. If more flow is deemed necessary, it is usually cheaper for engineers to spec a slightly larger valve. Wide seating surfaces are more forgiving to mass production mismatches, giving good sealing for the life of the engine even with a significant misalignment of the seating surfaces. As valves and seats wear, they tend to sink lower into the seats creating seating surface mismatches and valve shrouding (partial blockage or restriction caused by objects in close proximity of the valve seat; imagine having the valve having to lift itself out of a deep crater before it can start to flow). Wide seating surfaces tend to last a little longer when the valves sink under wear.

High-performance valve jobs use to be three-angled cuts, but with newer technology has allowed Williams performance to push the valve jobs where very few have been able to go in the past. We can do up to a 99 angle valve job with are newen contour bb cnc valve seat machine.

newen countour bbDue to wide production tolerances on a mass-production engine, the seat cut ends up being much wider than actually needed. For a high-performance valve job on a four-valve motor, the seat cut is usually made to be 0.040 inches or so on the intake side and 0.050 inches on the exhaust valve. The exhaust is made slightly wider to have more contact area so the exhaust valve can be cooled better by conducting heat into the head. With the valve seat width at a minimum and the seats matched to the valve, the incoming air or outgoing exhaust has less restriction due to the unshrouding effect the adjacent cuts produce. Simply put, as the valve moves away from the seat, more area for flow is exposed sooner. The third and final cut is called the top cut. The top cut is generally about 20 to 30 degrees and is made immediately after the seat. The top cut also helps reduce valve shrouding of the airflow past the valve (or before as in the case of the exhaust valve) as the valve starts to lift off of the seat.

Sometimes depending on the application we feel the very best valve jobs are radius valve jobs. A radius valve job has no sharp edges into the port wall and combustion chamber for a totally smooth transition from seat to port to combustion chamber.

Valve Seat CuttingSometimes larger valves can be installed in an engine to increase flow. However, the addition of larger valves should be checked on the flowbench when doing head R&D because larger valves can sometimes show a lesser flow due to an increase in valve shrouding. Heads with valves close to the periphery of the cylinder or engines with deep combustion chambers and small bores are prone to situation. This is because a larger valve can put the edge of the valve in close proximity to the combustion chamber wall, blocking the flow of air from the port or exhaust flow out of the port.


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