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The Magnums are assembled with positive guide-mounted valve-stem seals, quite an improvement compared to the umbrella seals of earlier engines. When measuring the valve-lift clearance, the retainer physically hits the seal at .540-inch lift on the exhaust side and .525 inch on the intake. This gives clearance for cams up to about .500-inch lift or so. This is plenty for lift ranges proportionally adequate for these ports in stock form.