Hitting the valve is a matter of lift and duration in combination. If you have a cam with a .490 lift and it's really advanced it could still hit the piston than a cam with .560 lift and installed "straight up". I have a 302 with a .570 lift solid lifter cam with 248 degrees at .050 and it never even came close to the piston. However my friends big block had a milder cam and it hit the piston because his block was decked and his heads were milled. There are many variables that affect the ultimate valve lift. The best way is actually check the clearance with modeling clay. If you don't check it and the valve hits the piston you're wallets going to get a lot lighter. From what I've seen, you can usually get away with around .500-.512 and around 220-230 degrees at .050 without any problems. These are still relatively mild.
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