A "safer" way of preventing valve drop is to get a cheap inspection camera and put the piston of the cylinder you're working on at tdc. Or, another old hot rodder trick is to fill the cylinder with string down the spark plug hole.

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2012 xDrive35d
2001 BMW X5 3.0i 5MT
and two 1987 Corvettes - Callaway Twin Turbo and Guldstrand Grand Sport 80
Block Tester