Abstract

A variable camshaft timing phaser for an internal combustion engine that varies rotation phase including a housing, a rotor, a locking pin, and a piston. The locking pin is moveable in a first direction, which urges a tapered end of the locking pin to engage a tapered recess in the rotor, and a second direction, opposite the first direction, to disengage the rotor. The is driven by an engine output in synchronism with the engine revolutions, the rotor is connected to the camshaft and has a fluid passage connecting a source of engine oil to the tapered recess in the rotor. The piston is located within the fluid passage and has a piston surface contacting the locking pin. The piston has a cross-section that is less than the cross-section of the locking pin, and blocks the fluid passage when the locking pin is locked, moving to a position which allows fluid to pass by the piston in the recess and press on the locking pin, so that higher pressure is required to unlock the locking pin than to hold it in its unlocked position.