An actuator for a fluid catalytic cracking unit slide valve, comprising: a hand-operated wheel operatively connected to a threaded drive shaft such that turning the hand-operated wheel causes the threaded drive shaft to rotate; a block having an engagement system encompassing the threaded drive shaft, the block comprising: a manual engagement lever; a first engagement shaft extending from the manual engagement lever on a first side of the threaded drive shaft, the first engagement shaft being operatively engaged with a first cam; a second engagement shaft extending substantially parallel to the first engagement shaft on a second side of the threaded drive shaft, the second engagement shaft being operatively engaged with a second cam; a split nut comprising: a first half disposed in the block on the first side of the threaded drive shaft; and a second half disposed in the block on the second side of the threaded drive shaft; a first cam follower extending from the first cam to the first half of the split nut; and a second cam follower extending from the second cam to the second half of the split nut; characterised in that: the bock is a traveling block which also comprises an engagement shaft linkage extending between the first engagement shaft and the second engagement shaft and being operative to translate a rotation of the first engagement shaft into a coordinated rotation of the second engagement shaft; whereby operating the manual engagement lever causes the first engagement shaft and the second engagement shaft to rotate, operating the first cam and the second cam to force the first half of the split nut and the second half of the split nut together around the threaded drive shaft or to release the first half of the split nut and the second half of the split nut from the threaded drive shaft. a hand-operated wheel operatively connected to a threaded drive shaft such that turning the hand-operated wheel causes the threaded drive shaft to rotate; a block having an engagement system encompassing the threaded drive shaft, the block comprising: a manual engagement lever; a first engagement shaft extending from the manual engagement lever on a first side of the threaded drive shaft, the first engagement shaft being operatively engaged with a first cam; a second engagement shaft extending substantially parallel to the first engagement shaft on a second side of the threaded drive shaft, the second engagement shaft being operatively engaged with a second cam; a split nut comprising: a first half disposed in the block on the first side of the threaded drive shaft; and a second half disposed in the block on the second side of the threaded drive shaft; a first cam follower extending from the first cam to the first half of the split nut; and a second cam follower extending from the second cam to the second half of the split nut; characterised in that: the bock is a traveling block which also comprises an engagement shaft linkage extending between the first engagement shaft and the second engagement shaft and being operative to translate a rotation of the first engagement shaft into a coordinated rotation of the second engagement shaft; a manual engagement lever; a first engagement shaft extending from the manual engagement lever on a first side of the threaded drive shaft, the first engagement shaft being operatively engaged with a first cam; a second engagement shaft extending substantially parallel to the first engagement shaft on a second side of the threaded drive shaft, the second engagement shaft being operatively engaged with a second cam; a split nut comprising: a first half disposed in the block on the first side of the threaded drive shaft; and a second half disposed in the block on the second side of the threaded drive shaft; a first half disposed in the block on the first side of the threaded drive shaft; and a second half disposed in the block on the second side of the threaded drive shaft; a first cam follower extending from the first cam to the first half of the split nut; and a second cam follower extending from the second cam to the second half of the split nut; characterised in that: the bock is a traveling block which also comprises an engagement shaft linkage extending between the first engagement shaft and the second engagement shaft and being operative to translate a rotation of the first engagement shaft into a coordinated rotation of the second engagement shaft; whereby operating the manual engagement lever causes the first engagement shaft and the second engagement shaft to rotate, operating the first cam and the second cam to force the first half of the split nut and the second half of the split nut together around the threaded drive shaft or to release the first half of the split nut and the second half of the split nut from the threaded drive shaft.