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Description
The reason behind Black Five number 44767 being named "George Stephenson" is that it features a unique motion and valve gear among all of the Black Fives that were ever produced. Whereas there were some Black Fives equipped with Caprotti gear, and mostly fitted with Walchaerts equipment, 44767 features Stephenson Outside Link Motion.

During the 1830s the most popular valve drive for locomotives was known as gab motion in the U.K. and V-hook motion in the U.S.A. The gab motion incorporated two sets of eccentrics and rods for each cylinder; one eccentric was set to give forward and the other backwards motion to the engine and one or the other could accordingly engage with a pin driving the distribution valve by means of the gabs: - vee-shaped ends to the eccentric rods supposed to catch the rocker driving the valve rod whatever its position. It was a clumsy mechanism difficult to operate that moreover only gave fixed valve events.

In 1841 two employees in Stephenson’s locomotive works, draughtsman William Howe and pattern-maker William Williams, suggested the simple expedient of replacing the gabs with a vertical slotted link, pivoted at both ends to the tips of the eccentric rods. To change direction, the link and rod ends were bodily raised or lowered by means of a counterbalanced bell crank worked by a reach rod that connected it to the reversing lever. This not only simplified reversing but it was realized that the gear could be raised or lowered in small increments, and thus the combined motion from the “forward” and “back” eccentrics in differing proportions would impart shorter travel to the valve, cutting off admission steam earlier in the stroke and using a smaller amount steam expansively in the cylinder, using its own energy rather than continuing to draw from the boiler. It became the practice to start the engine or climb gradients at long cutoff, usually about 70-80% maximum of the power stroke and to shorten the cutoff as momentum was gained in order to benefit from the economy of expansive working and the effect of increased lead and higher compression at the end of each strike. This process was popularly known as "linking up" or “notching up”, the latter because the reversing lever could be held in a precise positions by means of a catch on the lever engaging notches in a quadrant; the term stuck even after the introduction of the screw reverser.

It's considered to be quite a miracle that such a unique engine should survive the wrath of British Rail's steam clearout of the 1960s, and indeed it is an interesting feature to talk about and to watch in action.

Description
The reason behind Black Five number 44767 being named "George Stephenson" is that it features a unique motion and valve gear among all of the Black Fives that were ever produced. Whereas there were some Black Fives equipped with Caprotti gear, and mostly fitted with Walchaerts equipment, 44767 features Stephenson Outside Link Motion.

During the 1830s the most popular valve drive for locomotives was known as gab motion in the U.K. and V-hook motion in the U.S.A. The gab motion incorporated two sets of eccentrics and rods for each cylinder; one eccentric was set to give forward and the other backwards motion to the engine and one or the other could accordingly engage with a pin driving the distribution valve by means of the gabs: - vee-shaped ends to the eccentric rods supposed to catch the rocker driving the valve rod whatever its position. It was a clumsy mechanism difficult to operate that moreover only gave fixed valve events.

In 1841 two employees in Stephenson’s locomotive works, draughtsman William Howe and pattern-maker William Williams, suggested the simple expedient of replacing the gabs with a vertical slotted link, pivoted at both ends to the tips of the eccentric rods. To change direction, the link and rod ends were bodily raised or lowered by means of a counterbalanced bell crank worked by a reach rod that connected it to the reversing lever. This not only simplified reversing but it was realized that the gear could be raised or lowered in small increments, and thus the combined motion from the “forward” and “back” eccentrics in differing proportions would impart shorter travel to the valve, cutting off admission steam earlier in the stroke and using a smaller amount steam expansively in the cylinder, using its own energy rather than continuing to draw from the boiler. It became the practice to start the engine or climb gradients at long cutoff, usually about 70-80% maximum of the power stroke and to shorten the cutoff as momentum was gained in order to benefit from the economy of expansive working and the effect of increased lead and higher compression at the end of each strike. This process was popularly known as "linking up" or “notching up”, the latter because the reversing lever could be held in a precise positions by means of a catch on the lever engaging notches in a quadrant; the term stuck even after the introduction of the screw reverser.

It's considered to be quite a miracle that such a unique engine should survive the wrath of British Rail's steam clearout of the 1960s, and indeed it is an interesting feature to talk about and to watch in action.