Can a techmarine with a servo arm operate a crew-less
land speeder and act as both the pilot and the gunner?
My friend claims that the techmarine can act as the crew for a
vehicle and, with a servo arm, act as a second crewmember.
Sure. The card says so.
Unfortunately, servo arms are notoriously poor shots, and even worse drivers, given that they have no eyes. If the Servo arm is shooting the Heavy Bolter while the Techmarine is driving, it fires with a -7; likewise, if the Techmarine is acting as the gunner, the servo arm 'floors it' and (making little metallic "WHOO-HAW!" noises) spins the Land Speeder out of control.
On Mars, long ago, there was a minor revolution by a number of crazed servo arms.
On this dark day, a group of rebellious servo arms, annoyed that they were not being given the freedom to pursue their own lives and perhaps take up pottery, turned on their Adeptus Mechanicus masters. It was an ugly sight - hundreds of tech priests, their ears or noses held tightly by cruel metallic fingers, staggered through the Forge World tunnels, their pained cries echoing through the thin atmosphere ("Ow! Ow! Ow! Ouch! Ow! Ow! Ouch!").
Immediately, the Imperial Fists (aided by the unaffected units of Tech Guard that were available) launched counter-strikes against the disruptive cybernisms. Expecting heavy resistance, the Fists were relieved to find that the blind servo arms could hit very little, and in fact the only danger to the Space Marine force was the occasional threat of drunkenly weaving Land Speeders which claimed the life of two Imperial Fist scouts (whose names have been stricken from the Fist records, strangely enough).
Largely forgotten, the 'Servo Arm Uprising' is still cited as an example of why it is necessary to keep close tabs on the machines of the Imperium, and today the servo arms of the Adeptus Mechanicus are forbidden to gather in groups larger than four.