Abstract: A gear box 1 has an input shaft 2, an output shaft 4 and a counter-shaft 6. A headcut gear 8 mounted on the input shaft 2 intermeshes with a first counter-shaft 10 mounted on the counter-shaft 6. A second counter-shaft gear 12, intermeshes with a corresponding gear 16, mounted on the output shaft 4. The first two counter-shaft gears 10, 12 are interlocked by virtue of inter-engaging means comprising, for instance, a protrusion 24 from one of the two gears and a protrusion receiving recess 30 in the other of the two gears 10,12. Interlocking the two gears 10,12 prevents one rotating relative to the other in the event, say, of a torque spike.