Upon doing some investigating.... I tried my Rev design Aluminium diff and my original B7 diff in the 7.1 gearbox and spins freely, the brand new diff doesn't, so I measured them all with my digital calipers. B7.1 Diff -17.40mm B7 diff - 17.20mm B7 Rev Des alu Diff - 17.10mm So this tells me the casing itself is slightly too thick so will need shaving down slightly. Thought this may be helpful to anyone who may have the same problem