Finished:

I’ve smoothed some edges, as proposed by @spikeyxxx and collapsed the modifier stacks (with subsurf level 1) on the gearbox and its direct side parts (the latter two beeing one object in my file). Both objects now only have each one “Subsurf Modifier” level 1 and one “Shrinkwrap Modifier” for smoothing out slight grooves in the tube parts. File is here.