Right correct me if Im wrong here,
The crank has a plain shank at the nose end, ie pulley end, there is a jeyway, the keyway locates the timing/oil pump chain pulley/cog, the keyway only locates the cog it does not actually hold it as its not strong enough, then you fit the crank pulley and tighten the crank bolt, this effectively crushes the pulley and the cog tight against the step in the crank so the whole lot is locked together.
At one time crank pulley bolts coming loose was a 200tdi thing and the keyway used to rattle about so much it damaged the crank itself, casuing variable valve/injector timing.
They work exactly the same except they use a belt instead of a chain but the principle is the same.
If you think like a suspicious mechanic a loose crank bolt on its own will not cause an injector noise UNLESS it also had some means of effecting the valve/injector timing.
