My 1990 740 had the harmonic balancer replaced on the front of the engine about ten years ago. I installed a Scantech aftermarket unit instead of a Volve OEM part thinking, "what could go wrong?" Well, plenty. While replacing the timing belt last week, we found that the metal core of the balancer had disintegrated. The rubber sandwich and outer pulley was intact but the metal surrounding the crankshaft had apparently fatigued and fallen apart, damaging the crankshaft in the process. We installed a new Volvo OEM balancer and shimmed it with thin steel strips before tightening it down, but the damage was such that this fix has some potential of failing. We'll see what happens.

In my experience: the Volvo OEM part is almost always the best solution even if it costs more. Aftermarket parts are built to cheaper standards and can fail much sooner. In this instance, I think that Scantech's supplier (Chinese?) used cheap steel subject to fatigue. And fatigue it did.