[Fot] Assembling new engine with old crank pulley - timing mark 12-15 degress off - why?

[Scott Janzen]

I am assembling a new GT6 engine, re-using the vibration damper/crank pulley
that was fitted to the previous engine - TDC marked on my flywheel, triple
checked with dial indicator, etc.  Fitted the old damper/pulley and the TDC
mark is off by about 12-15 degrees.  did the keyways on the cranks vary that
much in location from Triumph or am I missing something?  Since it was in the
correct location on the old engine I doubt the exterior casing of the damper
has slipped