When I set the timing on my 2.5 L Subaru engine I can never get the timing marks on the belt to line up perfectly with the plastic cover marks.
On my chain drive Porsche engines, I can adjust the the cams to get them dead on by using the adjustment on the cam sprocket and a dial indicator on a valve. When the degree on the crank is at the set point and I have 1mm valve lift, I lock in the cam sprocket.
Using the plastic housing as a reference point doesn't appear the most exact way to make the adjustment.
I have seen the eccentric adjusters that can adjust the slack in the belt to make the timing adjustment closer but, if the eccentric should slip, there goes the engine. With the Porsche system, there is a series of off centered holes for a pin that when lined up will keep the adjustment from total failure if the locking nut should slip.
Is there a better way to precisely adjust the cams in the Subaru engine? Are all timing belts the same length? How do you compensate for a decked block?