not all cam s open past tdc this his to do with the cam size on dur .cam timing is not dist timming .cam timing should be done with a degree wheel and a dial indicator and can be 050 numbers or what the cam grinder use . you could do it other ways but this is the best way but of coarse you need to find true TDC before starting and then you can check TDC on the damper to timming tab is zero before you bolt the head down so what you time the engine you know timming marks are true