Re: Top end tappets knocking on start up? Help...
Based on some past experience with these things I have found that my problems were, low oil pressure, leak back of oil to the sump and contamination in the lifter or wear that required the lifters to be taken apart, cleaned, rebuilt or replaced.
Leak back occurs when the one way check valve does not seal properly, this allows the oil to flow out of the oil ways and back through the oil pump, these oil ways then have to be filled with oil to get the lifters back up to the correct height. I would think that if the check valve is stuck and does not open fully the volume pumped is diminished to a greater or lesser extent.
The longer the car is left idle I would imagine the leak back would be greater.
The older cars I saw had the check valve behind the external screw on filter, this one is close to the pump bodyI think.