There should not be a washer on top of the oil pump. I have seen some pumps that look like they have a washer built into the top of the pump. Maybe you have one of these and the washer has come loose.
Chevrolet sold as a replacement, for the earlier steel cam distributors, the distributor designed for cast came. I have not seen any abnormal wear by using this combo. I do not recommend going the other way around.


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