In May of 2012, one of the mounting holes in the rear main cap broke. I had a machine shop level it out and add a stud, used a gasket and a standard oil pump and drove it down the road until May of 2020. When I took it to the machine shop i snatched that pump off and noticed that gasket never made good contact all the way around that stud. So rather than a high spot like yellow rose mentioned, I think there's a low spot. I recon if one were to tighten that side first the opposite side might not seat properly. Additionally I think the dimensions of the high volume pump provides more of an angle if not exactly flat. My theory anyway.