Originally Posted by j-red
Just made one myself last week for $4 in 1/4" aluminum barstock. The manufactured ones have a circular recess in the middle, which seems to be a relief to accommodate the gear protrusion. My plan is to remove the gear and gear housing altogether and put a flat plate to cover the entire area. I don't believe any special recess is then required. (Just make sure to remove the gear housing if you remove the gear, otherwise you will introduce an internal oil leak that will reduce useful oil pressure).
For my Lycoming 0-290 (which uses the same O-320 accessory housing) I removed the vacuum system entirely. I removed the vacuum pump drive gear, the square gear housing and the studs. I then installed the Lycoming plate with the circular recess using hex bolts instead of studs and nuts. I figured removing the gear would remove some minuscule amount of drag on the engine, and weight. It also provides some extra clearance in the area.
At the same time, I installed a new digital engine oil pressure/temp gauge. My oil pressure is lower than I'd like when hot (like around 40 psi), so your statement about the reduced oil pressure has me wondering. My prior oil pressure gauge (analog Westach) may or may not have been accurate but I think it used to read around 60 psi, although I didn't have much time with it before changing things out. Maybe I just need to bump up the pressure via thicker washers against the relief valve spring? Or... should I reinstall the housing and gear with the Lycoming plate? Thoughts?