Like Jerry said, put a T into the block and hook up both your fancy gauge and a mechanical gauge at the same time. Makes for a much better comparison. I used a 4" diameter, oil damped, 270 degree sweep, +-1psi mechanical gauge when I checked mine. I'll loan it to you if you'll pay the freight.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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