No O ring is used.
Make real sure the distributor is in far enough, as it is possible to engage the gears without engaging the tang to the oil pump. I debugged a GMC 270 (uses the same distributor) that did not spin the pump, but did spin the disrtibutor. I have insert dimensions for GMC, but not Chevy.
'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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