The passage between the oil gallery in a GMC six cylinder head and the first rocker arm shaft bracket has a small copper washer used as a seal. No tube there.
And the funny part is the washer is shown in the 1955.2-1960 GMC Master Parts Book drawing, but has no reference number. (See ^ on page below.) It isn't shown at all in the 1939-53 GMC Master Parts Book.
My 302 has a "pinched down" oiler tube for the crank to cam gears.
'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.
