To answer your question about why on intakes only? Because the intake sucks and the exhaust blows meaning the intake can suck oil down the guides and the exhaust blows oil back up the exhaust guide. So, many people put a umbrella seal on the intakes only to help with oil seal. If your guides are nice and not too loose then the o-ring will work just fine like Dave said. And no they didnt put on umbrella seal from factory.

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1936 1.5 wrecker,1937 gmc coe,1939 Chevrolet coe,1942 4x4 coe, 1942 coe,1946 dump,1947 2 ton dump,another 1947 2 ton dump,1950 coe,1967 c30,1937 cat 22,1936 Chevrolet 5 window foremans coupe, 1914 ford speedster.