Thread: Taper VS Gasketed Plugs ???

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1981 Chevy C-10 pickup
427ci tall deck motor
heads are date stamped 1973

Ok after a little bit of research this morning, I managed to find some literature that says..."All cast-iron heads made prior to 1970 use 13/16 inch hex plugs with gasket, 3/4 inch reach." Then, obviously, says that all heads built afterwards use " small 5/8 inch tapered plugs with a 1/2 inch reach. Since being purchased, I have been using gasketed 13/16 plugs, my thoughts are that maybe im using the wrong plugs... Any final thoughts on this???

Originally Posted by zero_presence

1981 Chevy C-10 pickup
427ci tall deck motor
heads are date stamped 1973

Ok after a little bit of research this morning, I managed to find some literature that says..."All cast-iron heads made prior to 1970 use 13/16 inch hex plugs with gasket, 3/4 inch reach." Then, obviously, says that all heads built afterwards use " small 5/8 inch tapered plugs with a 1/2 inch reach. Since being purchased, I have been using gasketed 13/16 plugs, my thoughts are that maybe im using the wrong plugs... Any final thoughts on this???

A 13/16" plug has 18mm threads, and a 5/8" plug has 14mm threads, right?

Not neccissarily. I work at a parts house and i was comparing them earlier vand there's several that are the same threads wether they were 13/16 or 5/8

For example an A/C delco R43T is a small, 5/8 plug...And an A/C delco R43S is a 13/16 large plug but they have the same threads...

You should be able to pull a plug and look to see if its flat or tapered on the head.

Lol that's easier said than done as cold as it is outside, i can bearly feel my finger tips... I was really wondering if there is truth to the information in the literature i found... If ALL cast iron heads are taperd after 1970..

Dude....Your in sunny Alabama, weather channel says 50 degrees today. Ya wanna see cold come on up here to St.louis or South Dakota where Dave Severson lives.......