New ARP set of header bolts for Small Block Chevrolet engines using 3/8"-16 threaded holes.  Size is 3/8"-16 x .875" UHL.  These are 12-pt head 8740 chromoly (black oxide) steel rated to 180,000 PSI.  Set of 12 bolts/washers.  Fits 67-92 Small Block Chevrolet and 92-97 LT1 engines.  Rule of thumb is you want 3/8" minimum thread engagement into the cylinder head.  Bolts that are too long will bottom out into the head and leak.  So you would measure your header flange and gasket and add 3/8" to determine the minimum length bolt you need.