Norton ES2, Model 50 iron head engine barrel through bolts, head nuts to suit Norton ES2 and Model 18 1937 - 54, set of 4.Original part number:3680; A3/2 - crankcase cylinder stud x 4 [1936-54]8936;A3/156 - cylinder head holding down stud (sleeve) nut x 4 [1937- 54]