set of (16) RAS3 studs and set of (16) poly locks from Spotts Performance see my other auction for studs only US MADE SCREW IN Studs and US MADE Poly Locks KIT These replace Press In Studs on 389 and 421 heads. Very Popular for 64-66 GTO's and Big car 421's High quality studs are 1/2" course thread at base and stronger then factory 7/16 threads at top. Heads will need machine shop to pull studs and tap to 1/2" thread Do not hand tap your heads !!! Your studs may not be perfectly straight when installed. In the past, I've had to tap and heli coil for a 7/16 base thread, machine stud boss and then cut down the base of the screw in stud. to allow rocker arms to fit under stock valve covers. This kit is a time and money saver!!! Studs do not have oil thru holes on certain pre 64 heads, simply use rocker arms with oil hole (see may other auctions) On oil thru stud applications, I also plug the block with 2- 1/8 npt pipe plugs at deck to keep oil away from heads. This set up will allow factory valve covers to fit! This kit is for all stamped steel rocker arms (factory with oil thru hole) and also roller tip rocker arms. Some aluminum and stainless steel rocker arms will work as well. Not all aluminum rocker arms will allow the use of these poly locks. Fast and free shipping!