1/2 inch head dowels
Anyone fitting half inch head studs to an RB (should be all rb25 and 30 blocks) They suit OEM replacement also.
We have recently discovered that many people don’t use dowels when converting to 1/2 inch head studs, they are absolutely crucial to stop the head from walking around and hurting your gasket seal!
The reason you can’t run an RB26 type M12 / 12 mm Stud in an RB20, 25, or 30 is because they are originally 11mm thread, And if you drill it out and tap half-inch UNC you need around .7 mm per side of the material to tap through to form a reliable thread.
End result with 12mm thread is that you end up with roughly .4mm at best, threads do pull being grey cast, then you have to pull your whole engine down again and fix the problem but once you have gone and pulled the stud thread through, you have to go to 9/16 which is too big or helicoil/ threadsert.
We see it happen all the time so please don’t risk it and just go a 1/2 inch stud, use these dowels problem solved.