i would guess its 5/8 - 24. I just took the brake off my long range hunter in 6.5x284 and it had 9/16-28. but bigger bore size should have a bigger thread size in theory. but i was a little surprised to see 9/16-28 on mine. not a very common thread in the gun world. the brake came off pretty easy just find a couple screwdrivers to put through the holes ( i suggest 2 because for the hole size and the tightness of the brake you will probably break one off, it least i did.) and lefty loosy and its off! hope this helps a little.