These blocks serve two purposes for the racer, they space a competition block mounted idler bracket out from the engine block 1.00" and they allow the racer an -8 ORB port to plumb water thru.
Blocks are O-ringed on the engine block side to provide a superior leak free seal. Water blocks use the standard .375" two bolt mounting pattern and fit most all factory BBC and aftermarket KB, Donovan, World, Dart, CN, AJ engine blocks.
Blocks are sold separate so if ordering remember you need to order two for a complete set.