20 The metal retainer, often called a spider, is positioned over the lifters retainers to prevent the lifters and retainers from coming out of the block too far at high rpm. The spring steel of the spider puts pressure on the H-bar retainer to do this. The spider is retained to the block by two Allen head bolts, which must have holes drilled and tapped to accept them. John made the two necessary holes during the machining operation, so we were good to go. Don't make the mistake of having a completely built short-block and having to take it all apart to drill these holes.