Rear Axle Blocks. Bullet Proof Designs Axle Blocks are an easy way to bling up the area around the rear axle while providing a more accurate way to adjust the chain on your bike. They hold the wheel precisely in place while you tighten the chain, and they stay in position when the axle is removed! The unique design means only one wrench is needed to make your chain adjustments. One block is threaded per O.E.M. specs to fit your stock axle. Each anodized 6061 aluminum block has bright laser etched alignment marks. Anodized billet 6061 blocks. Stainless steel threaded adjusters do not oxidize or corrode. More accurate readings for adjustment. Super lightweight.