Milodon main caps are CNC machined from high strength ductile material, which has the proper balance of strength yet is not too rigid that allows the transfer of increased shock loads to the crank and bearings. In this case, more is not better when it comes to hardness.Only .030" of material removal is required upon initial installation, which will allow 100% cleanup on a damaged or crooked block. The drill bushing used to drill and tap the side bolt holes is included in the kit. The side bolts can be installed by anyone with a hand drill and a tap. If the block has square main registers, the block needs only a standard align bore to finish the job. Milling the mains is only necessary if the main registers are not square.The side bolts are angled so they pull from the side walls of the block and not the flimsy main web area. A bolt that is angled will require a massive amount of additional load to flex or stretch, let alone break. The side bolts heavily load the cap against the register step, which stops "cap walk". It is cap walk that causes a great amount of unforeseen bearing damage and horsepower loss.All necessary bolts, washers and the drill bushing are included in the center three cap kit. For maximum strength the installation of Milodon main studs are highly recommended.While mild steel main caps may look like main caps and are economical, they are the wrong material for a performance application. True "deductile" iron caps take a beating and return to their original shape (deductile) saving the bearings and continuing to perform well. Very rigid mild steel caps act differently and do not absorb the impact from the crank and usually pound out the bearings if run hard. Its like if you had to jump out of a second story window.Deductile iron caps also cut better during align bore. They heat and expand at the same rate the block material does so the crank bore stays round instead of going oval.