The cylinder heads are machined from aluminum billets. All receivers were machined from castings of AISI 8620 alloy steel. Many castings are machined in CNC milling centers. The block is machined from a piece of forged aluminum. Crankshafts can also be machined out of a billet of steel. Monel 502 can be machined similar to austenitic stainless steels. Both components are constructed from machined magnesium. The boxes were machined from aluminium stock and filled with Kapok sacks for added buoyancy. In most cases, the spindle is machined to accept Morse taper tooling for greater flexibility. The higher the rating, the more precise tolerances the bearing has been machined to. Unlike conventional optical grinding, complex aspheric surfaces can be machined easily. The ends may be machined for butt welding, threaded (usually female), or socketed. The F136 engines had crankshafts that were machined incorrectly. After grinding, any surfaces that require tight dimensional control are machined. They have historically been machined to gross shape, case hardened and then ground.