PORTLAND, Ore. (AP) — Defense contractor Precision Castparts Corp. said on Monday that it has agreed to buy Synchronous Aerospace Group, which makes mechanical assemblies for commercial and military planes.Precision Castparts said it was paying an undisclosed amount in the cash deal. It said the deal should close by the end of December and would add immediately to earnings.Synchronous makes flight controls, wing ribs, bulkheads, and other airplane parts. It employs about 700 people in Santa Ana, Calif., where it has its headquarters, and in Kent, Wash., Wichita, Kan., and Tulsa, Okla.