TM  10-3930-638-24&P 4-3.   FUEL   SYSTEM   MAINTENANCE   (cont) b. Fuel Injection Pump (cont). INSPECTION 17 18 Housing a.  Housing (133) b. Drive shaft (5) Seal seats, bores and threads Measure distance across flats of drive tang (see below); distance must not be less than  0.305  inch.  Shaft  dia- meter in drive shaft seal area must be free of nicks and scratches.  Moderate  shaft wear from the seal lips is normal Central and metering valve Hydraulic a.  Hydraulic head (81) head  (81) b. Vent wire (132) and retainer (131) bores  and  ports Check vent wire for freedom of movement. If wire is free, flush head and blow out all passages with clean, dry air (see below). If wire is stuck in screw remove and replace after a thorough cleaning of the unit and hydraulic head passage 4-106