What was found
IN CRUISE AT ALTITUDE FL340, WITH RIGHT AUTOPILOT ENGAGED. FO ALTIMETER DROPPED ABOUT 330 FT. WITH EICAS ALT DISAGREE, AUTOPILOT ATTEMPTED TO CLIMB BACK TO RECAPTURE FL340 WITH CAPT ALTIMETER SUBSEQUENTLY CLIMBING TO FL342. DISENGAGED AUTOPILOT AND RETURNED TO FL340 USING CAPT ALTIMETER AND STBY ALTIMETER. SWITCHED TRANSPONDER TO LEFT SIDE AND VERIFIED WITH ATC ALTITUDE REPORTING CORRECT. ALSO SWITCHED TO CENTER AUTO PILOT. COMPLETED QRH PROCEDURE BY SWITCHING FO AIR DATA TO ALTN. AMM: 34-11-00-5 PERFORMED CAPT'S AND F/O'S ADM TEST REF B767 MM34-26-03-4, OPS CKS NORMAL WITH NO FAULT CODES DISPLAYED. PERFORMED VISUAL INSPECTION OF ALL PITOT PROBES AND STATIC PORTS, NO DEFECTS NOTED REF B767 FIM 34-26-00-2. PERFORMED LOW AND HIGH RANGE TEST WITH NO LEAKS NOTED. COMPARED CAPT'S AND F/O'S ALTIMETERS WITH TEST SET AT 340. HAD A SPLIT OF 160 FT WITH F/O'S SIDE INDICATING 33840 FT. PERFORMED CAPT'S AND F/O'S ADM TEST REF B767 MM34-26-03-4,OPS CKS NORMAL WITH NO FAULT CODES DISPLAYED. PERFORMED VISUAL INSPECTION OF ALL PITOT PROBES AND STATIC PORTS, NO DEFECTS NOTED REF B767 FIM 34-26-00-2. CLEANED FO'S PITOT STATIC PROBE AND LEFT AUX PITOT PROBE NO DEBRIS FOUND, IN REF. AMM 34-26-04-4 PERFORMED LOW AND HIGH RANGE PITOT/STATIC SYSTEM TEST. ALL CHECKS GOOD PER AMM #34-11-00-5.NO DISCREPANCIES NOTED