B747 CREW OVERSHOT ALT DURING ENRTE CLB.

Narrative

ON A TRANS-PACIFIC FLT FROM OSAKA; JAPAN TO LOS ANGELES; CA. I HAD JUST MOVED FROM THE L (FLYING) SEAT TO THE R WHEN THE OTHER CAPT CAME FORWARD AFTER HIS BREAK. HE THEN BECAME THE PF WHILE I HANDLED THE RADIOS AND POS RPTING. WE STARTED TO GET MODERATE TURB AT FL360; SO I REQUESTED THE NEXT HIGHER FLT LEVEL (FL370). OUR CLRNC TO CLB CAME A FEW MINS LATER; JUST AS THE NEW CAPT WAS HANDED HIS MEAL TRAY. HE SET THE AUTOPLT FOR CLB AND THE THRUST INCREASED AS THE ACFT PITCHED UP. THE RATE OF CLB SEEMED HIGH FOR A 1000 FT ALT CHANGE; BUT THE AUTOPLT IS A GOOD ONE AND ALWAYS WORKS. AS WE CLBED; WE ENCOUNTERED A STRONG UPDRAFT AND THE ACFT SHOT THROUGH FL370; MUCH TO OUR AMAZEMENT. THE FLYING CAPT DISCONNECTED THE AUTOPLT AND SMOOTHLY LOWERED THE NOSE; NOT WANTING TO SUBJECT THE ACFT; FLT ATTENDANTS; PAX AND GALLEY EQUIP TO NEGATIVE G LOADINGS. THE END RESULT WAS AN ALT OVERSHOOT OF APPROX 700 FT. AFTER THE INCIDENT; I WISHED I HAD TAKEN CTL OF THE ACFT WHEN I NOTICED THE HIGH CLB RATE. BUT WITH 2 CAPTS OCCUPYING THE FRONT SEATS; THIS CAN BE AWKWARD. YOU REALLY HESITATE TO TAKE THE AIRPLANE FROM THE OTHER GUY. IN SITS LIKE THIS; IT SEEMS THERE ARE ALWAYS CONTRIBUTING FACTORS: THE EVER PRESENT FATIGUE ENCOUNTERED ON EBOUND TRANS-PACIFICS; DISTRS LIKE THE MEAL TRAY COMING UP; UNEXPECTED WX CONDITIONS; AND DEPENDENCE ON AUTOMATION THAT WORKS GREAT 99.99 PERCENT OF THE TIME. YOU JUST CAN'T BE TOO CAREFUL OUT THERE!