A B777 FLT ON AN EXTENDED OVERWATER OP TO LHR HAS TO REVERSE COURSE AT 50W BECAUSE OF AN UNKNOWN CAUSE FOR A FUEL IMBALANCE XFER PROB AND DIVERT TO BOS; MA.

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DURING CRUISE; FUEL IMBALANCE EICAS MESSAGE ILLUMINATED. WE STARTED FUEL TRACKING AND FOUND R TANK QUANTITY DECREASING TWICE AS FAST AS L TANK; BY 100 LBS EVERY 2 MINS. OVER NEXT HR WE DISCUSSED THE PROB WITH LINE MAINT AND DISPATCH. WE WERE NEVER SURE WHAT WAS CAUSING PROB. ATLANTIC ABORT DECISION DELAYED TO 80000 TOTAL FUEL VALUE; TO WORK OUT POSSIBLE TIE-IN WITH BULLETIN 02-02. WITH OUR ABILITY TO BAL LIMITED TO XFEED OP; UNKNOWN XFER PROB THAT MAY GET WORSE; ABORT DECISION MADE AROUND 50W. SEEMED A LONG TIME BEFORE ATC COULD GET US TURNED AROUND; AND THEN HAD TO DECLARE EMER TO GET RETURN CLRNC. 100+ KT HEADWINDS WBOUND. ARRIVING JFK WAS OUR INITIAL PLAN; BUT CLOSE LOOK AT WX AFTER TURN AROUND STARTED CREW DISCUSSION. JFK TERMINAL FORECAST TRENDED DOWN WITH VOR APCH IN USE; FORECAST WINDSHEAR; AND IN AREA OF CONV WX. BOS CLOSER; WITH TERMINAL FORECAST TRENDING UP. DURING DISCUSSION WITH DISPATCH THEY FAVORED JFK; BUT CREW FELT MORE COMFORTABLE WITH BOS. WE ALSO DISCUSSED THAT WE FELT SUBTLE PRESSURE FROM THEM FOR JFK; BUT WANED TO GO TO BOS. THEY THEN AGREED BOS OK. WITH FUEL TRACKING; ATC CLRNC LOADS; FLYING; PAX AND FLT ATTENDANT CONTACT; DIVERSION; CREW WORKING UNDER LOAD. LINE MAINT ASKED US TO LEAVE FUEL PUMPS RUNNING; XFEEDS CLOSED AFTER BLOCK IN. FUEL XFER CONTINUED AT BLOCKS. UNSURE FEELINGS OF WHAT WE HAD AND IF IT COULD WORSEN; CAUSED OUR ABORT DECISION. ONCE TURNED AROUND; IN LIGHT OF OUR FUEL MONITORING; AND DISCUSSION WITH LINE MAINT; GAVE CREW CONFIDENCE IN TOTAL FUEL QUANTITY. PROB CONTINUED THROUGH LNDG REQUIRING 4-5 FUEL XFEED OPS. BELIEVE PROB STARTED 1 1/2 TO 2 HRS AFTER TKOF AND WAS NOT IN EFFECT ON GND AT ORD. WE TERMINATED FINAL XFEED AT TOP OF DSCNT; AND LANDED 34000 LBS OUT OF BAL AT BOS WITHOUT INCIDENCE. SATCOM QUALITY OK WITH DISPATCH; BUT LINE MAINT FADED IN AND OUT ON THE PATCH. CREW FELT SOME UNEASE THAT LINE MAINT COULD NOT PUT FINGER ON EXACT PROB; BUT THAT IS UNDERSTANDABLE ON THEIR PART. WE FELT EVERYONE ON GND HELPED AS BEST THEY COULD. FLT ATTENDANTS WERE ADVISED THROUGH CABIN ADVISORY AROUND XA45Z. THEY DID GOOD JOB LEAVING US ALONE TO WORK PROB. THEY DID PICK A GOOD TIME TO CHK ON US TO SEE IF WE NEEDED ANYTHING. GOOD CREW INPUT AND QUESTIONS FROM OTHER PLTS. BOS OPS SAID THANKS FOR EARLY HEADS UP ON DIVERSION.