A Mechanic reports he had unknowingly used incorrect length bolts when he installed a Fuel Flow Transmitter on the # 1 engine of a B777 aircraft. At least one bolt backed-off with the O-ring causing an inflight fuel leak and a Return to Field. The Maintenance Manual installation procedure did not reference specific bolt locations.

Narrative

I was told when I came to work on December 2009; that there had been an air interrupt on a B-777; aircraft 'X'; because of a fuel leak from the left hand engine Fuel Flow Transmitter. In November 2009; I was assigned to install a Fuel Flow Transmitter (FFT) on the left hand engine on another B777; aircraft 'Y'. The original FFT was borrowed and installed on aircraft 'Y'. It was removed from aircraft 'X' and installed on aircraft 'Y' by another Mechanic. This was the first time I have installed an FFT and so I talked to other Mechanics about what the job entailed and I read through the Maintenance Manual. During installation I don't recall any major problems with the installation. I installed all new o-rings that came with the FFT and used the old bolts that were on the old FFT. I laid out the bolts in the same pattern as they came off the engine. I installed them in the same order. I used Maintenance Manual (AMM 73-31-11-4).I have been a Mechanic for 23 years and this is the first time something like this has happened with me involved. I take the time to recheck important stuff like Engine components or Flight controls and I am very troubled by this incident.