A Line Mechanic reports a B737-300 experienced an in-flight shutdown due to loss of #2 engine oil pressure. Mechanic did not notice his 3/4' inch adapter had remained in the CFM-56 engine AGB (Accessory Gearbox) drive pad after he replaced the (Main Engine Control) MEC and engine fuel pump.

Narrative

I was performing a Fuel Pump package R/R (Remove/Replace) on a B737-300 aircraft; number two engine. On the installation portion of the package; I removed the five bolts securing the drive pad cover and removed the cover to gain access to the AGB (Accessory Gearbox). I used an extension and a 3/4' inch drive adapter to rotate the gearbox to align the fuel pump shaft. The alignment was successful; the package was aligned and then proceeded to torque the quad ring per AMM (Aircraft Maintenance Manual). Afterwards I removed my extension from the gearbox and installed it in my toolbox. Apparently the adapter had separated from the extension; unaware of this I prepared the cover for installation and proceeded to install.I am very consciously observant to small details and I did not observe any problems installing the cover. It seated fine and torqued fine so I safety wired the hardware. The complete job went very smoothly; and my mental checklist was very satisfied; even the completed leak checks were good. I was notified by my Manager of Maintenance that an event had occurred with aircraft #2 engine with an 'In-flight Shutdown' due to loss of oil pressure.He briefly began to explain what happened and asked me if I removed the gearbox cover to rotate for alignment and I said 'yes.' He then explained in more detail and that's when my heart sank. Grateful and relieved that the aircraft had landed safely with no injuries; I told him what happened. I can't explain why I was not aware of the adapter remaining in the gearbox; and with no excuses. On our engine borescopes; the extension is one solid chrome 3/4' inch piece and that may have been programmed into me. My extension was chrome as well as the company's; my adapter was black; similar color to the gearbox and that might have had factor in not observing it. The AMM just says use a 3/4' inch drive to turn the drive pad; with no exceptions to adapters. I am very saddened this event took place; I consider myself a very safe and conscientious worker and I'm the sole person responsible for this event.