The majority of tandems use an eccentric bottom bracket to allow chain line tension adjustments between the front captain crank and rear stoker crank. The eccentric uses wedges and internal tension to hold the bottom bracket in place within the frame.
The eccentric bottom bracket is a simple piece of equipment that is easy to service and adjust. For chain tension changes the crank arms can remain in place. For servicing you will need to remove the crank arms.
During the service illustration below our editors were changing the bottom bracket hence it was removed before loosening the eccentric in the frame. This step can be skipped if you will not be replacing the bottom bracket.
Step 1: Clean the area. Given the eccentrics location it tends to collect a lot of road grime, grease, and dust. Before beginning service attack the area with a cleaning rag and some elbow grease. Also use a smaller size Allen tool to clean the allen head within the unit itself.
Step 2: Loosen the Allen bolt. This should be performed with care. As noted above the area collects grime. Take caution to make sure the dirt within the Allen head does not cause you to strip the head. The Allen head is located with the center hole (The other two holes allow leverage tools to be used to adjust chain tension).
The allen bolt will not come out of the unit. It is apart of the wedge itself so stop loosening when tension is met.
Step 3: Remove the eccentric. With the Allen bolt loose you should be able to lightly press the unit and slide it out of the frame.
Step 4: Remove lock ring from the unit. The ring can be lightly pried with a small flat head crew driver and slid from the body. Disassemble and clean old grease from unit. Dirt accesses the internals via the bolt holes and adjustment ports.
Step 5: Re-assemble unit with fresh grease. The unit should assemble back together smoothly.
Step 6: Lightly grease body and slide back into the frame. Tighten Allen bolt loosely. Reinstall crank arms and chain then adjust tension by loosening and swiveling the body. (Hot tip: once you have identified the proper tension use a sharpie to lightly mark the frame in-line with the eccentric Allen head adjustment hole. Future adjustments can be made quickly with the mark as your baseline)
Care and patience should be exercised throughout the maintenance process. Damage to the eccentric could cause irreparable damage to your frame.