14 CFR § 91.421 - Rebuilt engine maintenance records.
(b) Each manufacturer or agency that grants zero time to an engine rebuilt by it shall enter in the new record -
(1) A signed statement of the date the engine was rebuilt;
(2) Each change made as required by airworthiness directives; and
(3) Each change made in compliance with manufacturer's service bulletins, if the entry is specifically requested in that bulletin.
(c) For the purposes of this section, a rebuilt engine is a used engine that has been completely disassembled, inspected, repaired as necessary, reassembled, tested, and approved in the same manner and to the same tolerances and limits as a new engine with either new or used parts. However, all parts used in it must conform to the production drawing tolerances and limits for new parts or be of approved oversized or undersized dimensions for a new engine.