Save for factory remanufactures, all overhauls are permitted by the FAA to be performed to either new or service limits, but what’s the difference between new limits and service limits?
New Limits vs. Service Limits
Every engine design is assigned a published table of both new limits and service limits for all critical parts.
When new, these parts are required to be within a certain new limit size tolerance, but inevitably, the parts will wear through use to dimensions outside those limits. In order to provide for this, each part also has a service limit, which helps define tolerances for a part in service. When a part no longer meets new or service limits, it is considered to be out of service limits and the FAA requires that it be either repaired or discarded.
The limits are usually defined by a minimum and maximum size often as small as thousandths of an inch. Dimensional limits are measured by the use of precision measuring tools, such as micrometers, bore gauges, or calipers and are alternately defined as a part measurement or the clearance between two parts.
Service Limit Overhaul
By definition, an aircraft engine overhaul involves cleaning, carefully inspecting, and
repairing or replacing parts to meet service limits. It is therefore entirely legal to place
a used part that meets only service limits into an overhauled engine as a replacement.
This type of overhaul is known as a service limit overhaul, and is commonly seen in factory engine overhauls. Unfortunately, this practice allows manufacturers to reinstall a part that checks out during inspection with a service limit reflecting considerable wear before that part is considered to be out of service limits.
New Limit Overhaul
While in some special cases installing a service limit part might be acceptable, given the
expense and time cost of performing an overhaul, it’s usually a good idea to request that
parts used in the overhauled engine meet new limits. This type of overhaul is known as