Key-Locking Inserts for Aerospace Engine Casings
We have machined threaded inserts for engine and gearbox housings for more than twenty years, and casings are where threaded fastening gets hard: thin walls, cyclic loads, and high temperatures. A stripped thread on a finished aluminum or titanium case is expensive to repair. Key-locking inserts solve this by decoupling the load-bearing thread from the parent material and adding a positive mechanical lock.
What Is a Key-Locking (Pin-Locked) Insert?
A key-locking insert - also called a pin-locked insert or by the brand name Keensert - is a solid bushing with an internal thread for the bolt and an external thread that screws into the tapped hole. Its defining feature is a set of locking keys held in slots on the outside diameter.
After the insert is seated below the surface, the keys are driven down and wedge through the crests of the parent-material thread, mechanically locking the insert against rotation. This is what gives the insert its high torque-out and pull-out resistance, and why it survives vibration far better than a plain bushing. Smaller, lighter-duty sizes carry two opposed keys; larger, higher-load sizes carry four.
For context, a double-end stud is the companion part on these joints - a permanent male thread for nutting components on and off - but the insert is the harder problem and the focus here.
Installation, Step by Step
- Drill. Select the drill for the insert size; depth equals insert length plus about 1–2 mm. Chamfer the hole mouth (typically 1 × 45°), deburr and clean.
- Tap. Cut the external thread with the dedicated insert tap that matches the insert OD thread - never a standard machine tap. Inspect with go/no-go gauges; the hole must be free of chips.
- Run the insert in. Using the correct driver, run the insert in squarely until its top face sits below the surface, typically 0.1–0.3 mm down or per spec. An insert standing proud is a reject.
- Drive the keys. With the matching key punch, drive the locking keys straight down until they seat fully and flush, with no tilt. This is the step that locks the part, and where most field failures start.
- Inspect. Confirm no insert movement, flush keys, face below surface, and the internal thread passing go/no-go gauging.
Drilling, tapping, run-in and key pressing can all be integrated into a CNC/servo station with force-controlled key pressing. The real gain is repeatability on the key-pressing step plus traceable torque and depth data - not just speed.
Materials, Threads & Standards
Inserts are commonly supplied in carbon steel (often plated), 303/304 CRES for general aerospace use, and A286 or nickel-base alloys for hot-section joints. Internal threads follow the application: Unified inch (UNC/UNF) to ASME B1.1, metric to ISO 261/965, or the controlled-radius-root profile of NASM8879 for demanding joints.
The governing spec is MIL-I-45914A (Insert, Screw Thread - Locked In, Key Locked), with part standards such as MS51830 and NAS1394 referenced against it. Quote against the spec and the parent material, not a brand name.
Common Defects We Prevent
- No chamfer on the hole → the insert cross-starts and galls the new thread. Always chamfer and deburr.
- Insert standing proud → interferes with the mating face. Seat below surface, verify every time.
- Crooked or partially driven keys → can crack the boss or fail to lock, then loosen under vibration. Use a square punch and check the keys are flush.
Most of these are process-discipline failures, which is exactly why semi-automation pays off.
Buyer Notes
Key-locking inserts cost more and add a step over wire (helicoil) inserts, and they are overkill for low-load, low-vibration repairs. For structural, high-vibration and high-temperature casing joints, that cost buys reliability you will not regret. When sourcing, state the spec, the internal thread (locking vs. non-locking), and the parent material, and ask whether the tap and installation tools are included. For flight hardware, buy the certs, not just the parts.
FAQ
Key-locking insert vs. helicoil?
A helicoil is a coiled wire - cheaper and lighter, but no positive lock unless a locking coil is used. A key-locking insert is a solid bushing with driven keys for much higher torque-out and pull-out resistance.
How deep should it sit?
The top face should sit below the surface, commonly 0.1–0.3 mm or per spec. Proud inserts are rejected.
Two keys or four?
Two opposed keys for lighter-duty sizes; four for larger, higher-load inserts.
Which standard applies?
MIL-I-45914A, with MS51830 / NAS1394 part standards.
