Lycoming and Continental spark plug helicoils use a serrated teeth to lock the helicoil in place. Spark plugs come in "short reach" and "long reach". Shown here is the short reach plug with a Continental part number helicoil.
Following a Major Defect Report investigation involving two in-flight incidents of blown spark plugs from the same cylinder position on the same aircraft, there is clear evidence a large number of approved workshops are not aware of the correct procedure for replacing spark-plug-helicoil-inserts.
The Lycoming published procedure for replacing spark-plug-helicoils is detailed in Service Instruction-No. 1043A. The Continental (TCM) procedure for replacing spark plug helicoil inserts is in the respective engine's overhaul manual. Lycoming in SI 1043A recommends all spark plug helicoil defects should be repaired by tapping oversize and fitting an 0.010" oversize helicoil, regardless of whether the defect is helicoil thread damage or the helicoil is loose, the TCM procedure only addresses a damaged spark plug helicoil. Thereby lies one of the problems; there is a difference between a damaged helicoil thread and a loose helicoil.
As with all aircraft defect rectification tasks, it is important to recognize the primary defect. Is it a damaged helicoil thread resulting from improper spark plug installation? Or, is it a loose helicoil resulting from erosion of the cylinder head material? Replacing a damaged spark plug helicoil with a standard size helicoil will fix the damaged helicoil. Repairing an eroded spark plug helicoil insert thread with a standard size helicoil will almost certainly result in a blown out spark plug (complete with the new helicoil).
The second issue is; not all commercially available helicoil repair kits, such as those produced by Heli-Coil, have oversize helicoil inserts. The kits may not contain the expanding and staking tool, as required by Lycoming Service Instruction 1043A and, in most cases, do not have the 0.010" oversize helicoil inserts or tap. Many maintenance organizations buy these kits from their local supplier and repair damaged or loose spark plug helicoils by simply replacing the helicoil. Again, what is the defect, what is the correct repair?
When carrying out a spark plug helicoil repair, specific care should be taken to prevent further damage to the cylinder head. Certain cylinders, such as high time cylinder heads with short reach spark plugs, are particularly susceptible to spark plug helicoil insert thread damage. Care should be taken when replacing the helicoil to reduce the possibility of further damage to the cylinder head material.
According to Textron Lycoming-Service-Instruction-No-1043A, if a spark plug Helicoil insert replacement is performed with the cylinder assembled on the engine, "some provision must be made for preventing the aluminum shavings from entering the combustion chamber."
