Two types of Heli-Coil thread repair kits are available for correcting tapping errors to STI and standard tapped holes. Oversize inserts and Twinserts allow use of the original bolt size after the repair has been made.
Oversize inserts are primarily used to correct Heli-Coil insert assemblies that gage oversize due to tapping errors. The effective correction is achieved through a larger wire cross section.
Twinserts offer greater correction to oversize tapping errors because of the larger diameter that results from installing one insert inside another. This larger diameter allows repair of stripped STI holes as well as off-center STI and standard tapped holes. Holes tapped with the incorrect diameter and/or pitch can also be repaired.
Heli-Coil Oversize inserts and Twinserts have been approved for use by many U.S. Government agencies. U.S. Air Force T.O. 44H1-1-117 prescribes their use in maintenance and salvage operations at major overhaul bases.