Story and Photography by Ryan Lee Price

Strip a thread? Break off a bolt head? Nothing can halt a project on its heels quicker than a broken bolt or fastener, stripped nut or rounded off capscrew. The weakest point of any design is where two pieces come together, specifically, where the bolts meet the metal, and more so if that bolt is incorrectly tightened. Bolt (and/or nut) failure on a vehicle can have potentially disastrous consequences, so it is important to closely follow the tightening sequences for the specific vehicle.

Why Bolts Fail

A threaded fastener generally fails in one of three ways: 1) through the threaded section of the fastener; 2) thread stripping of the external thread; or 3) thread stripping of the internally threaded member.

1. Stress and Friction

The vast majority of fastener failures occur because of the first reason, failure through the bolt thread. Under static loads, the area most stressed determines the strength of the thread. When tightening a bolt, the shank sustains a direct stress, due to the elongation strain, together with a torsion stress of tightening due to the torque acting on the threads. If the values of these stresses, added to the frictional conditions that all bolts encounter, become greater than the design characteristics of the bolt, it simply bends, twists, or snaps off completely.

2. Stripping Saps Thread Strength

In many designs thread stripping is a problem where tapped holes are required in low tensile material, such as magnesium and aluminum. In general terms, thread stripping tends to be gradual in nature but greatly diminishes the usefulness of a fastener. The following factors all have an important effect on the strength of a thread:

  • The variation in the dimensions of the thread, (such as pitch and diameter of the thread);
  • Tensile and shear strength variations in the material for both the internal and external threads;
  • Known as nut dilation, the radial displacement of the nut or tapped component reduces the strength of the threads. The force on the fastener (like a wedging action) generates a radial displacement which reduces thread strength;
  • The effect of radial displacement, which causes bending of the threads;
  • Production variations in the threaded assembly, such as slight hole taper or bellmouthing, affect thread strength, creating essentially, a lemon bolt.

Solutions for Damaged Threads

Several methods of repairing damaged threads are available on the market today and these involve the same principle-drilling out stripped threads, tapping the hole and installing a pre-wound insert -- and it makes welding, plugging and oversize fasteners unnecessary.

Thread Repair Inserts

A kit usually supplies two types of thread repair inserts: a standard type for most inch-coarse, inch-fine, metric-coarse and metric-fine thread sizes and a spark plug type to fit most spark plug port sizes. Typical thread repair kits contain a selection of pre-wound threaded inserts, a tap (corresponding to the outside diameter threads of the insert) and an installation tool. Spark plug inserts usually differ because they require a tap equipped with pilot threads and a combined reamer/tap section. Most manufacturers also supply thread repair inserts separately plus a master kit containing a variety of taps and inserts plus installation tools.

Before repairing a threaded hole, remove any snapped, broken or damaged bolts, or studs. Use penetrating oil to free frozen threads. Remove the offending item with locking pliers or with a screw or stud extractor. Once the hole is clear, repair the thread as follows: Drill out the damaged threads with the specified bit. Drill completely through the hole or to the bottom of the blind hole. With the tap supplied, rethread the hole to receive the threaded insert. Keep the tap well oiled and back the tap out frequently to avoid clogging the threads. Screw the thread insert onto the thread installation tool until the tang engages the slot. Screw the insert into the tapped hole until it is a quarter to half turn below the surface. After installation break the tang off with a hammer and punch.

If you don't have the time or ability to pull the head or install heli-coils to repair a damaged thread, try a back tap-style tool similar to the ones shown here. The slim profile slips into the striped hole and then expands to the exact size, and if you apply a grease to the tip, it will catch filings and debris. These are available in 10, 12, 14 and 18mm for a wide variety of applications.


It is a good idea to consider applying antiseize compound to most all nuts and bolts during your projects. It prevents corrosion and resists moisture. 

In the best of times, working under the hood can be rewarding, satisfying and even fun, but in the worst of times, it can be downright frustrating, as unforeseen incidents can pile on time, resources, and stress to any project. A subscription to ChiltonDIY, is the right tool to grab -- designed for do-it-yourselfers to save many hours of frustration and immediately pay for itself.

Not only is Ryan Lee Price a freelance writer specializing in automotive journalism and a former long-time magazine editor, he is part of the technical editorial team that provides content for most all of the Chilton products. He currently resides in Corona, California, with his wife Kara and their two children.