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How to Remove Broken Bolts and Repair Stripped Threads

HOT ROD logo HOT ROD 19/08/2018 Hot Rod Network Staff

When a bolt or screw breaks off it leaves the hot rodder feeling befuddled and without hope. But take heart—Summit Racing Equipment, Harbor Freight, and Dave Akard of Burbank Speed are here to help get you back on the road.

Fasteners typically break off due to material failure, improper installation, corrosion, and the galling of threads. Material failure is a weakness in the fastener where it fails and breaks off at the weakest point. This is normally caused by a weakness in the material to begin with or improper torque (tightening) causing undue stress.

Corrosion is normally galvanic in nature (dissimilar metals such as a steel fastener in an aluminum casting) where corrosion eats away at the fastener and casting causing a weakness in the fastener.

If conditions are dry you're less likely to experience Galvanic corrosion. When conditions become wet the civility between dissimilar metals goes right out the window. Fluids such as water and acid serve as electrolytes to create an electrochemical reaction between dissimilar metals. Salty air is another culprit and highly corrosive. Electrons begin their journey from one metal to the other and metals begin to break down. When a fastener and casting have been screwed together for years and exposed to the elements, failure of the fastener is inevitable.

There are also stress issues that go with fasteners because they're under considerable tension for a long time. Stress corrosion comes from exposure to the atmosphere, loading, tension, and cyclic fatigue. Engine, driveline, and chassis components are subjected to extreme loads and cyclic fatigue. This is another reason why bolts and screws fail and break off. Engine fasteners, as a prime example, experience a tremendous amount of stress and heat cycling, which is why they can fail when it's time for removal.

A fastener's purpose is to provide a clamping force between two components to hold them together. If the proper fastener is used and is not torqued beyond its design limits the fastener should perform as engineered.

Once you know why fasteners fail it becomes easier to understand how to choose and properly install them to begin with. Fastener installation should always begin with clean undamaged threads on both the fastener and bolt hole along with proper lubrication during installation. Thread type and pitch should be uniform meaning coarse threads in a coarse thread bolt hole or fine thread in fine thread. Ideally, you will begin with new fasteners instead of working with bolts that have already been stretched and stressed. This may seem tedious at first glance, but which is more tedious—painstaking attention to detail now or broken bolts when it's time to take it apart?

Bolt threads should be lubricated during installation to reduce stress and achieve an accurate torque reading. ARP Ultra Torque Fastener Assembly Lubricant should always be used when you're installing fasteners because it helps yield a proper torque reading without stressing the fastener. If you install fasteners dry you're not going to get an accurate torque reading because you can get into thread binding and galling.

When bolts and screws fail you're faced with how to get them out. However, bolt, screw, and plug extraction need not be difficult if you're patient and think the process through. If the fastener or plug cannot be driven out with a punch or chisel, you will have to drill it out in phases until it can be removed with an extractor. Sometimes, heat has to be applied to the area around the fastener or plug, which causes the area to expand and loosen up. Soaking the area with a penetrating lubricant day ahead of time offers some hope and reduces the likelihood of failure. Another adversity can be a nut seized on a stud or bolt, which can be rescued with penetrating lube, applied heat or a combination of both.

Why Bolts Break

Automotive Racing Products (ARP) tells us there are common causes for fastener failure. In fact, each type of fastener has unique identifying physical characteristics that define them—and ARP can tell you all about them.

Tensile overload will cause the bolt to stretch and "neck down" prior to breakage. What this means to the layman is excessive tension or stretch to the fastener. You get torsional shear (twist) in a fastener when galling takes place between the male and female threads (normally due to using the incorrect lubricant or no lubricant) or when the male fastener's threads (bolt) misalign with the female threads (bolt hole).

Impact shear is similar in appearance to torsional shear with flat failure faces and obvious directional traces from the impact. With impact shear, the failed bolts not only had to clamp the parts, they were asked to also locate these parts, which is normally a job for the humble dowel pin. Most of the time these bolts were not sufficiently preloaded during installation meaning they were improperly torqued.

Another reason for bolt breakage is Cyclic Fatigue in two forms—rust pit or improper installation according to ARP. Many of the high strength steel alloys are susceptible to stress corrosion. L-19, H-11, 300M and Aeromet metals are particularly susceptible to stress corrosion and must be kept well lubricated. They can never expose to moisture including human sweat. By contrast Inconel 718, ARP 3.5 and Custom age 625+ are immune to both hydrogen embrittlement and stress corrosion.

ARP adds many bolt failures are caused by insufficient preload (proper torque). When a fastener is insufficiently tightened and loaded during installation the dynamic load may exceed the clamping load resulting in cyclic tensile stress and eventual failure. In other words the bolt moves (cycles) back and forth and round-and-round constantly because it was not properly torqued to specifications.

If a fastener is not tightened properly to begin with, it cannot apply the required preload (pressure) on the components it is enlisted to clamp together. By the same token, if a fastener is over tightened and stretched beyond allowable limits, it is likely to fail by exceeding its maximum yield point, ARP tells us.

Bolt breakage can be avoided with the use of the correct fastener, clean threads, bolt lubricant, and the proper use of a torque wrench. Never overtighten. Never jerk a torque wrench. And never under-tighten. Always use a calibrated torque wrench and torque fasteners in one-third torque values in order to bring on tension gradually. ARP also stresses the use of new fasteners instead of recycling old ones. In some instances, it is safe to reuse old fasteners. Engine builds, as one example, should always be treated to new fasteners especially in high-stress areas like cylinder heads, main caps, connecting rods, and valvetrain. Chassis components should always get new Grade 8 fasteners.

When Bolt Breakage and Thread Damage Are Beyond Salvage

Sometimes bolt breakage goes beyond simple bolt extraction to where the bolt and threads must be completely drilled out. Damaged threads can sometimes be chased and cleaned up with a thread chaser or tap. When they're so badly damaged they cannot be cleaned up your only choice is to drill out and replace them with a Heli-Coil insert or a Time-Sert.

The difference in Heli-Coil and Time-Sert is both convenience and cost. The Heli-Coil insert is the more affordable of the two and more easily found locally. Time-Sert is an incredible innovation and, therefore, more expensive. Both Heli-Coil and Time-Sert repair kits are available from Summit Racing Equipment.

When you are performing thread repair or replacement you have to pay close attention to detail. Make sure you're using the correct-sized drill bit for the stripped out hole. The drill bit must be absolutely parallel to the hole. You may use a straight edge or a precision steel block as a guide or even a drill press and vice if the part can be removed from the vehicle. Always confirm accuracy before drilling begins.

Once you have drilled the damaged threads out, remove all debris from the hole with compressed air or a vacuum. When you grab the correct-sized Heli-Coil tap, lubricate the threads with a Permatex Fast Break Super Penetrant and slowly run the tap in and out. Run the tap a full revolution and slowly back out one-half turn to clear debris. Continue running the tap until threads have penetrated the full depth of the hole. Wash the hole out with brake cleaner and allow it to dry.

The Heli-Coil insert is screwed into the tapped threads until seated. It is suggested you use Permatex Threadlocker on the outside diameter of the Heli-Coil insert to ensure security. Once the Heli-Coil is seated, break the tang off and you're ready for assembly. 

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