How to Repair Stripped Bolt Hole Threads
The guy down the street forced a standard-thread starter bolt into his block when it really needed a metric. That worked great for about two weeks. Then, while driving down the road vibration finally worked the incorrect bolt free and the increased tension snapped the remaining starter bolt flush with the block. After extracting the broken bolt he realized the cross-threaded hole now loses grip on the correct bolt before reaching the required torque. He had to repair the block's stripped bolt hole threads before reinstalling his starter.
If my neighbor had understood bolt-thread patterns, he would never have attempted to force the wrong bolt into his engine block. Most modern bolts have either ISO Metric Thread or American Standard Thread classifications, often referred to as simply metric or standard respectively. ISO Metric Thread measures the thread pitch, or distance between each set of threads. American Standard Thread counts threads per inch. Standard bolts measure the greater diameter and length in inches, while metric uses millimeters.
A common 3/8-16X1-78 starter mounting bolt measures 3/8 inch across its greater diameter and has 16 threads per inch. Lengthwise it measures 1-7/8 inch. The similar looking metric starter bolt M10-1.5X45 is 10mm thick with a 1.5mm thread pitch. It measures 45mm lengthwise.
A blind hole stops before it passes through the block. Debris packed into the bottom of a blind hole can make a bolt bottom-out prematurely. This common error initially leads to loose fitting parts and leaking gaskets. Trying to tighten a bolt in a debris packed blind hole often leads to stripped threads. Because a thru-hole, sometimes called a clearance hole, passes completely through the block, debris cannot build up.
Partially Stripped Threads
Partially stripped bolt-hole threads often vibrate loose or let go before applying maximum torque. Many times this happens when the wrong bolt was started, but not overtightened. The amount of damage depends directly on how much force was applied with a wrench. Occasionally vibration from a loose bolt wears or reshapes the hole, which destroys the hole's integrity. Depending on the severity and the application, many mechanics save partially stripped bolt holes without using a thread repair kit.
Remove all dirt and grime from the work area with a strong engine cleaner. Choose a thread tap that matches the original bolt's thread pattern. Dip the tap in a light oil; motor oil works great for this. Work the tap into the hole until hand-tight. Place a wrench on the square end of the thread tap and run the tap into the damaged hole a couple of times. With every turn the thread tap re-cuts and cleans the damaged portion. Remove the tap and clean it in oil. Continue to work the tap in and out, keeping cutting teeth constantly covered in oil. Do not over-tighten the tap. Running the thread-tap too deep can cause catastrophic damage to the part. Clean hole and work area with an engine cleaner. Coat the bolt's threads with a thread-locking material and complete the repair.
Badly Damaged Threads
Symptoms of a badly damaged bolt-hole range from a bolt that loses grip when applying moderate torque, to one that spins freely. The repair methods also vary greatly. When used appropriately, a simple chemical thread-filler works remarkably well. The popular thread-insert repair method returns a hole to its original size. Sometimes using a longer nut and bolt solves the thru-hole issues that cause inserts problems, such as insert pass through.
All blind hole thread repairs require a great deal of care. Never drill deeper than the original hole. Use a feeler gauge to measure depth of hole. Transfer the measurement to the drill bit. Wrap masking tape around the bit, keeping the edge closest to the bit's blade on the mark. The edge of the tape acts as the drill bit's depth gauge. Never drill deeper than this point.
A chemical thread-filler repairs moderately stripped bolt holes without much effort. This easy solution does not require a drill bit. Before starting, remove all grease and grim from the work area and read the manufacturer's instructions. A badly damaged hole often needs its threads re-cut with a tap before using a chemical filler. Spread the release agent on a new bolt's threads. Fill the stripped bolt hole with the thread-filling material. Immediately screw the bolt into hole. Wait for the filler to dry. Remove the bolt. Clean the release agent from the work area and the bolt.
Thread Repair Insert
A thread-repair insert acts as a bushing between an over-sized hole and a properly sized bolt. A popular insert has an expanding coil design; others look like a standard bushing. A basic thread repair kit contains a thread tap, insert and an installation tool. Thread-insert installation requires a properly sized drill bit.
When working on a blind hole, set a depth mark on the drill bit. Clearance-hole applications do not require this depth marks. Clean the work area. Work the drill bit the full depth of the hole, using extreme care to avoid going too deep in a blind hole. Coat the kit-supplied thread-tap with a light oil. Run the thread-tap into the hole, creating new female threads. Remove all residual oil from the hole and the surrounding area. Place the thread-insert on the end of the installation tool. Place thread locker, if manufacturer recommended, on the appropriate surface. Screw the insert into the hole, stopping when the insert bottoms out. Remove the installation tool. Complete the repair and install a new bolt.
A coil-style insert compresses during installation and does not usually need a thread-locking material. During installation, the tool-applied torque lengthens and compresses the coil. Removing the tool releases the compression, letting the coil swell against the over-sized hole. Installing a bolt into the insert forces the coil against the hole's over-sized threads.
Install Bolts Correctly
Thoroughly clean all working surfaces before starting. Test fit the bolt in its hole. It should travel in and out of the hole freely. Never cross-thread the wrong type of bolt into a hole. A bolt with an incorrect thread pattern will only twist a few times before binding. Once the bolt binds, the mating surfaces start to deform. Continuing to apply torque will strip the bolt-hole's threads.
Use thread-locking material on bolts susceptible to vibration. After the thread-locking material dries, a semi-permanent bond forms, preventing the bolt from vibrating loose. When working with a blind hole application, apply the thread-locking material to the bolt's male threads. Excess thread-locker in a blind hole can stop the end of the bolt from bottoming out. In thru-hole applications, cover the hole's female threads before inserting the bolt.
Press the bolt into the hole and twist clockwise. The bolt should turn easily. Once hand-tight, place the proper sized wrench or socket on the bolt's head and tighten as needed. When applicable, use a torque wrench and turn the bolt clockwise until it reaches the manufacturers recommended torque limit.
Thru-Hole Repairs Using a Nut
If the original bolt held a part against a flat flange, consider using a longer bolt, nut and washer set. Some applications allow slightly thinner bolts. Choose a thinner bolt that slides through the threaded hole. If using the same diameter bolt, remove the bolt-hole's stripped threads with a drill bit. Placing a lock washer against the nut limits movement.