Oh boy! Now you’ve done it. You tightened that bolt a quarter turn too much and stripped the bolthole in your engine’s aluminum block. Bummer.
What are you going to do?
Cheer up, partner. There’s an easy solution to your dilemma.
There are two ways to fix it. You can use the next-largest tap to chase new threads, and then use a correspondingly larger bolt, or you can mend the existing hole by installing a thread repair kit. Offered under brand names such as Perma-Coil and HeliCoil, these allow you to use the existing hardware, which is advantageous in certain cases.
BIGGER ISN’T ALWAYS BETTER
Although chasing new threads and upsizing the fastener will provide a strong connection, there are situations in which this isn’t a viable solution. You may not have room for a larger bolt, nor have sufficient access to swing the appropriate wrench or socket set. Then, too, a larger fastener may complicate the installation. For example, if the new bolt has to pass through a flange, you’ll have to enlarge the hole in the flange, as well.
In these and similar cases, a thread repair kit is an answer. Available at hardware or auto-parts stores, the kits generally include a tap, repair coils (also known as inserts) and a coil installation tool. Packages of inserts are also sold individually. Other tools needed include a drill, matching drill bit and a T-handled tap wrench (or, alternately, a socket driver or open-end wrench).
CHASE AND REPLACE
Basically, the job entails tapping a slightly larger hole to accept the repair coil. Once inserted, the coil maintains the stripped hole’s original diameter and serves as the threads against which the bolt will seat.
Since you’re now tightening the fastener against an insert, you’ll want to be careful not to overtighten — or you’ll strip the threads and be back to square one. The best way to avoid this is to use a torque wrench and abide by factory torque specifications.
Let’s walk through the job in five simple steps:
STEP 1: Using the drill bit that matches your thread repair kit, carefully enlarge the stripped hole. Be sure to keep the drill straight, so you’re not drilling off-axis.
STEP 2: We taped off open passageways to prevent shavings from getting inside, and placed a rag over the work area to catch debris and cutting oil. Next, we ran the tap into the block to cut new threads. This is the most critical step. It’s imperative to maintain a straight axis or the threads will be off-kilter and won’t properly accept the new bolt. Use a T-handled tap wrench or socket driver and plenty of oil (such as WD-40) to help the tap do its work.
STEP 3: Slip the repair coil onto the installation tool. We used two coils to match the depth of the hole. Carefully insert the coils into the cavity. Again, a little oil will ease installation.
STEP 4: In our application, once the repair coils were fully inserted, a portion of the second coil extended beyond the hole. We carefully nipped it off with wire cutters.
STEP 5: Here, we’re using a mill bastard file to smooth the sharp edge of the cut coil and true the mating surface. All that’s left now is to install the original bolt. With the completed repair, you’re assured a tight fit. Job well done. You will need a white 9012 led bulb for the illumination to help you fix the stripped bolt holes.