Nuts and Bolts Tech | MC GARAGE

Cool Threads, Dude or, When and How To Tap Out and Die.

Stripping threads is like running out of gas during rush hour, getting a flat tire at midnight, or forgetting to put the sidestand down in front of a crowd—if it hasn’t happened to you yet, it will. And it’s not just ham-fisted garage chimps who turn good hardware into scrap metal—even careful wrenching sometimes results in stripping a bolt or a nut, turning a quick job into an all-day affair. The more you know about thread care the less likely you are to take that trip yourself.

The primary causes of thread damage are installer error or impatience—often both. Starting a bolt or screw out of alignment is the most common, and it’s easy to do when the hole it threads into is hard to see. Fastener alignment is especially tricky with those that pass through several components like body panels that all need to be aligned before you can even see the hole behind them. Juggling all those pieces while trying to get the threads to mesh is where the trouble usually starts. The bolt goes in, makes it half a turn, maybe a whole one, and then you feel resistance. When this happens the urge is to bear down on the wrench or get a longer one, which only makes it easier to strip the threads. Stop right there.

For most weekend mechanics, the trick is to start the fastener by hand and make sure it goes in two or three turns easily before you put a tool on it. If at any point after that you feel resistance, stop and figure out why. You could just be squeezing those body panels together, or a couple of holes could be off center, in which case a little resistance is normal; shift the parts to align them and it’ll probably disappear. On the flip side, you might be encountering damaged threads deep in the nut or on the fastener; in that case you need to back out the fastener and see what’s wrong. Experienced mechanics learn to differentiate misaligned panels or normal torquing and the onset of problems.

If you see stripped threads on the fastener don’t try to clean them up by forcing it into the nut; you’ll just bugger up both pieces. If it’s a common fastener just buy a new one, or replace it from that jelly jar full of miscellaneous hardware under your workbench. Can’t find the right one? If the damage is limited to one or two threads, you can use a thread file to clean up the damage on the old one or a die to repair the threads. You can use hardware-store bolts as a temporary fix, but if it’s a special bolt get a new one. It’s special for a reason.

Uncaptured, “loose” nuts are likewise easy to replace, unlike captured nuts, which are pressed in or welded to the frame. To repair these, run a tap through the nut to clean up the threads. Double-check that the bolt you’re trying to shove through the nut is the right size by looking in the manual or consulting an online parts fiche. You’ll need this information to choose the correct tap.

Nutserts, or rivnuts, are threaded blind rivet-like fasteners that are available in metric and SAE sizes.

If there's room behind the old nut, you can drill it out, put in a slightly longer bolt, and put a regular nut on it. As a last resort, grind down the nut and weld on a new one. Just make sure you take better care of the threads from now on. If the threads in a stripped blind hole—like the ones for the bolts that hold the engine covers on—don't clean up with a tap, you're not out of options. A Heli-Coil insert replaces the stock threads and works best when the material is thick, like an engine case. (see Heli-Coil Installation here) For thinner material like frame tabs and body panels, Rivnuts—a combination pop rivet and threaded insert—are the way to go. They're also the repair method of choice where you can't easily reach around behind the trouble spot.

THREAD REPAIR: Unscrewing The Pooch

To clean up the damaged threads in a hole, start by making sure the tap you’re using matches the threads’ diameter and pitch. Use a double-ended tap handle, not a wrench, to keep the tap aligned as you turn it. Rotate the tap one-half to three-quarters of a turn then back it out of the hole to remove the chips. If you’re tapping into a blind hole make sure you know how deep it is before you start or you could ruin all the threads when the tap bottoms out and you force it to turn; a piece of tape wrapped around the tap makes a useful depth gauge.

Clean up the damaged threads on a fastener with a die. Clamp the fastener in a vise and run the die down the length of the fastener, occasionally backing it up a turn or two to remove the debris. Use a drop of light oil on the threads to prevent galling or tearing. If the threads are damaged severely, or in several places, the fastener can lose some of its strength even after repairing it. Replace it instead.

JUST-RIGHT TIGHT: Torque Isn’t Only For Engines

Every fastener on your bike has a recommended torque; you’ll find the values for specific ones in the shop manual, but you can spitball it based on the fastener’s diameter. Use too little torque and the fastener can back out; too much, and it can break off or strip the threads in the hole. Either way, you’ll have to dust off that tap-and-die set.

Don’t know what torque to use? Your bike’s service manual is the first place to look, but as a general rule bolts can be torqued based on their diameter. Class 4.6 bolts are non-structural (bodywork), while Class 8.8 bolts are used to secure items like the engine.

Good torque wrenches aren’t cheap, but they pay for themselves quickly. They range from the inexpensive beam-type to the pricier click-type and digital models. (Beam-style torque wrenches are wonderfully rugged but are subject to viewing errors; click-types are as easy as it gets.) Buy one calibrated in both foot-pounds and Newton-meters. Make sure the torque range fits the jobs you need to do. Ten to 60 foot-pounds ought to cover most common tasks. Beware that torque wrenches are most accurate in the middle of their range, so choose a tool accordingly.

Nutserts, or rivnuts, are threaded blind rivet-like fasteners that are available in metric and SAE sizes.
Don’t know what torque to use? Your bike’s service manual is the first place to look, but as a general rule bolts can be torqued based on their diameter. Class 4.6 bolts are non-structural (bodywork), while Class 8.8 bolts are used to secure items like the engine.