When working on dirt bikes, damaged threads are something you're likely going to come across eventually so the tips below may just be what you'll need someday.
Although I have made an attempt to cover the most common scenarios, it is virtually impossible to cover all possible scenarios of damaged or stripped threads and keep the article at a reasonable length.
It's almost guaranteed that sooner or later, if you're servicing your own or someone else's dirt bike, you're likely going to be faced with threads that are damaged, so you should probably learn how to perform these type of repairs yourself and if you're lucky, you may just be able to reuse the engine case(s), cylinder, side, ignition or clutch cover, swingarm or other threaded part again (with the correct nut or bolt) as it's not hard to perform a high quality thread repair and doing so can almost be categorized as easy if you're paying attention and using care.
Normally, when faced with damaged threads on a dirt bike AKA: "Pulled Threads" or "Stripped Threads"... this concern usually resides inside a hole, at the end of a shaft or on a bolt and this is frequently caused by carelessness during assembly and / or over tightening, although the good news is that there are tips below that may just get you through this with a successful repair.
One of the most common problems occur when over tightening a bolt (or Oil Drain) in aluminum and the threads get pulled out of the aluminum resulting in whatever it was you were tightening just becoming loose all the sudden and this is usually realized upon removal of the bolt or plug to see what went wrong, as the aluminum threads are generally attached to the bolt or drain plug at this point.
If you need a little bailing out on why the bolt you were just tightening became loose all the sudden, or it won't tighten at all, you may be able to find a legit excuse in the list below. If you're curious as to all the ways (or at least most of them) that threads in a hole or on a shaft can get to looking hammered, here are some of the most common causes:
Starting a nut on a bolt or shaft, or having a bolt started into a threaded hole while cross threaded NEVER ends up with a desirable result, unless the result you're after is that of metal shavings falling on the floor or getting into places that metal shavings shouldn't be, so it's really important that you have the parts which are being assembled, assembled with care and attention being given to being "straight".
First lets ensure you know that if the threads are too far gone, it's likely going to require a Heli-Coil for threads that are internal such as the oil check bolt that won't tighten, and if this problem resides on a shaft or bolt it may require replacement.
If the threads are only slightly damaged and look salvagable but won't thread together easily...
There are thread chaser sets which are available that are usually equipped with the most common thread sizes and pitches along with specialty files designed to restore threads on the outside diameter (OD) of a shaft and these thread restorer sets can usually be used to clean up damaged threads on a shaft or inside a cavity if the hole is not too deep for the reach of the thread chaser as well as these chasers being ideal for restoring damaged threads inside a nut.
Before getting too far ahead, it's important to determine the original thread pitch of the threads in question. If the damage is on the end of a shaft or a bolt, it may be possible to use a thread pitch gauge and measure the thread pitch beyond where the bolt or shaft is damaged to determine the appropriate thread chaser that needs to be used to restore the damaged threads, although if the damage is in a hole too small for a thread pitch gauge to be inserted into to measure the thread pitch, or there are not any threads which are measurable due to damage, the thread pitch gauge can also be used to measure the thread pitch of the part that threads into the hole to determine the thread pitch inside the hole at which point, if use of the proper sized thread restorer cannot clean the threads to a reusable condition, the bolt, shaft or nut is going to need to be replaced or a Heli-Coil inserted into the hole.
Also, It's important that you don't out do me here and bring out the tap and die set for restoring threads, even though a tap and die set may be just the ticket because you've used it before and you have a nice set out in the shop, using a tap or die to restore damaged threads is NOT the recommended method, reason being is that a tap or die will cut new threads which will weaken the gripping power of a nut, threaded hole, a bolt or shaft and a tap or die should not be used for restoring threads except in a very last resort type of situation such as while at a track, riding area, a compound or a race and you nor anyone else has a thread chaser set, also considering that a tap or die will cut new threads is why specifically designed thread chaser sets are available and should be used in lieu of a tap or die.
If the problem is a bolt or shaft and the threads are slightly damaged, maybe "rolled over" as can occur when a part is dropped or struck, such as can happen when using a hammer and punch in the vicinity, and you either don't have a thread chaser, or there is likely not one available for the particular application such as a steering stem here's a trick method that may just be what saves you:
When working with external threads (such as on a bolt or pivot shaft) that only have minor damage which prevents the nut from being threaded on cleanly by hand and there is not a thread chaser available for the application, you may want to pick up a thread file to clean up any external threads, provided they are only slightly damaged as a proper thread file can usually save whatever part or shaft it is that moments ago seemed like it was destined for the trash.
If the damaged part is a something such as an aluminum case or part, it is likely that the hole had either a 6mm x 1.0 a 12mm x 1.5 or a 10mm x 1.25 which are all the most common fastener sizes used on a dirt bike and to repair one of these, it will likely be necessary to use a Heli-Coil to restore the damaged threads.
Once everything is apart and evaluated, If there is a threaded hole in an aluminum part such as an engine case, it is likely going to be necessary to use a Heli-Coil insert to properly repair this, as once the threads are damaged in an aluminum part, it is generally not possible to properly restore these type of damaged threads without the use of a Heli-Coil insert, but the good news is that installing a Heli-Coil insert is easily accomplished and results in a stronger hole than was original. Be sure to see the article on Installing a Heli-Coil Insert for more on this.
If it's determined that a Heli-Coil insert is in order, the first step will be to determine if the affected hole is the type which is closed at the end, or if the hole goes through to the inner parts of the engine and an example of this would be the oil level check bolt which is standard on a lot of dirt bikes as this is the type of situation where the right cover would need removed before any drilling or insertion of a Heli-Coil insert. If the hole has a bottom to it as can be seen by looking into the hole with a flashlight and visibly seeing the end of the hole and the hole is not the type which has a long bolt in it, then the Heli-Coil can usually be inserted without any problems and without any further disassembly of the bike, although if the affected hole is the type that goes through to the inner parts of the engine than the affected part will need to be removed and / or disassembled to access the affected threads. (Yes, this could mean complete engine disassembly)
Once the affected hole has been determined to be safe to drill in (meaning it is a closed end hole) or if the affected part has been removed or disassembled, it's now going to be necessary to drill the hole to an adequate depth to allow insertion of the Heli-Coil insert and at the appropriate size as indicated on the packaging of the Heli-Coil kit that was purchased for this repair.
Note: When drilling the hole to prepare for installation of a Heli-Coil, it is critical that the hole is drilled straight and this is generally best done on a drill press although if the part is still on the bike, this can be accomplished using a hand held drill if extreme care is used as well as it being critical that the hole is not drilled too deep whereas the part can be further damaged as would happen if the hole were to be drilled to a depth that was deeper than the hole was originally.
If the Oil Drain hole is what the threads are damaged in, I understand that most people would rather not completely disassemble the engine and split the cases to repair this so here's how to repair this in the bike using a Heli-Coil Insert.
Once the oil drain hole has been repaired, if the original drain plug was in place, a magnetic oil drain plug should be used to replace the original and inserted, torqued to the manufacturers recommended torque, the bike filled to the proper level with oil, the oil filter changed if this is a 4 stroke and the bike put back on the track for some continued flogging.