Thread Head

You can tell a lot about a dirt bike's life by the type and quality of the fastening hardware used on it. Find titanium or aluminum bolts and drilled washers; you've got a moto-head and a bike that's probably spent its weekends racing 30 minutes at a time. Diverse, oddball and/or non-metric ¼-20 fasteners on the other hand should be a red flag, signaling a little shop of maintenance horrors and overall casual approach to bike repair. Fact is, there's a plethora of different types of fasteners out there, and their proper choice and application is essential to maintaining satisfactory function. Trodding far from the beaten path of OEM replacements can confound proper torquing of fasteners, lead to collateral damage like stripped threads or snapped bolts, and even present a safety hazard to the rider.
Terms like bolt, machine screw, and cap screw are often used interchangeably to describe the threaded fasteners that hold your scoot together. Though similar, there’are not quite synonymous, as machine screws are most often mass produced, screwdriver driven low grade fasteners, while cap screws generally denote quality hex or socket head bolts used for machine assembly. For simplicity's sake, we'll stick with the term bolt herein.

There are four basic parameters that differentiate bolts from one another. Three of them are fairly obvious; head design, length and thread pitch. Head types, e.g., flat, pan, fillister, button, socket, hex, phillips, slotted, flanged, etc., as well as bolt length designations are pretty much self explanatory. Thread specifications can be tricky (we’ll discuss threads further on) while the fourth is bolt strength, which greatly affects a fastener's application and associated price.

There are three common bolt strength designations used for both SAE ("American") and metric fasteners. They are, from weakest to strongest; standard duty, medium strength, and high strength. Generally speaking, all flat, pan, fillister, phillips and slot headed bolts are standard grade. Socket or button head bolts are most often medium or high strength bolts, while hex headed bolts can be found in all grades. Surprisingly, the strength designation of most fasteners is easily determined, shown by marks or numbers cast into the tops of bolt heads (or lack thereof) for both SAE and metric fasteners. Common SAE bolt strength designations are grade two (standard duty) which have unmarked heads, grade five, medium strength, shown by three oppositely opposed hash marks cast into the bolt head, and high strength, grade eight, designated by six hash marks. SAE standard duty, grade two bolts are manufactured from metals with a minimum tensile strength of 60,000-74,000 pounds per square inch (psi). Medium strength, SAE grade five bolts have minimum material tensile strengths of 90,000-120,000 psi, while high strength grade eight bolts designate material tensile strengths of greater than 150,000 psi.

Metric strength designations run along the same lines as the SAE designations. Standard duty bolts have no marking, while medium strength bolts will have the number 8.8 cast into the top of the head (comparable to SAE grade five) and high strength bolts will have the number 10.9 cast into the head (comparable to SAE grade eight). In metric fasteners, you’ll also find a stronger high strength bolt, indicated by the number 12.9, which is said to exceed the strength of SAE grade eight and have minimum material tensile strength greater than 175,000 psi. Socket head cap screws, like the ones typically used on European sleds, are often these highest strength fasteners. Hence, the old adage that Euro bikes were made with better fasteners was not unfounded. It should be noted that the strength grades noted above are those that are most commonly available. Be advised, there are numerous specialty bolts manufactured and sold for specific applications, some that might have even found their way into the motorcycle parts bin for highly stressed applications like shock and linkage fasteners.

Threads

A decade or two ago, different thread schemes used on motorcycles of different origins could give you a real headache. These days, you'll find nothing but metric fasteners on everything, including US built ATKs. The global marketplace has spoken. However, you still find SAE threads on some aftermarket products manufactured here in the USA, like some metal handguards, and other bolt-on accessories, so you still have to be careful. There are some tricks to determining the thread you're dealing with, the simplest being mechanical savoir faire. Hash marks on the top of a hex head bolt are a dead giveaway, identifying it as SAE, while numbers like 8.8 or 10.9 are metric designations. For the mechanically challenged, standard duty bolts (read: no marking) or those special and/or odd ball threaded fasteners that you find on old BSAs, Bultacos and the like, there's a low tech tool called a thread gage that’ll ID the problem fastener. A thread gauge is essentially a collection of thread size templates which may be matched to the thread in question. Even today, with SAE threads in course and fine pitch, as well as metric designations, a thread gauge is sometimes a handy tool to have around. On occasion, there's some confusion regarding proper metric thread designations. Since motorcycle fasteners are nearly 100% metric these days, an aside on specific metric threaded fastener sizing is in order. Metric threads are designated by the letter "M" followed by a number, which correlates to the nominal shaft diameter of the bolt in question. Standard metric thread designations are available in a single "pitch", which is the industry term for defining the number of threads per unit length. SAE thread designation, conversely state the nominal diameter and the number of threads per inch, e.g. ¼-20 UNC. Typical metric fastener threads used on dirt bikes are M5, M6, M8, M10, M12 and M14, shown in the table below.
 

Note that several of the primary fasteners used on dirt bikes, i.e., the M6 thread with an 8mm head or the M8 thread with a 12mm head, are listed as alternates. This is because these fasteners are primarily motorcycle manufacturer specific, and essentially specialty bolts in their own right.

Rules of Thumb

For all practical purposes, male and female thread designations can be considered the same, unless you’re specifying threads on a blueprint. When dealing with fasteners that attach plastic covers and fenders to the frame, these are almost universally standard duty strength fasteners and you can pretty much get away with whatever like-threaded replacement you can find. When it comes to the bolts used in the triple clamps, engine mounts, all engine bolts, subframe, shock and shock linkage mounting, it's probably best to replace any of these lost or broken bolts with OEM fasteners. For the most part, these fasteners are more highly stressed and often sized to a particular application. Aside from your local dealer, quality metric fasteners can be obtained at better hardware and automotive stores, so long as you pay attention to the grade designations. If you’re buying in quantity, there’s numerous fastener supply houses that offer a multitude of fastener options, through which mail order purchases can be made.

Threaded Fastener Dos and Dont's:
· Do use like quality fasteners when replacing damaged ones.
· Don't ever substitute non-standard, non-OEM bolts for those used in shock and shock linkage.
· Do use a torque wrench for tightening all motor fasteners.
· Don't forget to observe proper tightening sequence when tightening head bolts and clutch basket retaining screws.
· Do use thread lubricant on all fasteners.
· Don't use thread lubricant as a marital aid.
· Do use proper flat and lock washers where applicable.
· Don't try to use plug washers in the truck stop condom machine.
· Do clean out threads with proper sized taps and dies whenever dirt and corrosion are evident.
· Don't tap your boss on the shoulder and tell him to get socket head cap screwed.

By Mark Uth

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