Let's get right down to the basics, the most important group of items that hold your whole scooter together; fasteners. Nuts and bolts can be very confusing to the novice. The concept of assembling components held together with socket-head, button-heads, Torx-head, setscrews, elastic lock nuts (Nylocks®), flex-nuts, 12 points and cap screws could become a mind-boggling choice. Add to these decisions the variations of sizes, SAE, metric, grades, finishes and materials and the task to choose the proper fastener can become daunting. I'll try to cut through the minutiae here so you won't have to have an engineering degree to understand what type fasteners are appropriate for your motorcycle.
When it comes to hardware, size does matter. Whether you are building a bike from scratch or just replacing some cadmium plated dull looking bolts on your Harley for polished chrome ones, you should understand how bolts are sized along with the proper terms for the different applications and styles.
A) Coarse Thread, B) Fine ThreadHere in the USA we use standard types of fasteners and wrenches and tools to install these fasteners. The Society of Automotive Engineers, SAE for short, developed these standards early in the last century so we Americans who use fractions of inches, instead of tenths-of-meters, would all have a common structure to base our hardware sizes.
One of the most common fasteners on your Harley is a 1/4x20 socket cap screw. Now be truthful; can you go out to your bike right now and point out just one 1/4x20 cap screw? Don't feel bad, there's a large cross section of guys who can't point one out either, and some work on bikes everyday. To understand bolt sizes all you'll need is a basic understanding of fractions and inches.
Bolts are described by the diameter of the shaft, or shank, and the number of threads per inch. So a 1/4x20 socket head cap screw, or commonly referred to as an Allen bolt, is ¼ of an inch in diameter and has 20 threads per inch. All you need is the length of the fastener or how deep it goes into the material for the final dimension, i.e. 1/4x20x1" is one inch long. Too short of a bolt for your application and you don't have enough thread engagement to properly hold your bike together or too long of a bolt and it will 'bottom out' and not hold your components together tightly.
The number of 'threads per inch' determines the difference between 'Fine' and 'Coarse' threads, both of which have their advantages when chosen properly. On your American bike there are typical uses for coarse vs. fine threaded fasteners. There are exceptions to these rules, just like in the English language, but in general, coarse threaded bolts are used when the fastener is threaded into a solid piece, billet or casting, while fine threaded bolts are used when a nut is used on the backside of the pieces that are being held together. The coarse threaded fastener has more 'meat' between the threads to grip better in cast materials while the fine thread has more surface area so you can maintain a higher torque value (tightness) on a forged or hardened nut.
C) Flat Head Cap Screw, D) Socket Cap Screw, E) Button Head Cap Screw,Lets' go back to our 1/4x20 example. That's a coarse thread fastener. If you wanted a fine threaded fastener that is a 1/4" in diameter you would ask for a 1/4x28. Which means it has 8 more threads per inch than the coarse threaded 1/4x20. This holds true for all basic sizes of SAE fasteners. For example 5/16x18 is coarse threaded while 5/16x24 is fine threaded, 3/8x16 is coarse while 3/8x24 is fine threaded and so on. Got it now?
F) Hex Cap Screw
Bolts are also graded, designating how 'hard' they are. Maybe you've heard of a 'Grade 8' fastener and wondered what that was all about. Well, wonder no more. Everything has trade-offs and bolts are no exception. The "harder" bolt you choose, the higher the grade number, and the more brittle or "less shear resistant" it becomes. Just because you are using a higher grade, or harder bolt, it might not be the appropriate fastener. Think of bolts as plastic. If you have a softer plastic rod it will flex and stretch and bend. If you have a brittle or hard plastic rod, you can pull on it and it won't stretch or break, but if you put side tension on the hard rod, it will snap in two with very little effort. These two characteristics are important to remember when choosing a bolt. The standard scale of bolt hardness ranges from Grade-2 (soft) to Grade-12 (hard), but a grade 8 is about as hard as you'll find on a Harley. Make sure you are using the same grade washer as the fastener. If you use a lower grade washer (softer) it will 'give' or deform in time resulting in a loose bolt.
On a standard 'Hex-Cap' or bolt with 6 sides on top, kind of shaped like a stop sign (only different), you'll see markings on the head, cap, or top of the fastener, so you can tell what grade the bolt is. This gets a little confusing but most of the bolts you should be using on your motorcycle will be either a grade 5, which will have 3 slash marks on the top, or a grade 8 which will have 5 slash marks radiating on the top. Don't ask me why the SAE decided to mark them this way (probably just to confuse the uninformed). Other types of fasteners are not marked clearly, so it is important to purchase your fasteners from reputable dealers. Cheaper fasteners are not hardened the same, or might not have the quality that want to stake your life on. Can you say 'imported?' The process used to add 'tensile strength' or 'harden' fasteners adds to the cost, so the harder the bolt the more expensive it becomes. Most of the inexpensive standard hardware at the 'Home Improvement' stores that are unmarked are grade 2, which are very soft and flexible. This is great for holding lumber together, but not what you want on your daily ride.
The head or 'cap' of a fastener also determines what 'style' the bolt is. The different heads are chosen for installation, accessibility and the ability to take torque or tightness. The most common fastener is a 'Hex-Cap' screw. These are the 6 sided standard looking bolts that you use a 'box wrench' to tighten. '12-Point' screw is very similar to a hex cap except they have twice as many points and you'll need a 12 point socket or box wrench to tighten them. 12 points are commonly used to attach your lifter-blocks and head bolts on a 'Big-Twin' engine because generally you can apply a higher torque value to a 12 point fastener. 'Socket Cap Screws' or Allen head bolts are used all over American bikes. They have a round rod shaped head with a recessed hex in the head which has 6 sides that you insert an 'Allen' style wrench into to tighten it. 'Torx' head bolts are found on many new Harleys and have a rounded cap with a modified star shaped recess in the cap for a tool similar to an Allen wrench but with more surface area to allow for more torque without stripping the head. A Torx bolt is almost just the opposite of a 12 point cap screw. Torx bolts are also used for quicker assembly in manufacturing situations because they are more stable on the end of the tool installing them, which makes for a faster installation and are suited well for pneumatic (air) tool installation. 'Button-Head' bolts have a rounded cap top, which also has a hex 'Allen' style socket or a Torx drive socket. Button-Heads are very cosmetically appealing especially when chromed, but do not take well to higher torque values without stripping out. Finally a 'Flat-Head' bolt has a cone under the head, which recesses flat into the surface that it is mating into. Flat heads are also commonly called 'counter-sunk' Allen's. Flatheads also use an Allen (trade name) wrench to tighten. NOTE: The "Allen" in the description of the fastener is describing the tool used to tighten the bolt, not the style of fastener. There are tons of different "ALLEN" bolts, to use the commonly misused term. So it's important to understand the difference and the vernacular to select the proper fastener.
Finally we get to the shiny part; everybody likes chromed bolts on their cycle. The guys at the factory in Milwaukee would rather save a couple of cents per fastener and use cadmium or galvanized bolts just to prevent rust and keep the profits higher for their share-holders. High quality chrome fasteners are available to replace your dull looking ones on your factory machine. One of the top quality US suppliers of chromed and polished hardware is Gardner-Wescott. They have been in the business of making chromed fasteners for your Harley or Hot-Rod for decades and have a huge variety of domestic high-quality fasteners for your appropriate customizing needs.
G) Torx Cap Screw, H) 12 Point Cap ScrewChrome is a durable finish until it is exposed to excessive moisture or salt. If you live by the ocean or in areas where they salt the roads, you may want to consider stainless steel fasteners that are polished. Or you might just want to move somewhere more conducive to a good days ride on a motorcycle. The choice is yours. Polished stainless-steel fasteners are almost (the key word 'almost') as attractive as their chromed steel counterparts. Stainless steel is generally a stronger fastener and will not show rust but will dull as time and oxidation takes their toll, but much slower than a chromed bolt in a damp salty environment. Chromed bolts will perform and look great here in the dry Southwest, which is a great place to live too. If you take your standard cadmium plated fasteners to your local bumper shop to get them chrome plated it usually costs more than just replacing them with new production-chromed fasteners. Another thing to consider about the chrome plating process is that if it's not done correctly, it may change the hardness characteristic of your fasteners. The final point that also bears consideration when using chromed or stainless-steel fasteners is the appropriate use of 'Anti-Seize' to avoid galling and binding of the bolts when tightening (see last months Tech-Tip) which may result in the use of foul language too.
If you are still a little confused and want to replace some of your hardware on your bike with chrome pieces, it's a safe bet to just pull your old fastener off your bike and bring it into a shop that stocks and understands hardware such as Surgical-Steeds (more shameless self promotion). By bringing in your old hardware, as an example piece, you'll take the guesswork out of getting the correct fastener when you want to replace it with chromed ones for cosmetic reasons. If you're planning on building a bike from scratch, hopefully I've helped you make informed decision on what type of bolts to choose for your project. Regardless, it's very important to choose the right fastener and have it installed properly to insure a safe, good looking ride for the long haul.
Keep the rubber side down.
- John at Steeds
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