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The Nuts and Bolts of Nuts and Bolts



Last year, a 37-foot sailboat broke away from a mooring during a winter storm. A marine surveyor was called in to investigate and found that the cleat had torn off because the owner used undersized 1/4″ bolts to install a new, larger cleat. The surveyor noted that the proper bolts for the job were 3/8″, which are two-and-a-half times stronger, and would have almost certainly withstood the storm. The mistake caused the boat to spend most of the season undergoing extensive repairs (Claim #0702436).

Boats are loaded with fasteners. Anytime something is added to your boat or a repair is made, chances are nuts, bolts, or screws will have to be removed, changed or replaced. Choosing the correct fastener is critical; a failure could be devastating.

Getting The Right Size And Other Considerations

If you’re replacing a fitting, like the cleat mentioned above, don’t assume that the old fasteners will work. Check the packaging, which will usually specify the recommended size. If you’re not sure, it’s a safe bet to use the largest fastener that will fit. A few other considerations that may make your job easier: Bolts are measured by the diameter of the threaded part, though the head (usually hex-shaped) is nearly always a different size. So, for example, you might have a 1/4″ bolt, but the head size, and therefore the wrench size, might be 3/8″ — don’t get the two confused. Bolts are sold with specific measurements; if you see a bolt described as 5/16 x 18 x 1, it means the bolt diameter is 5/16″, the thread count is 18 (which means 18 threads per inch), and the length is 1 inch. The length of a bolt is correct when about two threads extend past the nut after it’s been tightened; this is because the first couple of threads on a bolt are not as strong. Having a bolt extend too far through the nut is just asking for something to catch on it. Bolts need to fit snugly in the hole they go into — if the hole is too big, the parts being held together can shift, and the fastener can shear. For practical purposes, the thread count doesn’t mean much as long as nuts and bolts have the same one. Nuts are measured by the bolt they fit as well as the thread count.

For metric fasteners, the sizes always start with an “M”. If you see one sold as M3 x 1.5 x 10, it means the bolt is 3 mm in diameter, the thread count is 1.5 mm from crest to crest, and the length is 10 mm. Metric and standard nuts and bolts and can never be mixed.

A screw is a type of bolt, but can also be self-tapping, like a wood screw. They’re measured as a number from 0-12 with larger numbers representing larger diameters, up to 1/4″ when they start being measured by diameter, like bolts. The thread count per inch is also a part of their measurement, which is why you see screws sold as 6-32 x 1; 6 is the screw size, 32 is the thread count, and 1 is the length in inches. For woodscrews, at least half of the length of the screw should enter the wood. For machine screws (used in metal), the rule is that about two times the diameter of the screw should go into the metal.


Look at the head of a bolt and you’ll learn something about it. Usually, they’re stamped with markings — either a number (metric) or a series of lines (standard) indicating their strength — more lines or a higher number indicates higher strength. The lack of stamps generally tells you they are, at best, of so-so quality. Stamps are useful because when you’re replacing bolts, it’s important to use the same or greater strength to prevent a failure. And nothing good ever happens when a fastener fails.


Most common hardware store fasteners are carbon steel and not suitable for the majority of uses aboard because they corrode. The exception is engine fasteners. Stainless steel, while corrosion resistant, is not as strong as carbon steel and shouldn’t be substituted in drive trains. Stainless steel is the best choice for all other uses aboard except where they will be continuously wet, because even good stainless will corrode in an oxygen-starved environment. In such cases fasteners should be bronze. Stainless-steel fasteners should be either type 18-8 (the same thing as 304) or 316. Most quality stainless-steel fasteners are not attracted to a magnet, but that method is not foolproof, since even good stainless can have magnetic properties.

Charles Fort

Contributing Editor, BoatUS Magazine