Thread
sizeSize of a bolt or large screw is three numbers like 3/8 x 18 x 1 where the first is the diameter of the threaded shaft (not the wrench size), the second is the pitch in threads per inch, and the third is the length, which must allow at least two or three threads to protrude from the nut because the first few threads are tapered and not as strong. Too much Nut size has just two numbers—inside diameter and pitch—and both must match the bolt size. Diameter of the hole must be only slightly larger than that of the bolt to give a snug fit and prevent shifting. Metric fastener sizes begin with M and are like M5 x 1.75 x 10 where the first number is the shaft diameter, the second is the thread separation, and the third is the length, all in mm. Below about a quarter inch, screw diameter is a number from 0 to 12 where larger is bigger. A screw size like 8-12 x 1 is size 8 in 12 pitch and one inch long. Screw heads can be slotted, Phillips, torx, etc. Use a correctly sized screwdriver of the same type lest you damage the head.
Thread
strengthOn the head of a bolt will be hash marks or, if metric, a digit up to 5 that shows the strength, where more or larger is stronger. A bolt with no markings has low strength. Most fasteners on a boat will be marine quality stainless steel except in the engine and drive train where stronger high carbon steel is used. Nut and bolt must be of the same material to prevent corrosion.
Almost every nut and bolt needs a Washers washer of the same size and material to provide a flat bearing surface and distribute the tightening load. If there is any vibration, the nut should have a lock washer, which is usually slotted but can be toothed, for use in soft materials, or sometimes cupped. At the head of the bolt, use only a flat washer of the same size and material. Never put more than one washer at either end of a fastener.
not tighten too much or use an excessively large wrench. An over tightened fastener is already half broken. If in doubt, use a torque wrench. Apply sealant to any threads that are exposed to water at either end, as it will prevent moisture from wicking into the threads. How to tighten How to loosen Forcing a stuck nut or bolt risks damage that can be difficult to repair. Use only a 6-point socket to avoid rounding off. Two inexpensive tools that rarely fail, especially when used together, are an impact driver, the kind you tap with a hammer, and a spray called PB Blaster. If it does not work right away, apply the spray repeatedly every few days and give it time.