How to Install Hex Cap Screws Right: Washers, Torque, and Why Bolts Loosen

A correctly chosen bolt can still fail if it’s installed wrong. SC Fastening Systems covers length, washers, tightening, and the habits that keep a bolted joint tight.

A hex cap screw doesn’t hold a joint together by being strong on its own. It works by stretching slightly when it’s tightened, which creates clamping force that squeezes the parts together. Too little tension and the joint can slip or loosen. Too much and the bolt can yield or snap. Most “bad bolt” problems are really installation problems, and most of them are easy to avoid.

Get the Length Right

• Add up the grip. Combine the thickness of every plate, plus washers and the nut, to find the length you need.
• Let the nut fully engage. The bolt should extend past the nut by at least a couple of threads so the full nut is carrying load.
• Mind the thread runout. On a partially threaded bolt, try to keep the unthreaded shank across the joint faces, since that’s the stronger section in shear. A bolt that’s too long can bottom out or leave threads where you wanted shank.
• Don’t overdo excess length. A long tail sticking out is a snag hazard and adds nothing to the joint.

Washers: Which Kind and Where

• Flat washers spread the clamping load over a larger area and protect the surface from being marred by the head or nut. They’re especially helpful on softer materials and oversized holes.
• Hardened washers are the better match for higher-grade bolts, since a soft washer can flatten and lose tension under the load of a Grade 8 fastener.
• Place the washer under the part you turn. If you tighten the nut, the washer goes under the nut. If you turn the head, it goes under the head. That way the turning part slides on the washer instead of galling the surface.

One caution on split lock washers: they’re widely used, but their ability to resist vibration loosening is limited. For joints that see real vibration, a prevailing-torque (locking) nut or a thread locker is a more dependable choice.

Tightening: Torque Is a Starting Point, Not a Guess

• Use a torque wrench for anything that matters, and look up the recommended value for the bolt’s size, grade, and finish rather than going by feel.
• Lubrication changes everything. Torque charts assume a certain friction condition. Oil, anti-seize, or plating on the threads reduces friction, so the same torque produces more clamping force and can overload the bolt. Use values that match how the bolt is actually being installed.
• Tighten in stages and in a pattern. On joints with several bolts, such as a flange or a bracket, snug them all in a crisscross sequence first, then bring them up to full torque in steps. This keeps the parts seating evenly instead of cocking to one side.
• Keep threads clean. Dirt, rust, or damaged threads create friction that gives a false torque reading, so you end up with less clamp load than you think.

Special Cases Worth Knowing

• Stainless steel galling. Stainless bolts and nuts can seize when driven fast under pressure, because the threads cold-weld together. Slow down, use a suitable lubricant or anti-seize, and avoid impact tools on stainless hardware.
• Dissimilar metals. A stainless or zinc-plated bolt in aluminum, or in a wet environment, can set up galvanic corrosion. Isolating washers or the right finish for the application help prevent it.
• Reusing high-strength bolts. A bolt that has been tightened near its limit may have stretched. In critical or high-load joints, replace it rather than reusing it.
• Don’t mix thread types. Coarse and fine threads won’t engage properly, so confirm the thread pitch matches the nut or tapped hole before forcing anything.

For structural or engineer-specified connections, follow the torque, tightening method, and hardware requirements called out on the drawings.

A Quick Pre-Install Checklist

1. Length covers grip plus full nut engagement.
2. Bolt, nut, and washer grades are compatible.
3. Threads are clean, undamaged, and the same pitch.
4. Washer sits under the turning part.
5. Torque matches the size, grade, finish, and lubrication condition.
6. Vibration risk is addressed with a locking nut or thread locker where needed.