A through bolt is a fastener that passes completely through two or more pieces of material and is secured on the opposite side with a nut. Unlike a screw or tap bolt that threads directly into one of the workpieces, a through bolt relies on a nut and washer assembly to clamp the joined pieces together. This simple design makes it one of the most common and versatile fasteners in construction, woodworking, and mechanical assembly.
How a Through Bolt Works
The concept is straightforward. You drill a clearance hole through every piece you want to join, slide the bolt through all the layers, then thread a nut onto the exposed end and tighten it. The bolt shank doesn’t grip the material by cutting threads into it. Instead, the head on one side and the nut on the other squeeze the pieces together with clamping force.
A standard through bolt assembly consists of one threaded rod or bolt, two washers (one under the head and one under the nut), and a nut. The washers distribute the clamping pressure across a wider area, which prevents the bolt head and nut from digging into the material surface. In structural applications, hardened washers are common because they resist deformation under high torque.
This approach differs from fasteners like lag bolts or self-tapping screws, which thread directly into one of the connected materials. Because a through bolt doesn’t depend on the material itself to hold threads, it works well in softer materials like wood, composite panels, and hollow steel sections where tapped threads might strip or fail under load.
Clearance Hole Sizing
The hole drilled for a through bolt is always slightly larger than the bolt’s outer diameter. This clearance allows the bolt to slide through without forcing it, while still keeping the fit tight enough for a solid connection. Industry standards define two common fits: close fit and free fit.
For a 1/4-inch bolt (0.250″ diameter), a close-fit hole measures about 0.257″, while a free-fit hole opens up to roughly 0.266″. For a 1/2-inch bolt (0.500″ diameter), the close-fit hole is about 0.516″ and the free-fit hole about 0.531″. The pattern holds across sizes: the clearance hole is typically 2% to 6% larger than the bolt diameter, depending on how precise the alignment needs to be. Close fit is used when parts need to align accurately. Free fit gives you more room for adjustment during assembly, which is practical when you’re working with multiple bolt holes that need to line up across large pieces.
Getting the hole size right matters. A hole that’s too tight makes insertion difficult and can damage threads. A hole that’s too large reduces the bolt’s ability to resist side-to-side (shear) loads because the bolt can shift inside the hole before it bears against the material.
Where Through Bolts Are Used
Through bolts show up anywhere you need a strong, removable connection between two or more pieces. In wood framing, they join beams to posts and connect structural members at critical load points. In furniture and cabinetry, they hold knockdown joints together, the kind you tighten during assembly and can take apart if you move. In automotive and mechanical work, through bolts hold engine components, brackets, and mounting plates in place.
In steel construction, through bolts can connect hollow structural sections (tubes and rectangular sections) where welding isn’t practical or desired. The American Institute of Steel Construction permits through bolts in bearing connections on hollow steel members, though these connections are typically installed snug tight rather than fully pretensioned. The bolt passes through both walls of the hollow section, and the primary design consideration is bolt bearing, meaning the bolt’s ability to transfer load without crushing the steel around the hole.
Through bolts are also the standard choice when you need to disassemble and reassemble a connection. Because the nut simply threads off, you can take the joint apart without damaging any of the connected pieces. Screws and tap bolts that thread into the material risk stripping those internal threads over repeated removal cycles.
Bolt Grades and Strength Ratings
Through bolts come in a range of strength grades, and the markings on the bolt head tell you what you’re working with. The two main classification systems are SAE (used mostly for smaller mechanical fasteners) and ASTM (common in structural and construction applications).
At the lower end, SAE Grade 1 and ASTM A307 bolts are made from low or medium carbon steel. These are general-purpose fasteners suited for light loads and non-critical connections. Grade 2 bolts offer slightly higher strength and cover most everyday hardware store applications for sizes up to 3/4 inch in diameter.
For higher-strength work, SAE Grade 5 bolts use medium carbon steel that has been quenched and tempered, a heat treatment process that significantly increases tensile strength. Grade 5 bolts are common in automotive and machinery applications. In structural steel construction, F3125 Grade A325 bolts (which replaced the older A325 specification in 2016) serve a similar role, with a Type 1 version for standard environments and a Type 3 version made from weathering steel for outdoor exposure without paint.
Stainless steel through bolts fall under ASTM A193/A320. Grade B8 uses 304 stainless, which handles most corrosion concerns. Grade B8M uses 316 stainless, which offers better resistance in marine or chemical environments. Stainless bolts generally have lower tensile strength than heat-treated carbon steel bolts, so they’re chosen for corrosion resistance rather than raw strength.
You can identify a bolt’s grade by the markings stamped on its head. Three radial lines indicate Grade 5, six radial lines indicate Grade 8, and no markings typically mean Grade 2 or lower. ASTM structural bolts carry their specification marking directly on the head.
Installing a Through Bolt
Start by drilling clearance holes through all the pieces you plan to join. Clamp the pieces together or hold them in alignment so the holes line up. Insert the bolt from the side that gives you the easiest access to the nut on the other side. Slide a washer over the bolt before seating the head, and place another washer on the exit side before threading the nut on.
Tighten the nut while holding the bolt head stationary with a wrench (or vice versa). For general carpentry and light mechanical work, hand-tightening with a wrench until the joint is snug is usually sufficient. For structural steel or high-load applications, specific torque values or turn-of-nut methods may be required to achieve the correct clamping force.
When bolting through wood, avoid over-tightening. Excessive force will crush the wood fibers around the washer and weaken the joint over time. In softwood, a large flat washer or even a steel plate can help spread the load. When bolting through metal, ensure there are no burrs around the drilled holes, since rough edges can prevent the pieces from sitting flush and reduce clamping effectiveness.
Locking Hardware for Vibration Resistance
One drawback of through bolts is that vibration can gradually loosen the nut. In machinery, automotive, and outdoor structural applications, this is a real concern. Several types of locking hardware address the problem. Nylon-insert lock nuts (nyloc nuts) use a plastic ring inside the nut that grips the threads and resists rotation. Prevailing torque nuts achieve a similar effect through a deformed thread section. Split lock washers add spring tension under the nut. For critical applications, you can also use two nuts jammed against each other or apply thread-locking adhesive.
Choosing the right locking method depends on whether you need the joint to be removable. Nyloc nuts and lock washers allow disassembly. Thread-locking adhesive comes in removable (medium-strength) and permanent (high-strength) formulas.