Two Ways to Anchor an Exit Device -- and Why the Choice Is Not Interchangeable
This article explains the practical difference between through-bolt and sex bolt (also called barrel bolt or Chicago screw) installation methods for rim and mortise exit devices. It is written for commercial installers, hardware coordinators, and facility maintenance teams who need to confirm the right fastening approach before the device goes on the door -- or before a replacement order is placed.
What Is a Sex Bolt, and What Is a Through-Bolt?
Both terms describe how an exit device case is anchored to the door stile. Understanding the hardware anatomy first saves troubleshooting time in the field.
- Through-bolt: A machine screw that passes completely through the door from the inside face of the lock stile to the outside face, where a nut (or cap nut on fire-rated applications) is tightened against the door surface. The bolt puts the fastener in tension across the full door thickness.
- Sex bolt (barrel nut / Chicago screw): A two-part fastener -- a slotted or Phillips-drive male screw threads into a hollow female barrel that is inserted from the opposite side of the door. No nut protrudes on the outside face. The barrel sits flush or near-flush in a counterbored hole.
Sex bolts are sometimes called barrel bolts, sex screws, or Chicago bolts depending on the region and the manufacturer. The term appears explicitly in the Securitron EASB (Energy Absorbing Sex Bolt) product line designed to reinforce magnetic lock mounting points -- illustrating that the fastener type shows up beyond exit devices alone.
When Through-Bolts Are the Right Call
Through-bolt installation is the dominant method for most commercial rim and mortise exit device applications, and for good reason.
- Hollow metal doors: The bolt and nut draw the stile reinforcement (strike plate or internal channel) tight against the door skin. Without it, the case can pull away under panic-load cycles.
- Fire-rated openings: UL-listed fire exit hardware commonly requires steel cap nuts and machine screws specifically for labeled door applications. Standard wood screws do not satisfy the listing. Always verify the device manufacturer's listed installation instructions -- the fastener is part of the listing.
- High-cycle and high-abuse environments: School corridors, retail back-of-house, and industrial egress doors experience repeated loads. Through-bolts distribute that stress more evenly than a fastener seated only in a counterbored hole.
- Narrow-stile aluminum doors: Manufacturers of exit devices for narrow-stile aluminum frames specify through-bolt patterns that tie into the stile extrusion. Sex bolts may not engage the extrusion wall reliably.
One important note for hollow metal doors: when through-bolting at the top rail for a closer-plus-exit-device combination, a bridge-type reinforcement is required to prevent the rail from collapsing as fasteners are tightened. This is a prep coordination item, not a field improvisation.
When Sex Bolts Are Used -- and What the Trade-Offs Are
Sex bolts show up in exit device installation for specific reasons, usually appearance or access constraints.
- Finished wood doors in architectural interiors: A sex bolt eliminates the visible nut and washer on the push side. Healthcare corridors, hospitality, and education projects with stained wood doors sometimes specify this approach for aesthetics.
- Replacement on existing preps: If an existing door was originally drilled for a sex bolt pattern and the stile has already been counterbored, re-drilling for through-bolts may not be practical without patching.
- Rim cylinder attachment: The Sargent 34 Series rim cylinder, for example, is furnished with break-off screws that pass through a back plate -- a functionally similar concept where the fastener is captured on both sides of the door material.
The key trade-off: a sex bolt relies on the barrel staying engaged in the counterbore. Over time -- especially on high-traffic openings -- the barrel can loosen or spin if the counterbore tolerances are loose. Through-bolts, properly torqued with a lock nut or cap nut, maintain clamping force more reliably over the device's service life.
Fire Door and Code Considerations
The fastener method is not decorative -- it is part of the fire listing on labeled openings. Key points for specifiers and inspectors:
- On fire-rated doors, use only the fastener type specified in the device's UL-listed installation instructions. Steel cap nuts and machine screws (not sheet metal screws, not wood screws) are required on labeled hollow metal doors.
- Dogging is not permitted on fire-rated exit devices, and the same discipline applies to fastener substitution: field changes to fastener type can void the listing.
- If an authority having jurisdiction (AHJ) questions the installation, the answer lives in the device manufacturer's listed installation sheet, not in field habit.
Choosing the Right Method: A Quick Field Guide
- Hollow metal door, fire-rated: Through-bolt with steel cap nut -- verify listed instructions for the specific device.
- Hollow metal door, non-fire-rated: Through-bolt preferred; confirm stile reinforcement is in place.
- Wood door, architectural finish, non-fire-rated: Sex bolt acceptable if stile thickness supports counterbore and device instructions permit it.
- Aluminum narrow-stile door: Follow device manufacturer template exactly -- stile extrusion engagement is critical.
- Replacement on existing prep: Match the existing method unless the door is being re-prepped; document any deviation for the hardware schedule.
Exit Device Lines Worth Specifying
Whether the job calls for through-bolt or sex bolt installation, the device itself needs to be spec-grade hardware. Sargent, Corbin Russwin, Hager, PDQ, and Accentra (formerly Yale) exit device lines available through DoorwaysPlus.com are engineered for commercial applications and come with published installation documentation that specifies fastener requirements clearly -- so there is no guesswork on the job site.
If you are replacing a device and need to confirm which fastening method the existing door prep supports, or if you are putting together a hardware schedule for a new build, the team at DoorwaysPlus.com can help you match the right device to the right opening.