Stop stripping threads and wrestling with wobbly hardware. Here is exactly how to match the through-bolt span to door thickness.
If you have ever installed a panic bar, pull handle, or lever trim only to find the bolt is too short to catch the nut—or so long it bends on the inside—you know the frustration. Choosing between stock (standard) length and custom length through-bolts isn’t just about convenience; it is about security, durability, and code compliance.
Below is a practical guide to mastering through-bolt span selection for any door thickness.
Why the Right Through-Bolt Span Matters More Than You Think
A through-bolt connects the exterior trim to the interior chassis of your lock or exit device. The span (the usable threaded length between the head and the start of the threads) must match your door’s total thickness.
Consequences of wrong span:
- Too short: Nut falls off, hardware loosens, creates a security gap.
- Too long: Bolt bottoms out, strips door prep, damages internal wiring (in smart locks).
Stock Length Through-Bolts – When Are They the Right Choice?
The Standard “Sweet Spot”
Most commercial and residential doors in North America are 1-3/4″ thick. Manufacturers design stock through-bolts for this exact dimension, typically offering a span of 1-7/8″ to 2-1/8″.
Choose stock length when:
- Your door is a standard hollow metal, wood, or fiberglass slab (1-3/4″).
- You are using name-brand hardware (Schlage, Yale, Sargent, HES) on new construction.
- The door has no added laminate, soundproofing, or armor plate.
Pros of stock:
- ✔ Readily available (same-day pickup)
- ✔ Cheaper (0.50–2.00 per bolt)
- ✔ No lead time
Cons of stock:
- ✘ Only fits ±1/8″ tolerance – fails on overcut doors
- ✘ Often incompatible with retrofit applications
Custom Length Through-Bolts – When You Absolutely Need Them
The Most Overlooked Fix for Problem Doors
If your door thickness is anything other than 1-3/4″, stock will fail. Custom-length through-bolts are not a luxury; they are a requirement for:
- Thin doors (1-3/8″ or less): Closet doors, imported residential doors.
- Oversized doors (2″ – 2-1/2″): Fire-rated, acoustic, bullet-resistant, or maritime doors.
- Doors with backer plates: Stone, metal-clad, or laminated surfaces add 1/8″–1/4″.
- Retrofit projects: Old doors may have warped cores or filled prep holes.
Pro tip: Measure door thickness at the edge (use a caliper, not a tape). Add 1/4″ to that number. That is your minimum through-bolt span.
How to Order Custom Span (Without Confusion)
- Measure total door thickness – at three points (top, middle, bottom).
- Subtract the recess depth of your trim (usually 1/8″–1/4″).
- Add 3/16″ for washer and nut engagement.
Formula: Span = Door thickness + 0.25″ (minimum)
Example: A 2-1/4″ fire door → need 2-1/2″ span custom bolt.
The “Will It Fit?” Checklist (Before You Buy)
Use this fast decision tree:
- Is your door exactly 1-3/4″ ± 1/16″?
- → Yes → Buy stock.
- → No → Go custom.
- Does your hardware include trim plates thicker than 1/8″?
- → Yes → Add 1/4″ to stock span → custom.
- Are you installing on an exterior door with weatherstripping or armor?
- → Yes → Custom (add 1/2″ total).
Conclusion – One Rule to Never Break
Always match the through-bolt span to the actual measured door thickness plus ¼ inch. Stock bolts are for ideal conditions. Custom bolts are for real doors.
Investing in the correct span eliminates loose hardware callbacks, prevents core damage, and maintains fire ratings. When in doubt, order one size longer and use a spacer sleeve—but never go shorter.
FAQ
Q1: Can I cut a long through-bolt to make it fit?
Yes, but carefully. Use a hacksaw and re-thread the end with a die. Never cut a hardened security bolt (e.g., Grade 8) – it will crack. For most brass or zinc bolts, cutting is fine.
Q2: What is the difference between “through-bolt span” and “overall length”?
Span = usable thread from under head to start of threads.
Overall length includes the threaded tip. A 3″ bolt may only have a 2″ span. Read spec sheets.
Q3: Will a wrong span void my fire door certification?
Yes. NFPA 80 requires hardware to be mounted without modification that changes the door’s integrity. A bolt that is too long can split the core; too short fails under positive pressure.
Q4: Do smart locks need special through-bolt lengths?
Often yes. Smart locks (August, Level, Schlage Encode) have internal wires or batteries. A bolt that is 1/8″ too long will short the PCB. Always use the manufacturer’s custom bolt or approved kit.
Q5: Where can I buy custom-length through-bolts fast?
Specialty suppliers:
- Trimco (stock + custom, 2-day lead)
- Ives (for exit devices)
- McMaster-Carr (cut-to-length stainless)
- Local locksmith (can thread rod for you in 10 minutes)
Q6: My door thickness varies by 1/8″ – what do I do?
Order custom for the thickest measurement. Use a spring washer or Belleville washer to take up the difference on the thinner side.
Final thought: Stock is fast. Custom is correct. Measure twice, order once, and your hardware will outlast the hinge.
