Let me tell you a quick story. The first time I tried tapping threads into a piece of steel, I grabbed a drill bit that looked close enough. Big mistake.
The tap broke halfway through the hole, and I had to scrap the whole project. Since then, I’ve learned a lot about matching drill bits to tap sizes, especially the 5/16 tap. If you’re wondering, “what size drill bit for a 5/16 tap?”—you’re in the right place.
I’m breaking it all down for you: the exact size you need, why it matters, and how to make sure you get perfect threads every time. Let’s drill in (pun totally intended).
Article Summary
- Understanding Tapping and Why Drill Bit Size Matters
- What Size Drill Bit for a 5/16 Tap?
- Why There Are Two Drill Bit Sizes for a 5/16 Tap
- How to Tell If You’re Using Coarse or Fine Threads
- How to Drill for a 5/16 Tap
- Tap Drill Chart for Other Common Sizes (Just in Case)
- Choosing the Right Tap Set
- Metric Equivalent of 5/16 Tap Drill Sizes
- Common Mistakes I’ve Made (So You Don’t Have To)
- It’s All About the Right Match
- What Size Drill Bit for a 5/16 Self-Tapping Bolt?
- What Size Drill Bit for 5/16 Tapcon Heavy Duty?
Understanding Tapping and Why Drill Bit Size Matters
Tapping is the process of cutting threads inside a hole so that you can screw a bolt or screw into it. Pretty straightforward, right? But here’s the catch: if your hole is too small, the tap might break. Too large, and the threads won’t hold properly.
So what’s the golden rule?
You need to drill a hole slightly smaller than the outer diameter of the screw or tap size you’re using. That’s where the proper drill bit size comes into play.
Why the Drill Size Can’t Be a Guess
- Too small = broken taps
- Too big = weak threads
- Just right = perfect fit, strong hold
Let’s move on to the star of the show: the 5/16 tap.
What Size Drill Bit for a 5/16 Tap?
This depends on the thread pitch. There are two common 5/16 taps:
- 5/16-18 UNC (Coarse Thread)
- 5/16-24 UNF (Fine Thread)
Each has a different recommended drill bit size.
Quick Reference Table
| Tap Size | Thread Type | Drill Bit Size | Decimal Equivalent |
|---|---|---|---|
| 5/16-18 UNC | Coarse | F drill bit | 0.2570 inches |
| 5/16-24 UNF | Fine | I drill bit | 0.2720 inches |
Pro Tip: Don’t eyeball it. Use a drill bit gauge or check the markings to get the exact size.
Why There Are Two Drill Bit Sizes for a 5/16 Tap
The number after the dash (like -18 or -24) tells you how many threads per inch (TPI). The more threads, the finer the pitch.
- Coarse threads (18 TPI) are easier to tap and less likely to strip.
- Fine threads (24 TPI) are stronger and better for precision work.
Your project decides which one you need.
Example from My Workshop
When I built my DIY welding table, I used 5/16-18 bolts because I needed durability over finesse. But for a small electronics mount, I went with 5/16-24—it just felt more secure.
How to Tell If You’re Using Coarse or Fine Threads
Look at the threads:
- Wide spaced threads? Coarse.
- Close together threads? Fine.
Still unsure? Use a thread pitch gauge. They’re cheap, and they’ve saved me from a lot of head-scratching.
How to Drill for a 5/16 Tap
Here’s how I do it, step by step:
- Mark your hole. Use a center punch to keep the bit from wandering.
- Secure the material. Clamp it down tight.
- Use the correct drill bit. F for coarse, I for fine.
- Drill at a steady speed. Don’t force it.
- Deburr the hole. Use a countersink to clean the edges.
- Tap slowly. Use cutting oil and turn the tap 1/4 turn forward, then 1/8 turn back.
Tap Drill Chart for Other Common Sizes (Just in Case)
Here’s a quick chart I keep taped above my bench.
| Tap Size | Thread Type | Drill Size |
|---|---|---|
| 1/4-20 | UNC | #7 |
| 3/8-16 | UNC | 5/16 |
| 1/2-13 | UNC | 27/64 |
| 10-24 | UNC | #25 |
| 10-32 | UNF | #21 |
Print this out. Trust me, it’ll save you time.
Choosing the Right Tap Set
A cheap tap set might save money now, but I’ve broken enough taps to know better. Look for:
- High-speed steel (HSS) taps
- Marked sizes on bits and taps
- A sturdy tap wrench
Budget Tip: You can find decent quality tap-and-drill sets at hardware stores or online bundles. Just make sure they list tap AND bit sizes.
Metric Equivalent of 5/16 Tap Drill Sizes
Just for the international folks or if you’re using a metric drill set.
| Tap Size | Thread Type | Metric Drill Bit Size |
|---|---|---|
| 5/16-18 | UNC | ~6.53 mm |
| 5/16-24 | UNF | ~6.91 mm |
Metric bits often come in 0.5mm steps, so round up carefully or get a full fractional set.
Common Mistakes I’ve Made (So You Don’t Have To)
- Using wood bits on metal – bad idea. Use twist bits.
- Forgetting cutting oil – leads to dull bits and broken taps.
- Not clamping the material – leads to wandering holes.
- Drilling too fast – overheats and dulls the bit.
Learn from me. These little mistakes can cost you hours.
It’s All About the Right Match
To wrap it all up, here’s what you really need to remember:
- For a 5/16-18 tap, use an F drill bit (0.257″).
- For a 5/16-24 tap, use an I drill bit (0.272″).
Choosing the right size drill bit isn’t just a detail—it’s the difference between frustration and a perfect fit. After messing up more than a few projects, I always double-check the chart before I drill. It takes two seconds and saves a ton of stress.
If you’re serious about your DIY game, print out a tap drill chart, label your bits, and always take your time. You’ll get clean, strong threads every time.
What Size Drill Bit for a 5/16 Self-Tapping Bolt?
When I first used a 5/16 self-tapping bolt, I made a big mistake—I used the wrong drill bit. The bolt stripped halfway, and I had to redo the entire job. Trust me, choosing the right bit saves a lot of time and frustration.
Here’s what I learned the hard way.
Self-tapping bolts vs regular bolts
Self-tapping bolts don’t need a pre-cut thread. They make their own. But they still need a pilot hole, or you’ll risk cracking the material or snapping the bolt.
So, what size drill bit should you use?
For a 5/16″ self-tapping bolt, the size depends on what you’re drilling into:
| Material | Pilot Hole Size |
|---|---|
| Softwood | 1/4″ (0.250″) |
| Hardwood | 17/64″ (0.266″) |
| Sheet metal (mild) | Letter F (0.257″) |
| Aluminum | 9/32″ (0.281″) |
Quick Tip: When in doubt, start smaller. You can always widen the hole, but you can’t un-drill it.
I usually go with a 9/32″ drill bit when working with sheet metal. It gives the bolt enough grip without splitting the material. But for soft wood, 1/4″ does just fine.
What Size Drill Bit for 5/16 Tapcon Heavy Duty?
When I worked on a concrete wall last year, I used 5/16 Tapcon heavy-duty screws. I thought they’d go in easily—just like wood screws. Nope! Concrete doesn’t play around.
Here’s what I learned (the hard way)
Tapcon screws are made for concrete, brick, or block. They tap their own threads as you drive them in. But the hole size must be exact—too tight, and the screw breaks; too loose, and it won’t hold.
So, what’s the right drill bit size for a 5/16 Tapcon?
| Tapcon Size | Drill Bit Required | Drill Bit Size (inches) |
|---|---|---|
| 5/16″ x 2-3/4″ | 1/4″ masonry bit | 0.250″ |
Always use a carbide-tipped masonry bit. That’s what works best for concrete and brick.
I personally use a hammer drill with a 1/4″ bit for Tapcon. It’s fast and gets clean holes. I always vacuum the dust out before screwing—trust me, it helps the screw grip better
FAQs
Can I use a 1/4 drill bit for a 5/16 tap?
Nope. It’s too small for both 5/16-18 and 5/16-24. You’ll likely break your tap.
What happens if I use the wrong drill bit size?
If it’s too small, the tap may snap. Too big, the threads won’t grip well. Always use the correct drill bit size.
What’s better: fine or coarse thread?
Depends on your project. Coarse threads are stronger in soft materials. Fine threads are better for precision and harder materials.
Do I need a special drill for tapping?
No. A regular hand drill works for making the hole. You tap manually with a tap wrench.
Final Thoughts
Figuring out what size drill bit for a 5/16 tap—whether it’s for a self-tapping bolt or a Tapcon screw—doesn’t have to be confusing. Once I learned the differences and tried things hands-on, it became second nature.
If you’re ever unsure, look up the product manual or do a quick test on scrap material. That’s saved me more than once!
Know More:
- How Much Does It Cost To Drill A Bowling Ball?
- What Size Drill Bit for 10 Screw
- Are You Supposed To Submerge Powered Orthopedic Drills For Decontamination?
- A 135 Split Point Cobalt Letter Drill Bits?
- How Long Does It Take to Drill a Water Well?
- What Type of Drill Bits Won’t Snap?
- What Are the Best Drill Bits for Stainless Steel?