Look, there is absolutely nothing worse in the shop than snapping a tap inside a nearly finished part. You’ve spent hours machining, fabricating, or wrenching, only to have a cheap piece of hardened steel snap off because the pilot hole was five-thousandths of an inch too tight.
If you are staring at a bolt that says “M8” and holding a box of drill bits, you need the right answer, and you need it fast.
You might be used to SAE (Standard) sizes, but the automotive and machinery world is increasingly metric—even here in the States. Whether you are fixing a stripped thread on a Ford F-150 engine block or building a custom rig, getting the drill size right is the difference between a perfect thread and a nightmare extraction job.
Here is everything you need to know about selecting the correct drill bit for an M8 tap, complete with Imperial conversions for those of us who organize our drill indexes by fractions.
The Short Answer (Cheat Sheet)
If you are in a rush and just need to chuck up a bit and go, here are the numbers.
For Standard Coarse Thread (M8 x 1.25)
This is the most common M8 bolt you will encounter at Home Depot, Lowes, or inside your car’s engine bay.
- Metric Bit (Best): 6.8 mm
- Letter Bit (Excellent): Letter “H” (0.2660 inches)
- Fractional Bit (Close): 17/64″ (0.2656 inches) Use with caution in hard steel.
For Fine Thread (M8 x 1.0)
Common in precision automotive applications, braking systems, or vibration-heavy areas.
- Metric Bit (Best): 7.0 mm
- Letter Bit (Excellent): Letter “J” (0.2770 inches)
- Fractional Bit (Close): 9/32″ (0.2812 inches) Produces a slightly lower thread percentage.
Pro Tip: If you don’t know the pitch (the distance between threads), it is almost certainly M8 x 1.25. However, always check with a thread pitch gauge before drilling.
The Science: Why “Close Enough” Doesn’t Cut It
You might be thinking, “Can’t I just grab a 1/4 inch bit and wallow it out a little?”
Please don’t.
Tapping is a precise operation. The drill bit creates the Minor Diameter of the thread. The tap cuts the v-shape grooves to create the Major Diameter.
If your hole is too small, the tap has to remove too much material. This generates massive torque and heat, leading to the dreaded “snap.” If the hole is too big, your bolt will fit, but the threads will be shallow and weak. Under load, that bolt will rip right out.
The Magic Formula
For standard 60-degree metric threads, the formula to find your drill size is simple:
$$Drill Size = Major Diameter – Pitch$$
For a standard M8 bolt:
- Major Diameter: 8mm
- Pitch: 1.25mm
- Calculation: $8 – 1.25 = 6.75mm$
While the math says 6.75mm, the standard industry recommendation rounds up slightly to 6.8mm to reduce tap breakage while maintaining about 75% thread engagement. 75% is the sweet spot—it provides 95% of the strength of a full thread but drastically reduces the force needed to turn the tap.
Imperial Conversions: When You Don’t Have Metric Bits
This is the biggest headache for US-based makers. You have a full index of fractional bits (1/16 to 1/2 inch) and maybe a Letter set (A-Z), but your metric selection is lacking.
Can you drill an M8 hole with American tools? Absolutely.
1. The 17/64″ Solution (For Standard M8 x 1.25)
The decimal equivalent of 17/64 inch is 6.746 mm.
Recall that the perfect theoretical size is 6.75mm.
This makes the 17/64″ bit incredibly accurate for M8 tapping. In fact, it’s slightly tighter than the recommended 6.8mm metric bit.
- Pros: widely available in standard US drill sets.
- Cons: Because it is tighter, you need to use plenty of cutting fluid (like Tap Magic) and back the tap off frequently to clear chips. If you are tapping stainless steel or titanium, this tight hole might be risky.
2. The Letter “H” Bit
If you own a Jobber drill set (A-Z), grab the Letter H.
- Size: 0.2660 inches (6.756 mm).
- Verdict: This is virtually perfect. It sits right between the math (6.75mm) and the standard loose fit (6.8mm). If you have it, use it.
3. The 9/32″ Mistake
Some charts might suggest a 9/32″ for a loose fit.
- Size: 0.2812 inches (7.14 mm).
- Verdict: Do not use this for M8 x 1.25. The hole is way too big (7.14mm vs the required 6.8mm). You will end up with barely 50% thread engagement. The threads will look fine, but they will strip the moment you torque the bolt down.
Material Matters: Adjusting Your Drill Size
Context is everything. A machinist drilling into 6061 Aluminum has different rules than a gunsmith drilling into 4140 Chromoly Steel.
Soft Metals (Aluminum, Brass)
Soft metals are “sticky.” As you drill and tap, the metal tends to flow or swell toward the center of the hole.
- Recommendation: Stick to the 6.8mm or Letter H.
- Warning: Use a lubricant designed for aluminum (like WD-40 or kerosene) to prevent the aluminum from welding to the tap flutes.
Hard Metals (Stainless Steel, Cast Iron)
Hard metals don’t give an inch. Tapping them generates immense friction.
- Recommendation: If you are struggling, you can cheat the hole size up slightly. A 6.9mm bit allows for easier tapping in tough stainless steel. You lose a tiny bit of thread strength, but you save the tap.
- Fluid: Use a high-sulfur heavy cutting oil.
Plastics
Plastics relax and shrink after drilling.
- Recommendation: You often need a slightly smaller hole because the material expands away from the drill and then shrinks back. A 6.7mm or the 17/64″ is perfect here.
Cut Taps vs. Form Taps (The Curveball)
Everything written above applies to Cutting Taps. These are the standard taps with flutes that cut chips out of the metal. 99% of DIYers and garage mechanics use these.
However, if you are working in a high-production machine shop, you might be using Form Taps (also called Roll Taps). These don’t cut; they displace metal to form threads (like molding clay).
If you are using a Form Tap for M8 x 1.25:
- You need a larger hole because the metal flows inward.
- Drill Size: 7.40 mm or Letter L.
- Note: If you use a 6.8mm drill with a form tap, you will break the tool instantly.
Step-by-Step: Drilling and Tapping M8 Like a Pro
You have your drill bit (let’s say you grabbed the 17/64″). Here is how to ensure the bolt fits perfectly.
Step 1: Center Punch
Drill bits wander. Use a center punch to mark your spot. If the bit walks, your hole won’t line up with the mating part, and no amount of tapping will fix that.
Step 2: Pilot Hole (Optional but Recommended)
If you are drilling through thick steel, start with a smaller bit (like 1/8″). This reduces the pressure needed for the final 17/64″ or 6.8mm bit.
Step 3: Drill Square
Use a drill press if possible. If you are drilling by hand, use a square guide. If the hole is crooked, the bolt head won’t sit flush, creating stress that snaps bolts.
Step 4: Chamfer the Hole
This is the secret sauce. Before tapping, take a larger countersink bit and deburr the top of the hole. This helps the tap enter smoothly and prevents the first thread from bulging out, which would stop the parts from sitting flush.
Step 5: Tap with Lubrication
Apply cutting fluid. Turn the tap clockwise one-half turn, then back it off (counter-clockwise) a quarter turn until you feel a “click.” That click is the metal chip breaking off.
- Rule: Turn, back off, turn, back off. Never force it.
Troubleshooting Common Issues
“My Tap is Spinning but Not Cutting”
You likely drilled the hole too big, or you are trying to tap into a hole that has work-hardened. If you used a dull drill bit and generated too much heat, the metal surface inside the hole may have become harder than the tap itself.
“The Bolt is Wobbly”
You used a drill bit that was too large (e.g., a 7mm or 9/32″ on a standard coarse thread). You can try to save it with a Helicoil repair kit, or drill it out to the next size up (M10).
“I Broke the Tap”
Welcome to the club.
- Do not hit it with a hammer (it will shatter, possibly into your eyes).
- Use a specialized Tap Extractor tool.
- If that fails, and the part is valuable, take it to a machine shop that offers EDM (Electrical Discharge Machining) removal.
Conclusion
Getting the right answer for what size drill bit for M8 tap prevents headaches and ruined projects.
To summarize for the US crowd:
- Standard (M8x1.25): Use 6.8mm, Letter H, or 17/64″.
- Fine (M8x1.0): Use 7.0mm or Letter J.
Don’t guess with expensive materials. A $5 drill bit is cheaper than a new engine block or a wasted afternoon trying to extract broken tool steel.
Next Step: Check your drill index right now. If you don’t have a sharp 17/64″ or a metric set, order a high-speed steel (HSS) metric drill and tap set today so you’re ready for the next repair.
Frequently Asked Questions (FAQ)
What is the SAE equivalent for an M8 tap drill bit?
The closest SAE (Imperial) fractional equivalent for a standard M8 x 1.25 tap is 17/64 inches. It creates a hole that is 6.74mm, which is very close to the ideal 6.75mm, providing strong thread engagement.
Can I use a 5/16 bit for an M8 tap?
No, a 5/16 inch bit is roughly 7.9mm. This is almost the same size as the bolt itself (8mm). If you use a 5/16 bit, the tap will not have any material to cut into, and the bolt will slip right through the hole without tightening.
Is an M8 tap the same as 5/16 thread?
No. While they are similar in diameter, the thread pitch is different. An M8 bolt will not screw into a 5/16-18 nut, and vice versa. Forcing them will strip the threads. Always use metric tools for metric fasteners.
What is the drill size for an M8 Helicoil?
If you are repairing a stripped M8 thread using a Helicoil (thread insert), you need a larger hole than standard. Typically, you will need an 8.3mm or 21/64″ drill bit, but always check the specific instructions on the Helicoil package as sizes vary by brand.
Do I need a different drill bit for Stainless Steel?
For hard metals like Stainless Steel, it is recommended to stick to the standard 6.8mm or go slightly larger to 6.9mm to reduce torque on the tap. Using a tight fractional bit like 17/64″ in stainless steel increases the risk of snapping the tap.
I'm Daniel Silva, writter of Toolsroute. We believe in empowering people with insanely detailed information to make woodworking a happier, safer, and more productive hobby.
