When working with metric fasteners, one of the most common questions is what size drill bit to use. M6 bolts show up in furniture, automotive parts, machinery, and general shop projects, and choosing the wrong drill bit can quickly lead to loose joints or damaged threads.
This guide explains the correct drill bit size for an M6 bolt and how that choice changes depending on the job. It covers drilling for clearance holes, tapping threads, and working with different materials, using straightforward guidance suitable for most DIY projects.
Quick Answer
For an M6 bolt, the most common drill bit size is 6 mm when you are drilling a clearance hole where the bolt passes through freely. If you are drilling a hole to tap threads for an M6 bolt, the standard drill bit size is 5 mm.
This rule changes depending on whether the bolt needs to slide freely, thread into the material, or fit tightly. Thicker coatings, soft materials, or precision assemblies may also require small adjustments to hole size to get the best fit.
How Drill Bit Size Works
Drill bit sizing is based on the relationship between the bolt’s outer diameter and how the bolt interacts with the hole. An M6 bolt has a nominal outer thread diameter of 6 millimeters, measured across the threads. A clearance hole must be slightly larger than the bolt’s solid core so the threads do not bite into the material.
When drilling a hole for tapping, the goal is different. The hole must be small enough that the tap can cut threads into the material, but not so small that the tap binds or breaks. For an M6 bolt with a standard coarse pitch of 1.0 mm, a 5 mm drill bit leaves the correct amount of material for proper thread formation.
In practice, drill bit selection also depends on tolerance. General DIY work usually allows for small variations, but mechanical or load-bearing assemblies require closer attention. Understanding whether the hole is structural or simply for fastening helps determine how precise the sizing needs to be.
Best Drill Bit Type for This Job
The drill bit type matters just as much as the size. For wood and soft plastics, standard twist drill bits are sufficient for both clearance holes and pilot holes. These bits cut cleanly and are easy to control with handheld drills.
For metal, high-speed steel (HSS) drill bits are the most common choice. They handle mild steel, aluminum, and sheet metal well when used at the proper speed. Cobalt drill bits are a better option for harder metals or stainless steel, as they resist heat buildup and stay sharp longer.
If you are drilling into masonry or concrete, standard drill bits are not suitable. A masonry drill bit paired with a hammer drill is required for clearance holes, although tapping threads directly into masonry is uncommon. In most cases, anchors are used instead of threaded holes in these materials.
Material Considerations
Different materials affect how accurately a hole holds its size. Soft materials like wood or plastic tend to compress slightly, which can make a hole feel tighter than expected. In these cases, a full 6 mm clearance hole usually works without adjustment.
Metals behave differently depending on hardness. Aluminum and mild steel drill cleanly, but thin metal can deform around the hole. For sheet metal, a slightly oversized clearance hole, such as 6.2 mm, can make bolt installation easier without sacrificing strength.
Hard materials require more care. Stainless steel and hardened alloys need slower drilling speeds and steady pressure. Using the correct drill bit size becomes more important here, since forcing an undersized hole can damage both the bit and the workpiece.
Common Mistakes to Avoid
A common mistake is confusing clearance holes with tapping holes. Drilling a 6 mm hole and attempting to tap M6 threads will result in weak or incomplete threads. Always confirm whether the bolt needs to thread into the material or pass through it.
Another frequent issue is using worn or low-quality drill bits. Dull bits can cut oversized holes, especially in metal, which reduces clamping force. For most DIY projects, replacing heavily worn bits improves accuracy more than changing technique.
Drilling too fast is also a problem, particularly with metal. Excessive speed generates heat, which dulls bits and causes holes to wander. Slower speeds with consistent pressure produce cleaner, more accurate holes that better match the intended size.
Frequently Asked Questions
What drill bit size should I use to tap an M6 bolt?
Use a 5 mm drill bit for tapping standard M6 threads with a 1.0 mm pitch. This size leaves enough material for strong threads without overstressing the tap.
Can I use an imperial drill bit instead of metric?
In some cases, yes, but it is not ideal. A 15/64-inch bit is close to 6 mm, and a 13/64-inch bit is close to 5 mm, but the difference can affect fit. For consistent results, metric drill bits are the better choice.
Should the clearance hole always be exactly 6 mm?
For most DIY projects, a 6 mm hole works well. For tight alignments or painted parts, a slightly larger hole can make assembly easier without reducing strength.
Does thread pitch affect drill bit size?
Yes. The standard M6 coarse thread uses a 5 mm tapping hole, but fine-pitch M6 threads may require a different size. Always check the thread specification before drilling.
Do I need a pilot hole before drilling to size?
For larger holes in metal or hardwood, a pilot hole helps keep the bit centered and reduces wandering. For a 6 mm hole, a small pilot hole can improve accuracy, especially with handheld drills.
Final Thoughts
For an M6 bolt, the correct drill bit size depends on whether the bolt passes through the material or threads into it. A 6 mm bit works for clearance holes, while a 5 mm bit is standard for tapping threads. By matching the drill bit type and size to the material and purpose, most DIY projects can achieve strong, reliable results without unnecessary trial and error.