Tapping a hole is only as good as the drill that made it. Choose a drill bit too small and the tap binds, stalls, or breaks off in the workpiece; choose one too large and the threads strip under load. Between those two failures sits a number called thread engagement, and it determines the exact drill size you need for any tap. The process is the same whether you are working with inch threads in a steel plate or metric threads in an aluminum extrusion: identify the thread, pick an engagement percentage for the material, and look up the drill size on a chart. Getting the first step right starts with selecting the right drill bit for the material, and this article walks through the full sizing routine with a real example.
Thread Engagement: The Number That Drives Everything
Thread engagement is the percentage of the thread height that actually bites into the mating material. General recommendations cluster around 50 percent for steel, 75 percent for aluminum, and 60 to 70 percent for most engineering applications. Higher engagement gives a stronger joint but puts more load on the tap; lower engagement taps easier and leaves more wall strength in thin sections.
Why 50 Percent Is Fine for Steel
Steel is strong enough that a 50 percent engagement thread still develops most of the pull-out strength of a full thread, and the lighter cut means fewer broken taps. Aluminum needs the extra engagement because the parent material is softer and the threads carry less material per turn.
The 70 Percent Default in Extruded Aluminum
Some manufacturers size their extrusions for 70 percent engagement without any extra drilling: a 0.262-inch through-hole in a 15-series extrusion taps straight to 5/16-18 at 70 percent engagement. That is a useful default when you are working with pre-drilled stock, and it explains why the same hole can be tapped directly or enlarged for a different thread.
Engagement also interacts with hole depth. A shallow blind hole tapped at 75 percent engagement leaves less usable thread than the same hole at 50 percent with a deeper tap, so depth and percentage are decided together, not separately.
Drilling the hole itself is the first quality gate. For masonry and concrete drilling, selecting a hammer drill for masonry and concrete drilling starts with the material, exactly as tap drilling does, because the wrong drilling method ruins the hole before the tap ever sees it.
Reading Tap Drill Charts
Tap drill charts convert thread size and engagement percentage into a drill diameter. The Little Machine Shop chart and the OSG thread engagement tables are two references that agree on the standard numbers: for an M10 x 1.5 tap at 75 percent engagement in non-ferrous material, the chart specifies an 8.50 mm drill; at 50 percent engagement in ferrous material, a 9.00 mm drill.
How the Charts Are Built
Each chart row is computed from the thread pitch and the engagement percentage. The formula subtracts the thread depth from the nominal diameter, so a finer pitch at the same percentage needs a drill closer to the nominal size, and a coarser thread needs a proportionally smaller drill.
The 75 Percent Default
When a chart lists a single size with no material note, it usually assumes about 75 percent engagement. That works for aluminum, brass, and plastics, and it is on the aggressive side for steel, where 50 percent is the safer starting point.
Charts also list clearance drill sizes for bolts and screws, which sit larger than tap drills because clearance holes let a fastener pass without cutting threads. Grabbing the wrong column is a common mistake, so read the header before you drill.
Consulting a reference before buying is a habit that carries across trades: selecting the right paintbrush size starts with the same measure-first discipline as picking a tap drill, and skipping the lookup costs time either way.
Metric vs Imperial: Converting and Choosing the Closest Bit
The M10 x 1.5 example exposes the metric/imperial gap quickly. An 8.50 mm drill is about 0.3346 inches. The closest imperial letter drill is R at 0.339 inches, or about 8.61 mm, and at least one reference chart lists Q as the closest. There is no fractional drill close enough, so you either buy a metric bit or work from a letter drill set.
The Close Enough Method
If the exact size is unavailable, choose the closest bit and accept a small shift in engagement. Going slightly larger, from 8.50 to 8.61 mm, drops engagement a few points; going slightly smaller raises it. Neither shift is fatal in most aluminum or steel work, but verify against the chart when the joint carries real load.
Letter, Number, and Fractional Drill Sets
Drill bits come in fractional, number, and letter gauges. Letter drills fill the gap between fractional sizes above 1/4 inch, which is exactly where metric tap drills land. A letter set plus a set of metric bits covers nearly every tap size in common use.
| Thread | Material | Engagement | Drill size | Closest imperial bit |
|---|---|---|---|---|
| M10 x 1.5 | Aluminum, non-ferrous | 75% | 8.50 mm (0.3346 in) | R, 0.339 in (8.61 mm) |
| M10 x 1.5 | Steel | 50% | 9.00 mm (0.3543 in) | T, 0.358 in (9.09 mm) |
| 5/16-18 | Aluminum extrusion | 70% | 0.262 in existing hole | No drilling required |
The closest-fit discipline shows up in home projects too: selecting and placing area rugs in a primary bedroom comes down to matching the closest available size to the room, and the same logic applies to drills and threads.
Step-by-Step: Sizing a Tap Drill for a Real Project
Here is the full sequence used to size an M10 x 1.5 tap drill for an aluminum extrusion project:
- Identify the thread: M10 x 1.5 means a 10 mm major diameter with a 1.5 mm pitch.
- Choose engagement by material: 75 percent for aluminum.
- Look up the drill size: 8.50 mm on the chart.
- Convert if you work in inches: 8.50 mm is about 0.3346 inches.
- Select the closest available bit: metric 8.50 mm or letter drill R at 0.339 inches.
- Drill the hole, then chamfer the mouth so the tap starts square.
- Tap with cutting fluid, reversing every few turns to break chips.
The measure-first sequence is the same one you use when planning a bathroom vanity size before installation: check the standards, measure the space, then buy. Skipping the measurement is what produces stripped threads and return trips.
Tap Types and Hole Preparation
The tap itself comes in geometries matched to the hole. A spiral point plug tap pushes chips ahead of the cut, which makes it ideal for through-holes. A spiral flute bottoming tap pulls chips back out of the hole, which makes it the choice for blind holes that cannot be tapped through.
Through Holes vs Blind Holes
Through-holes let chips exit ahead of the tap and simplify lubrication. Blind holes trap chips at the bottom, so the spiral flute geometry and frequent reversal matter more. When enlarging an existing hole, as with a 0.262-inch through-hole opened up to M10, the through-hole makes chip management a non-issue.
Cutting Fluid and Chip Control
Aluminum is gummy and loads flutes fast; a light cutting fluid keeps chips moving. Steel needs lubrication to keep the cut cool and the tap sharp. On short runs, a spray lubricant and a consistent tap-handle rhythm break chips and protect the threads.
Tap quality shows in the flute geometry and the cutting edges. A name-brand tap cuts cleaner and lasts longer than a bargain-bin special, and the extra cost disappears fast when you compare it with one broken tap in a finished part.
Fit is about geometry everywhere: matching a rug shape and size for sectional sofas matters as much as matching a tap to a hole, because a shape that does not fit the space fails at the edges.
Building a Sizing Habit That Carries Across Projects
The pattern behind tap drill selection is short enough to carry anywhere: identify the thread, choose engagement for the material, consult a chart, convert units, and verify the closest bit. That same pattern handles fasteners, fixtures, and fittings, and it extends to home installations like selecting the right aquarium size for a fish tank installation: measure, consult a reference, then commit.
Keep a tap drill chart posted in the shop and a letter drill set in the drawer. The 15 minutes it takes to size a hole correctly costs less than one broken tap, one stripped thread, or one re-drilled part. The reference materials are free, and the math fits on a scrap of paper, so the habit pays back on every threaded connection you make.
When in doubt, drill the test hole in scrap first: a five-minute trial with the actual tap tells you more than any chart.