Choose the Right Drill for Your M5 Tap

Use a 4.2 mm drill bit before an M5 × 0.8 cutting tap. That is 0.1654 inch; a No. 19 drill at 0.1660 inch is a close inch substitute. If the tap forms rather than cuts threads, or if its pitch is not 0.8 mm, use a different hole size.

Select the pitch and tap type to see the matching drill size.

Tap	Tap type	Drill size	Inch equivalent
M5 × 0.8	Cutting	4.2 mm	0.1654 in
M5 × 0.8	Forming/roll	4.6 mm starting point	0.1811 in
M5 × 0.5	Cutting, fine pitch	4.5 mm	0.1772 in

The 4.2 mm recommendation for M5 × 0.8 appears in both AutoDrill’s metric cut-tap chart and J.C. Gupta’s metric drill-and-tap chart. The J.C. Gupta chart lists 4.5 mm for the less common M5 × 0.5 fine thread.

The Standard M5 × 0.8 Cutting-Tap Drill Is 4.2 mm

Most requests for an M5 tap-drill size refer to an M5 × 0.8 conventional cutting tap. For that combination, 4.2 mm is the general chart answer.

The M5 designation gives the nominal thread diameter. It does not state the complete thread specification by itself. The 0.8 in M5 × 0.8 is the pitch: the distance from one thread crest to the next, measured in millimeters. The pilot hole depends on that pitch because the tap must leave enough material to create the intended thread form.

For ordinary metric cutting threads, a quick estimate is nominal diameter minus pitch equals tap-drill diameter. Applied here, 5.0 mm minus 0.8 mm equals 4.2 mm.

That calculation explains the chart value, but it remains a shortcut. A drawing, tap manufacturer’s table or controlled shop process takes priority where the thread class, finished-hole tolerance or workpiece material is specified.

A bit marked 4.2 mm is the direct choice. If only inch drills are available, a No. 19 drill is a close substitute: its published diameter is 0.1660 inch, compared with 0.1654 inch for 4.2 mm. Check the requirements of the job before substituting an inch size in work governed by a drawing or inspection standard.

Cutting Taps and Forming Taps Need Different Holes

A conventional cutting tap removes material to produce the internal thread. Its flutes provide cutting edges and space for chips. For the common M5 × 0.8 thread, the general chart drill for this type of tap is 4.2 mm.

A forming tap—also called a roll tap or fluteless tap—does not cut the same thread from a 4.2 mm hole. It displaces the surrounding material into the thread shape and therefore requires a larger, more tightly controlled pilot hole.

Haas lists 4.6 mm for M5 × 0.8 in its roll-tap chart. Haas identifies that figure as a starting point that may need adjustment for material, hardness and tolerance.

Jarvis likewise gives 4.6 mm as the nearest drill for a 75% M5 × 0.8 formed thread. Its forming-tap data provides different theoretical hole sizes for other thread percentages. This is why 4.6 mm should be treated as a starting point rather than a universal forming-tap specification.

Do not run an M5 × 0.8 forming tap into a 4.2 mm cutting-tap hole unless the tap manufacturer explicitly specifies that hole. Jarvis warns that an undersized forming-tap hole can create excessive torque and break the tap. When the tool is marked “form,” “roll” or “fluteless,” use its maker’s data for the workpiece material and required thread.

An M5 × 0.5 Tap Uses a 4.5 mm Drill

An M5 tap can have a pitch other than 0.8 mm. The draft chart data includes the less common M5 × 0.5 fine thread, which uses a 4.5 mm drill with a cutting tap. Its listed inch equivalent is 0.1772 inch.

The same metric shortcut shows why: 5.0 mm minus 0.5 mm equals 4.5 mm. Because the fine thread has a smaller pitch, its cutting tap starts with a larger hole than the M5 × 0.8 cutting tap.

This difference makes the full marking essential. A box, drawing or tap marked only in passing as “M5” does not provide enough information to distinguish the 0.8 mm and 0.5 mm pitches. Read the tap shank or its packaging before drilling. A 4.2 mm pilot intended for M5 × 0.8 is not the chart size for an M5 × 0.5 cutting tap.

The available source data here does not establish a forming-tap drill size for M5 × 0.5. If the tool is an M5 × 0.5 forming tap, use the manufacturer’s chart rather than transferring either the 4.5 mm cutting value or the 4.6 mm M5 × 0.8 forming value to it.

Some M5 × 0.8 Charts Specify 4.3 mm

Not every general-purpose tap chart targets the same amount of thread engagement. One published metric drill table specifies 4.3 mm for M5 × 0.8, while the cut-tap charts cited above specify 4.2 mm.

A larger pilot hole leaves less material for the tap to cut and produces a shallower thread form. That can reduce tapping effort, but it also changes the resulting thread engagement. The presence of 4.3 mm on a chart does not make 4.2 mm incorrect; the charts may be targeting different results.

Use 4.2 mm as the general chart answer for a conventional M5 × 0.8 cutting tap. Choose 4.3 mm only when the tap maker, engineering drawing or an established process calls for it. Do not enlarge the hole merely because a second chart presents a different general recommendation.

The nominal-diameter-minus-pitch calculation also points to 4.2 mm, but it cannot resolve every production requirement. Where the finished thread will be gauged, the specified hole tolerance and process instructions matter more than a quick chart lookup. The same distinction between pitch and tap type applies at larger sizes, as shown in our M8 tap-drill guide.

Verify the Bit and the Finished Hole

The drill’s marking identifies its nominal diameter, but the hole it produces is what determines how much material remains for threading. Drill condition, chuck condition, alignment and the way the work is held can affect the finished hole.

Before drilling, make sure a metric bit has not been mixed into an inch-index slot. A 4.2 mm bit and the No. 19 substitute are close, but they are not identical. For controlled work, use the size and measuring method required by the drawing rather than assuming the nearest available bit is acceptable.

Hold the work securely and keep the drill square to the intended thread axis. A visibly angled pilot hole makes it harder to start the tap straight. It can also leave the threaded fastener misaligned even if the tap itself does not break.

If hole size is critical, measure the resulting hole with a suitable method instead of relying solely on the diameter printed on the bit. This is especially relevant for a forming tap because its required pilot hole is more tightly controlled and its manufacturer may adjust the recommendation for material and thread percentage.

Drill and Tap the M5 Hole in Sequence

1. Confirm the complete marking. Look for M5 × 0.8 or M5 × 0.5, then determine whether the tool is a cutting tap or a forming tap. Do this before selecting the drill.
2. Choose the matching pilot drill. Use 4.2 mm for a conventional M5 × 0.8 cutting tap. Use 4.5 mm for the listed M5 × 0.5 cutting tap. For an M5 × 0.8 forming tap, 4.6 mm is a published starting point, subject to the manufacturer’s material and tolerance guidance.
3. Drill square to the work. Secure the workpiece so it cannot shift. The finished hole diameter and alignment matter, not only the bit marking.
4. Chamfer the entrance lightly. Remove the sharp edge without turning the entrance into an unnecessarily large countersink. The chamfer helps the tap start centrally and reduces the chance of raising a burr at the opening.
5. Use lubricant suited to the tap and workpiece. Start the tap square and follow the tap maker’s chip-control directions. A cutting tap produces chips; a forming tap displaces material and follows different hole-size requirements.
6. Match the tap geometry to the hole. Through-holes and blind holes may call for different tap geometries. Do not assume a tap that handles chips acceptably in a through-hole will behave the same way at the bottom of a blind hole.
7. Provide extra depth in a blind hole. The drill point and the tap’s incomplete lead threads occupy space below the required full threads. The drilled depth therefore has to account for more than the desired usable thread length.
8. Check the finished thread. Use the specified thread gauge when the thread must meet a controlled tolerance. For noncritical work, verify that the intended M5 fastener enters normally. Do not force it through damaged, crooked or incomplete threads.

If tapping resistance rises unexpectedly, stop rather than applying more force automatically. Recheck that the hole matches the pitch and tap type. A forming tap in a cutting-tap-sized hole is a specific mismatch that can create excessive torque.

A Clearance Hole Is Larger Than an M5 Tap Hole

A 4.2 mm hole is for producing an M5 × 0.8 internal thread with a cutting tap. It is not the normal choice when an M5 fastener is supposed to pass through the part without engaging threads.

A typical reference lists 5.2 mm for close clearance and 5.5 mm for free clearance around an M5 fastener. Those values appear separately from the tap-drill size in RivCut’s metric reference chart.

Use 4.2 mm when the part will be tapped M5 × 0.8 with a cutting tap. Use the specified clearance diameter when the bolt should slide through and engage a nut or a threaded component on the other side. Drilling a 4.2 mm hole for clearance will leave it too small for the nominal M5 fastener, while drilling a 5.2 mm or 5.5 mm clearance hole removes the material needed to cut an internal M5 thread.

The practical selection is therefore based on the intended feature: 4.2 mm for an M5 × 0.8 cut thread, 4.6 mm as the cited starting point for an M5 × 0.8 formed thread, 4.5 mm for the listed M5 × 0.5 cut thread, or the separately specified clearance size when no internal thread is required.