M8 x 1.25 Tap Drill Size (6.8 mm), Clearance Hole and Depth

The tap drill for M8 x 1.25 is 6.8 mm (0.2677 in), which gives about 74% of full thread depth. The exact 75% hole is 0.2670 in (6.78 mm). The table lists every number, letter, fractional and metric drill that gives between 55% and 90% thread, so you can work from the bits you own.

Drill	Type	in	mm	Thread	Note
6.6 mm	metric	0.2598	6.60	86%	deep, hard to tap
G	letter	0.2610	6.63	84%	deep, hard to tap
6.7 mm	metric	0.2638	6.70	80%	deep, hard to tap
17/64"	fraction	0.2656	6.75	77%
H	letter	0.2660	6.76	77%
6.8 mm	metric	0.2677	6.80	74%	chart drill
6.9 mm	metric	0.2717	6.90	68%	easier to tap
I	letter	0.2720	6.91	67%	easier to tap
7 mm	metric	0.2756	7.00	62%	easier to tap
J	letter	0.2770	7.04	59%	easier to tap

Thread % = (major diameter − drill) × TPI ÷ 0.01299 (Machinery’s Handbook), for M8 x 1.25; our arithmetic. Bold = the usual chart drill. Below about 68% the tap turns more easily, which helps in stainless steel and with small taps.

About the M8 x 1.25 thread

M8 x 1.25 is the coarse 8 mm thread on general machinery bolts, engine and motor brackets, larger furniture fittings, bike crank and stem hardware, and heavy-duty jig and fixture parts. The fine M8 x 1 thread is common on cars and in hydraulics, and M8 x 0.75 appears on special fittings. People look for all three, and they take different drills, so the first job is to measure the pitch.

For the coarse thread, 6.8 mm is the chart drill (8 minus 1.25 would be 6.75; 6.8 mm is the nearest standard bit). For the fine threads, see the fine-thread rows in the dimensions table below and the metric fine thread tap drill chart.

M8 x 1.25 thread dimensions

Dimension	Value	Also
Nominal diameter	8 mm	0.3150 in
Pitch	1.25 mm	20.32 threads per inch equivalent
Basic pitch diameter	7.188 mm	0.2830 in
Basic minor diameter (nut)	6.647 mm	0.2617 in
Tensile stress area	36.61 mm²	0.0567 in²
Fine thread M8 x 1: tap drill	7.00 mm	77% thread

Basic (nominal) dimensions from the ISO 68-1 profile: d2 = d − 0.649519 P, D1 = d − 1.082532 P, As = π/4 (d − 0.938194 P)². Our arithmetic.

Clearance hole for M8

The tapped hole is in the part that holds the thread; the part being clamped gets a clearance hole the bolt passes through. For M8 a normal fit is 9 mm. Never drill the clearance part with the tap drill: the screw will not pass.

Fit	Drill	Diameter
Close (ISO 273 fine)	8.4 mm	0.3307 in
Normal (ISO 273 medium)	9 mm	0.3543 in
Loose (ISO 273 coarse)	10 mm	0.3937 in

From the clearance hole tables our clearance hole calculator uses.

How deep to drill a blind M8 x 1.25 hole

Part material	Full thread wanted	Drill depth (full diameter)	Depth to drill tip (118°)
Steel or cast iron (1 × diameter)	8.0 mm	16.8 mm	18.8 mm
Aluminium or brass (2 × diameter)	16.0 mm	24.8 mm	26.8 mm

Plug tap (5 tapered threads) + 2 threads of chip room below the tap + the 118° drill point. Same rules as our tapped hole depth calculator, where you can change any of them; why 1 × and 2 ×: thread engagement chart.

Wrench size and torque for M8

Hex bolt or nut: 13 mm wrench (ISO)
Socket head cap screw (ISO 4762): 6 mm hex key

Bolt grade	Dry	Lubricated
Class 8.8	25.5 N·m (18.8 ft-lb)	19.1 N·m
Class 10.9	36.5 N·m (26.9 ft-lb)	27.3 N·m
Class 12.9	42.6 N·m (31.4 ft-lb)	32.0 N·m

T = K × D × F with F = 75% of minimum proof load (ISO 898-1) and K = 0.20 dry, 0.15 lubricated: a general starting figure, not a spec. See the bolt torque chart. The maker’s torque always takes precedence.

Threads easily mistaken for M8 x 1.25

Thread	Diameter (in)	Diameter (mm)	Pitch
M8 x 1.25 (this page)	0.3150	8.00	1.25 mm (20.3 TPI)
M8 x 1	0.3150	8.00	1 mm (25.4 TPI)
5/16-18 UNC	0.3125	7.94	18 TPI (1.41 mm)
5/16-24 UNF	0.3125	7.94	24 TPI (1.06 mm)

Threads within 6% of the same diameter, from our UNC/UNF and metric charts. None of them interchange with this one.

Tips for tapping M8 x 1.25

M8 x 1.25 and 5/16-18 are very close (0.315 vs 0.3125 in, 1.25 mm vs 1.41 mm pitch); they start in each other’s holes and then jam.
For M8 x 1 fine, drill 7.0 mm; for M8 x 0.75, drill 7.2 to 7.25 mm.
An M8 hex bolt needs a 13 mm wrench; an M8 socket head cap screw takes a 6 mm hex key.
In aluminium, give an M8 thread about 16 mm of engagement, or use a steel thread insert if the part is assembled often.

Frequently asked questions

What size drill bit for an M8 x 1.25 tap?

6.8 mm, which gives about 74% thread. The exact 75% size is 0.2670 in (6.78 mm).

How deep should a blind M8 x 1.25 hole be?

For a full thread of 1 × diameter in steel, drill about 16.8 mm at full diameter with a plug tap, or 18.8 mm to the drill tip; in aluminium, about 24.8 mm.

What drill for an M8 x 0.75 tap?

7.25 mm in theory (8 minus 0.75); 7.2 mm is the nearest common bit and gives a deeper thread, about 82%.

What size drill for an M8 tap in inches?

Letter H (0.266 in, 6.76 mm) is the closest inch drill to 6.8 mm; 17/64 in (6.75 mm) is almost the same.

Sources and method

Every dimension and percentage on this page is computed from the formulas listed, not copied from another chart. Values are basic (nominal); real parts are made to tolerance classes (2A/2B for inch, 6g/6H for metric).

ASME B1.1 and FED-STD-H28/2B (Unified inch), ISO 68-1 and ISO 261 (metric): basic thread profile and stress area formulas.
Machinery’s Handbook (Industrial Press): percentage-of-thread relation used by our tap drill size calculator.
ASME B94.11M twist drill series (number, letter, fractional); 0.1 mm metric drill steps.
ISO 273 metric clearance holes; inch close, normal and loose fits as published in the ASME B18.2.8 clearance classes.
SAE J429 and ISO 898-1 proof strengths; T = K × D × F torque relation (see our bolt torque chart for the full method).
Wrench and hex key sizes: ASME B18.2.1, ASME B18.3, ASME B18.6.3, ISO 4014/4017, DIN 933 and ISO 4762 values as widely published.

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Last reviewed: 24 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.