What size drill for an M10×1.25 tap?

8.8 mm (8.8 mm), which gives about 75% thread engagement.

What clearance hole for M10×1.25?

10.5 mm for a close fit — enough to pass the bolt without slop.

What spanner fits M10×1.25?

16 mm across the flats on a standard hex head.

M10×1.25 thread

M10×1.25 tap drill

8.8 mm

Clearance hole 10.5 mm · 16 mm hex

M10×1.25 is a fine metric thread with a 10 mm (0.394") major diameter and 1.25 mm pitch. Tap it with a 8.8 mm drill; drill 10.5 mm to pass a bolt through.

The numbers

Designation: M10×1.25
Major diameter: 10 mm / 0.394"
Minor diameter: 8.647 mm
Pitch: 1.25 mm pitch
Tap drill (75% thread): 8.8 mm — 8.8 mm
Clearance hole (close fit): 10.5 mm / 0.413"
Hex across flats: 16 mm

How to measure what you have

Measure across the thread crests with calipers. A used M10×1.25 reads a little under 10 mm — the crests are truncated by the standard, then worn and plated, so anything from about 9.6 mm up is consistent with this size.
Measure the pitch: put a rule against the threads and count the distance over ten crests, then divide by ten. This one should give 1.25 mm.
A thread pitch gauge settles it in seconds and costs very little. Guessing between a coarse and a fine thread of the same diameter is the usual way a thread gets stripped.

Tightening torque

Typical torque

43–58 N·m

31.7–42.8 lb·ft

Plain steel property class 8.8 or SAE grade 5, dry threads, tightened to about 65% of yield. Lubricated threads reach the same preload at 20-30% less torque. Where the manufacturer publishes a figure — spark plugs, wheel nuts, cylinder heads — use theirs.

Where this size turns up

Motorcycle brake banjo bolts
Oil drain plugs and sensor ports
Suspension and linkage adjusters

Other threads at this diameter

Same diameter, different pitch. Forcing the wrong one strips the thread in a turn or two.
Thread	Pitch	Tap drill	Clearance
M10×1.5	1.5 mm	8.5 mm	10.5 mm
M10×1	1 mm	9 mm	10.5 mm

Common questions

What size drill for an M10×1.25 tap?: 8.8 mm (8.8 mm), which gives about 75% thread engagement.
What clearance hole for M10×1.25?: 10.5 mm for a close fit — enough to pass the bolt without slop.
What spanner fits M10×1.25?: 16 mm across the flats on a standard hex head.

M10×1.5 8.5 mm tap drill
M10×1 9 mm tap drill
M8×1.25 6.8 mm tap drill
M12×1.75 10.2 mm tap drill
M8×1 7 mm tap drill
M8×0.75 7.2 mm tap drill
M12×1.5 10.5 mm tap drill
M12×1.25 10.8 mm tap drill
M12×1 11 mm tap drill
M6×1 5 mm tap drill
M14×2 12 mm tap drill
M6×0.75 5.2 mm tap drill

Sources

Official vendor product documentation — At One Place
Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked