Tap drills
M8 x 1.25 tap drill size
The tap drill for M8 x 1.25 is 6.8 mm, which is 0.2677 inches or 6.80 mm. That leaves about 89 percent thread engagement.
M8 x 1.25 tap drill, clearance and engagement
| Series | Metric coarse |
| Major diameter | 0.3150 in (8.00 mm) |
| Pitch | 1.25 mm |
| Tap drill | 6.8 mm (0.2677 in / 6.80 mm) |
| Thread engagement | 89% |
| Drill for 75% thread | 0.2750 in |
| Clearance, close fit | 8.4 mm (0.3307 in) |
| Nearest metric drill | 6.8 mm (0.2677 in) |
What M8 x 1.25 is used for
The general-purpose metric thread one size up from M6. Engine ancillaries, machine frames, motor mounts and structural brackets.
If you do not have that drill
7.0 mm drops engagement to about 74 percent and taps noticeably more easily in stainless, at a small cost in thread strength.
How to tap a M8 x 1.25 thread
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Drill the hole
Use a 6.8 mm drill, 0.2677 inches or 6.80 mm. Drill square to the surface and clear the swarf on deep holes.
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Start the tap square
Start with a taper tap and check it is square in both axes within the first two turns. A tap started crooked will cut a crooked thread all the way down and cannot be corrected.
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Cut and break the chip
Turn forward about half a turn, then back a quarter turn to break the chip. Use cutting fluid on steel and stainless; aluminium and brass usually need none, though a wax stick helps in aluminium.
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Check the fit
Run the mating fastener in by hand. It should turn freely without side play. If it binds, the hole was undersized or the tap wandered.
Common questions
What size drill do I need for a M8 x 1.25 tap?
- The tap drill for M8 x 1.25 is 6.8 mm, which is 0.2677 inches or 6.80 mm. That leaves about 89 percent thread engagement. 7.0 mm drops engagement to about 74 percent and taps noticeably more easily in stainless, at a small cost in thread strength.
What is the clearance hole for a M8 x 1.25 bolt?
- 8.4 mm, which is 0.3307 inches. That is a close fit. A normal fit is one drill size larger and a loose fit two, and the looser classes are the sensible choice when the parts have to line up without reaming.
How is the M8 x 1.25 tap drill worked out?
- Subtract the pitch from the major diameter: 0.3150 inches (8 mm) minus 1.25 mm gives 6.75 mm, and 6.8 mm is the nearest standard drill. This is the metric convention and it lands near 89 percent engagement, not the 75 percent an inch chart targets.
Can I use a metric drill for a M8 x 1.25 tap?
- The closest standard metric drill is 6.8 mm, which is 0.2677 inches against 0.2677 for 6.8 mm. That changes engagement to about 89 percent, so check that figure is acceptable for your material before substituting.