Can I Use Metric and Imperial Screws Interchangeably With Countersunk Magnets?
No, not as a rule. Inch (imperial) flat-head screws have an 82° head and metric flat-head screws have a 90° head, and each countersunk magnet is made for one of them. Use a screw with the same angle as the countersink, a head no wider than the countersink and a shank that passes freely through the hole. A few sizes are dimensionally close, such as M3 and #4, M3.5 and #6, M4 and #8, and M5 and #10, but even then the head touches the magnet on a thin ring instead of the whole cone. That concentrates the force on brittle neodymium and can crack it, so treat a close cross-match only as a hand-tight fallback for light loads.
- Inch flat heads
- 82° (ASME standards)
- Metric flat heads
- 90° (ISO and DIN standards)
- Closest cross-matches
- M3/#4, M3.5/#6, M4/#8, M5/#10
- Rule of thumb
- Match the angle first, then the size
82° vs. 90°: What’s the Difference?
The angle is measured across the cone under the screw head. Inch flat-head screws are made to 82° (ASME B18.6.3 for machine screws, ASME B18.6.1 for wood screws). Metric flat-head screws are 90° (ISO 7046-1 and ISO 2009 for machine screws, ISO 10642 for hex socket screws, DIN 7997 for wood screws). That’s only 4° on each side of the cone, but it decides where the head presses on the magnet.
- Matching angles: the head touches the whole cone and spreads the force evenly.
- A 90° metric head in an 82° countersink touches only at the top edge of the countersink, right at the magnet’s face, where the ring is thinnest. The head can look flush while all the force sits on that edge.
- An 82° inch head in a 90° countersink touches only near the bottom of the countersink and stands slightly above the face. Tightening it down to flush drives it in like a wedge and splits the ring from the inside.
Neodymium magnets are sintered, which makes them hard but brittle. Wood or soft metal crushes a little and lets a slightly wrong head settle in; a magnet can’t, so it cracks instead. See how to install countersunk magnets without breaking them.
Metric vs. Imperial Flat-Head Screw Size Chart
Closest matches by shank and head diameter. These are dimensional comparisons only. Every pair below still has the 82° vs. 90° mismatch, so none of them is a confirmed substitute.
| Metric screw (90°), shank / head in mm | Closest inch screw (82°), shank / head in mm | How close |
|---|---|---|
| M2: 2.0 / 3.8 | #2: 2.2 / 4.4 | Shank close; inch head 0.6 mm wider |
| M2.5: 2.5 / 4.7 | #2: 2.2 / 4.4 | Approximate; #2 is a little smaller |
| M3: 3.0 / 5.5 | #4: 2.8 / 5.7 | Close |
| M3.5: 3.5 / 7.3 | #6: 3.5 / 7.1 | Close |
| M4: 4.0 / 8.4 | #8: 4.2 / 8.4 | Close |
| M5: 5.0 / 9.3 | #10: 4.8 / 9.8 | Close; inch head 0.5 mm wider |
| M6: 6.0 / 11.3 | #12: 5.5 / 11.1 | Head matches; shank 0.5 mm thinner |
| M6: 6.0 / 11.3 | #14 (wood screw) or 1/4: 6.1 to 6.4 / 12.9 | Shank matches; head 1.6 mm wider |
| M8: 8.0 / 15.8 | 5/16: 7.9 / 16.1 | Close |
| M10: 10.0 / 18.3 | 3/8: 9.5 / 19.4 | Approximate; inch head 1.1 mm wider |
Shank is the thread’s outside diameter. Head is the maximum head diameter in ISO 7046-1 (metric) and ASME B18.6.3 (inch). #14 comes from ASME B18.6.1, because it’s a wood and sheet metal screw size, not a standard machine screw. Real heads usually run a little smaller than the maximum, and other styles differ: a 3.5 mm metric wood screw (DIN 7997) has a 6.5 mm head, not 7.3 mm. Measure the screw before you count on a match.
Why a Matching Head Size Isn’t Enough
M4 and #8 heads are both about 8.4 mm across, but they meet the countersink at different angles, so they seat at different depths and press on different spots. The shank matters too: it has to pass freely through the hole. An M5 (5.0 mm) won’t go through the 4.95 mm hole made for a #8 screw, even though the heads look close.
Which Combinations Work?
| Combination | What happens | |
|---|---|---|
| Compatible | An 82° inch flat head in an 82° countersink, or a 90° metric flat head in a 90° countersink. Head no wider than the countersink, shank passes through the hole | The head bears on the whole cone. One size smaller also works if the head still seats on the cone |
| Works with Limits | A close cross-match from the chart, such as M4 in a #8 countersink, with the head no wider than the countersink | Contact on a thin ring only. Tighten by hand, stop at snug, use it for light, steady loads, and swap in the matching screw when you can |
| Avoid | A head wider than the countersink, a shank that doesn’t pass freely through the hole, pan, round, truss or bugle heads (drywall and deck screws), or 100° flat heads | Loads the thin edge or wedges the ring apart. A cracked magnet is likely |
Which Screws Fit Our Countersunk Magnets?
Each product page lists the countersink as the diameter at the face, the hole diameter and the angle. Match that first; the other-standard screw is only a fallback.
| Our countersink | Matching screw | Closest other-standard screw | Too big |
|---|---|---|---|
| 0.242 × 0.136 Inch, 82° | #4 | M3 | M3.5 and up |
| 0.295 × 0.17 Inch, 82° | #6 | M3.5 | M4 and up (8.4 mm head) |
| 0.35 × 0.195 Inch, 82° | #8 | M4 | M5 and up (5 mm shank) |
| 0.40 × 0.22 Inch, 82° | #10 | M5 | M6 and up |
| 6.5 × 3.5 mm, 90° | M3 | #4 | #6 and up |
| 8.5 × 4.5 mm, 90° | M4 | #8 (a close fit) or #6 | #10 and up |
| 10.5 × 5.5 mm, 90° | M5 | #10 | #12 and up |
See which magnets use each countersink in how to install countersunk magnets.
If You Do Use a Cross-Match
Start it by hand, keep it square and stop as soon as the head is snug. Never drive it flush with a drill or impact driver: with a mismatched angle, the extra turns go straight into the magnet. If the head sits proud, use the right screw instead of tightening harder.
Takeaway
Match the countersink’s angle first, then its size: 82° inch screws in 82° countersinks, 90° metric screws in 90° countersinks. Similar head diameters don’t make screws interchangeable. Keep the close cross-matches for hand-tight, light-duty fixes when the right screw isn’t available.
- Countersunk Ring MagnetsScrew-Mount, Flush Head
- Countersunk Block MagnetsScrew-Mount Rectangles
- Pot (Mounting) MagnetsSteel Cup Focuses the Pull
More Questions About Countersunk Magnets
What angle are flat-head wood screws?
Inch wood screws are 82° (ASME B18.6.1) and metric wood screws are usually 90° (DIN 7997). Bugle-head drywall and deck screws have a curved underside that doesn’t seat in a countersink, so don’t use them.
How can I tell if a screw is 82° or 90°?
Check the package. Inch screws (sizes like #8 or 1/4, threads like 8-32) are made to 82°, and metric screws (sizes like M4, threads like M4 × 0.7) to 90°. Hardware stores usually shelve them separately.
Can I use a smaller screw than the one listed?
Yes, one size smaller with the same angle works as long as the head still seats on the cone. A wider head never does. See how to install countersunk magnets.
Are your countersunk magnets paired north and south?
Most of our countersunk packs are half north and half south so pairs attract. See how pairing works.