The calculator
Runs in your browser. Inches or millimetres at the top.
How the check works
Depth the head needs. An MS20426 rivet has a 100° head. Machine-countersunk to sit flush, the cone runs from the drilled hole up to the head diameter, so its depth is (head Ø − hole Ø) / (2 × tan 50°). For a 1/8 in rivet in a #30 hole: (0.225 − 0.1285) / 2.384 = 0.040 in.
Depth the sheet can spare. The FAA Airframe Handbook: “In primary structure and pressurized skin repairs, some manufacturers are currently recommending the countersink depth be no more than 2/3 the outer sheet thickness or down to 0.020-inch minimum fastener shank depth, whichever is greater. Dimple the skin if it is too thin for machine countersinking.”
Our reading. Both figures limit the straight hole left under the countersink: at least a third of the sheet, and at least 0.020 in. The calculator keeps whichever is greater, the conservative reading of the sentence. On thin sheet the 0.020 in governs; above 0.060 in the 2/3 rule does.
The old rule, that the head had to be contained in the outer sheet, was dropped because countersinks cut too deep caused fatigue cracks in pressurized skin. The tool guide on dimple dies and countersinks covers which alloys dimple cold and which need hot dimpling.
Worked example
MS20426AD4 rivets (1/8 in, #30 hole) in 0.040 in and in 0.063 in skin.
- Countersink depth = (0.225 − 0.1285) / (2 × 1.192) = 0.040 in.
- 0.040 in sheet: keep max(0.040/3, 0.020) = 0.020 in, so the deepest allowed is 0.020 in. The head needs 0.040 in, the whole sheet: a knife edge. Dimple.
- 0.063 in sheet: keep max(0.021, 0.020) = 0.021 in, deepest allowed 0.042 in. 0.040 ≤ 0.042: countersink, just.
A 1/8 in flush rivet needs about 0.061 in of outer skin to countersink: in practice 0.063 in sheet. Below that, dimple.
Limits — when NOT to use this
- Approved data governs. The Handbook reports what “some manufacturers” recommend for primary structure and pressurized skin. The SRM or repair drawing may set a different limit.
- Head diameter. We use the middle of the MS20426 tolerance band. A head at the top of the band needs about 0.002 in more depth.
- Other flush fasteners (NAS1097 reduced heads, Hi-Loks, blind rivets) have different heads and their own installation data.
- Dimpling has limits too. Hard alloys such as 7075-T6 crack when dimpled cold. See the dimple-die guide before you choose.
Questions people actually ask
Can I countersink the bottom sheet and dimple the top?
Yes, that is the usual answer when the top sheet is thin and the one under it is thick: the top sheet is dimpled and nests into a countersink cut in the sheet below. Check the lower sheet's thickness in this calculator.
Why not a 2/3 rule only?
On thin sheet two-thirds leaves almost nothing: in 0.032 in skin it would allow a 0.021 in countersink over 0.011 in of hole. The 0.020 in shank depth keeps enough bearing for the rivet.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4, Countersinking — the 2/3 outer sheet and 0.020 in minimum shank depth recommendation, quoted above; 100° as the common countersink angle; dimple when too thin to countersink.
- MS20426 rivet head diameter limits, from distributor data sheets — 3/32: 0.175–0.183 in; 1/8: 0.221–0.229 in; 5/32: 0.282–0.290 in; 3/16: 0.349–0.357 in; 100° head. Taken from listings that cite MS20426, not from the standard itself.
- Rivet drill size chart — #40, #30, #21 and #11 holes for 3/32 to 3/16 in rivets.