Alignment Pin Selection
MS16556 / CRES / Straight Dowel
Alignment Pin
Fits, tolerances and part numbers for straight, headless dowel pins (MS16556, superseded by NASM16556 [4]). A dowel pin locates mating parts precisely and repeatably across disassembly. The pin carries no load itself; the fit between pin, hole and mating hole does the work. Designers and machinists use the page to choose the fit, the tolerance and the process for each hole, then pick the pin by part number.
What the page covers
- Fit Classifications: the preferred clearance, transition and interference fits in hole-basis and shaft-basis form, from Shigley [1].
- IT Grade Tolerance Values: the IT5 to IT9 zone widths, in µm, for basic sizes up to 50 mm, from ISO 286-1 [2].
- Machining Process → Achievable IT Grade: the grade each process can usually hold, after Machinery’s Handbook [3].
- Alignment Pin Selection: pick a Diameter and the card lists every cataloged length with its MS16556 dash number and a Copy button [4].
Reading a fit
A fit such as H7/g6 names the hole first and the shaft (here, the pin) second. The letter places the tolerance zone relative to basic size. H holes start at basic and go up; h shafts start at basic and go down. The number is the IT grade, which sets the zone’s width. Hole-basis fits are the common choice, because reamers come in fixed sizes while a pin can be ground to any size. Lengths in the pin card show the fraction and its decimal in inches.
Limits and common mistakes
- The IT table is metric. Convert an inch diameter to mm before you pick the row, and convert the µm result back.
- An IT grade is only a width. The zone’s position comes from the fit letter, which needs the ISO 286 deviation tables [2].
- The process grades are approximate. Real capability depends on the machine, the tooling and the setup.
- Dash numbers are sequential in the standard, not coded from size, so a gap between diameters is not a missing pin.
Worked example
A 3/8″ thick fixture plate must come off a base and go back in the same place. It is located by two 1/4″ dowels, pressed 3/8″ into the base and slip-fit through the plate.
- From Fit Classifications, take H7/p6 (locational interference) for the base and H7/g6 (sliding fit) for the plate.
- 1/4″ is 6.35 mm, so read the 6–10 row of IT Grade Tolerance Values: IT6 9 µm · IT7 15 µm Each H7 hole has a 15 µm (0.0006″) zone.
- Machining Process → Achievable IT Grade gives Reaming as IT6–IT9 (precision reamers IT6–IT7). Use a precision reamer for both holes.
- In Alignment Pin Selection, choose Diameter 1/4″ (0.250″). The pin length is 3/8 + 3/8 = 3/4″: 3/4″ (0.750) · MS16556-646
Check: 15 µm ÷ 25.4 = 0.00059″, about half a thousandth, which is what a reamed precision hole holds. IT7 is about 1.6 times IT6 in every row (15 / 9 here), the standard step between ISO grades [2].
Fit Classifications
| Class | Hole Basis | Shaft Basis | Description |
|---|---|---|---|
| Clearance Fits | H11/c11 | C11/h11 | Loose running fit for wide commercial tolerances or allowances on external members. |
| H9/d9 | D9/h9 | Free running fit not for use where accuracy is essential, but good for large temperature variations, high running speeds, or heavy journal pressures. | |
| H8/f7 | F8/h7 | Close running fit for running on accurate machines and for accurate location at moderate speeds and journal pressures. | |
| H7/g6 | G7/h6 | Sliding fit not intended to run freely, but to move and turn freely and locate accurately. | |
| H7/h6 | H7/h6 | Locational clearance fit provides snug fit for locating stationary parts; but can be freely assembled and disassembled. | |
| Transition Fits | H7/k6 | K7/h6 | Locational transition fit for accurate location, a compromise between clearance and interference. |
| H7/n6 | N7/h6 | Locational transition fit for more accurate location where greater interference is permissible. | |
| Interference Fits | H7/p6 | P7/h6 | Locational interference fit for parts requiring rigidity and alignment with prime accuracy of location but without special bore pressure requirements. |
| H7/s6 | S7/h6 | Medium drive fit for ordinary steel parts or shrink fits on light sections, the tightest fit usable with cast iron. | |
| H7/u6 | U7/h6 | Force fit suitable for parts which can be highly stressed or for shrink fits where the heavy pressing forces required are impractical. |
IT Grade Tolerance Values
| Basic Size Range (mm) | IT5 (µm) | IT6 (µm) | IT7 (µm) | IT8 (µm) | IT9 (µm) |
|---|---|---|---|---|---|
| 0–3 | 4 | 6 | 10 | 14 | 25 |
| 3–6 | 5 | 8 | 12 | 18 | 30 |
| 6–10 | 6 | 9 | 15 | 22 | 36 |
| 10–18 | 8 | 11 | 18 | 27 | 43 |
| 18–30 | 9 | 13 | 21 | 33 | 52 |
| 30–50 | 11 | 16 | 25 | 39 | 62 |
A width, not a location — h6 on a 20 mm shaft is a 13 µm zone running from 0 to −13 µm off basic size (h is always at-or-under basic), while H7 on the mating 20 mm hole is a 21 µm zone running from 0 to +21 µm. See the Press-Fit / Interference Fit calculator to carry a chosen fit through to contact pressure and stress.
Machining Process → Achievable IT Grade
Consolidated, approximate ranges — different references (and different shops) draw the boundary between adjacent grades slightly differently, and a given process's real capability depends heavily on machine condition, tooling, and setup. Treat this as an ordering aid for picking a process to hit a target grade, not a guarantee.
| Process | IT Grade |
|---|---|
| Lapping | IT3–IT5 |
| Honing / superfinishing | IT4–IT6 |
| Precision (cylindrical) grinding | IT4–IT5 |
| Grinding, standard | IT5–IT6 |
| Diamond turning / diamond boring | IT5–IT6 |
| Precision boring, fine turning | IT6–IT7 |
| Reaming | IT6–IT9 (precision reamers IT6–IT7; standard IT7–IT9) |
| Broaching | IT7–IT9 |
| Turning / boring, standard | IT7–IT9 |
| Milling | IT7–IT11 (finish IT7–IT9; rough IT10–IT11) |
| Planing / shaping | IT9–IT12 |
| Drilling | IT10–IT13 |
| Sawing, flame/plasma cutting | IT14–IT16 |
| Sand casting, die casting, forging (as-formed) | IT12–IT16 |