The Marameter 844 D indicating plug gage was previously sold as the Dimentron plug. It is a two-point bore gage: two opposed carbide contacts are spring-loaded outward, and a carbide pin carries their radial movement to a dial comparator or digital indicator. The reading is the bore's deviation from the setting ring used to zero the gage.

A two-point gage reads the true diameter only when its contacts sit straight across the middle of the bore and square to it. Off to one side, the contacts measure a line that misses the center, which is shorter than the diameter, so the gage reads small. Tipped, they measure on a slant from wall to wall, which is longer than the diameter, so the gage reads large.

A split-ball gage centers itself side to side but can still tip, so the operator rocks it and takes the smallest reading, the reversal point. The 844 D handles both errors mechanically. Its guide cylinder is ground just under the bore size, so once it is in the hole it can neither slide sideways nor tip. There is no reversal point to find: the first reading is the reading.

• Measuring force comes from a built-in spring, so it does not vary with the operator.
• Diameter, roundness and conicity: one reading gives the diameter. Rotating the gage shows roundness deviation, and readings at several depths show conicity.
• Published performance: repeatability 1 µm (≈ .00004"); linearity deviation 1 % (min. 1 µm), 1.5 % on the DS types.

844 D

Ø 2–200 mm (≈ .079–7.87"). e = 1.5–4 mm. For fast checks of precise bores in general work.

Choose it when the bore is open and the characteristic is not right at an edge or bottom.

844 DR

Ø 4–200 mm. e = 6–10 mm. An extended guide cylinder runs past the contacts, so the gage stays aligned in through holes and in parts thinner than the guide.

Choose it when the bore is in sheet or a thin wall, or must be measured from the edge of a through hole.

844 DS

Ø 2–200 mm. e = 0.7–1.2 mm. The contacts sit close to the front face. Linearity deviation is 1.5 % (min. 1 µm).

Choose it when the diameter must be checked close to the bottom of a blind hole.

Holders, reach and measuring depth

• 844 Dg holders take Ø 8 mm indicator shanks and M6 × 0.75 or M10 × 1 plug gages, in lengths from 109 to 249 mm.
• 844 Dga adds a lifting lever that retracts the contacts. It eases a DS gage into a blind hole and is recommended for small, sensitive bores.
• 844 Dg-XL is the heavy holder for plug gages from Ø 60 mm, especially on long Ø 18 mm extensions.
• 844 Dv extensions run from 48 to 1000 mm (≈ 1.9–39.4"). 844 Dw turns the measuring axis 90° for side bores and for checks in a lathe or grinder.

Depth stops set a repeatable measuring depth and keep the gage from tilting on entry: the 844 Dt-R ring clamps on the guide cylinder (Ø 3–105 mm). The Dt-S disc (Ø 16–40 mm), Dt-B bridge (Ø 16–160 mm) and Dt-3 three-point stop (Ø 20–150 mm) clamp on the holder or extension shank.

Setting ring and temperature

The 844 D is zeroed on a setting ring at the nominal size, so the reading is only as good as the ring. It also depends on how closely the ring and the part share a temperature. A steel ring and a steel part at the same temperature grow together, and the error largely cancels.

Aluminum does not cancel. Take an aluminum bore (≈ 23 µm/m·K) checked against a steel ring (≈ 11.5 µm/m·K), both at 25 °C instead of the 20 °C reference. At Ø 100 mm the differential growth is 11.5 µm/m·K × 0.100 m × 5 K = 5.75 µm (≈ .00023") (calculated). That is more than five times the gage's repeatability.

The 844 K covers Ø 0.95–20.6 mm (≈ .037–.811") with interchangeable split-ball probes on one holder. Its repeatability (1 µm) and linearity deviation (1 % min. 1 µm, 2 % on Ø 1–1.4 mm probes) match the 844 D.

The trade-off is centering. The two halves of the split ball expand equally, so the contacts center side to side and land on a diameter rather than a chord. The short ball can't hold the probe square to the bore axis, though. The operator rocks the gage through the axis, and the smallest reading, the reversal point, is the one taken square to the bore. A 1087 BR/BRi digital indicator records that minimum with its MIN function, and the 844 KM measuring stand removes the rocking step altogether.

Each probe covers a band rather than one size: 0.2 mm (Ø 1–1.4), 0.4 mm (Ø 1.75–3.75), 0.6 mm (Ø 4–9.5) or 1.2 mm (Ø 10–20). Cased sets cover the full range in five groups. The probes come in hard chrome (844 K), DLC-coated (844 KC) and blind-hole (844 KS) versions. The 844 Kgz holder takes 3/8" indicator shanks.