Product Overview

The Marui Keiki SP-401 is a Japanese-made bevel protractor designed for precision angle measurement on the bench, surface plate, or machine. Built around a finely graduated dial and a stainless steel blade and stock, this is the kind of toolroom-grade instrument that earns its place alongside a surface gauge and a good straight edge. Whether you’re setting compound angles on a mill, checking taper work off the lathe, or verifying included angles on a formed cutter, the SP-401 delivers repeatable, readable results without the faff of a vernier protractor that has you squinting under a work lamp.

Circular dial graduation with magnifier-style readout — Unlike a traditional vernier protractor where you’re interpolating between two fine scales, the SP-401 uses a circular dial indicator-style readout that gives a direct, unambiguous angle reading. On the shop floor, that means faster setup and fewer misreads when you’re under time pressure.
Full 360° measurement capability via indexed blade and stock — The blade pivots through a full circle relative to the stock, letting you measure any included, complementary, or obtuse angle without repositioning the workpiece. That’s particularly useful when checking both faces of a dovetail or a chamfer in a single setup.
Stainless steel blade and ground reference stock — Both contact surfaces are ground stainless, meaning they sit flat and true against a workpiece without bowing or flexing under light clamping pressure. Ground references matter when you’re checking a 30° tool relief or a precision wedge — a soft or bowed blade will introduce error before you’ve read a number.
Clamp lock for holding a set angle — The blade clamps firm at any position, so you can transfer an angle directly to a workpiece, a sine bar setup, or a mill head without the reading drifting. Transferring angles is where a lot of cheaper protractors fall apart — the clamp here is positive.
Ideal for — setting compound slide angles on the lathe, checking taper and chamfer angles on turned or milled components, verifying tool grind angles (drills, form tools, end mills), setting a tilting vice or rotary table, and checking dovetail slides against nominal specification.

Condition

(Used — Good Condition / Pre-owned, fully functional, light bench patina)

Mechanically, this protractor is in good working order. The blade traverses smoothly through its full range, the clamp locks positively without slipping, and the dial reads cleanly without fogging, cracking, or parallax distortion to the face. The ground stainless surfaces on the blade and stock show the kind of light surface patina consistent with careful workshop use — there are no deep scores, dings, or edge damage that would compromise the reference surfaces or introduce measurable error. The dial mechanism engages and returns correctly, and the graduation markings are sharp and legible. As with any pre-owned precision angle instrument, it is good practice to verify the zero against a known square reference surface before putting it into critical work — confirm the blade reads 0° (or 90°, depending on orientation) when closed flat against a known ground reference such as a surface plate or precision square. This takes thirty seconds and gives you full confidence in the baseline.

Included:

1 x Marui Keiki SP-401 Single Protractor (bevel protractor with circular dial, stainless blade and stock)
Original wooden storage box
Original instruction sheet / manual (Japanese and English bilingual)

Not included: Height gauge holder accessory (referenced in the manual as a separately sold accessory — ハイトゲージホルダー — not present in this lot)

Brand Information

Marui Keiki (丸井計器) is a Japanese precision instrument manufacturer with a long history in the metrology and measuring tools segment of the Japanese industrial market. Japanese-made measuring instruments from this era sit in the same respected tier as Mitutoyo, RSK (Niigata Seiki), and Shintom — characterised by tight manufacturing tolerances, properly hardened and ground reference surfaces, and instruments built to survive daily toolroom use rather than sit in a cabinet. Marui Keiki tools are less commonly seen on the Australian market than Mitutoyo, which makes them a solid find for the machinist or toolmaker who values Japanese quality without the premium brand markup.

Specifications

Feature	Specification
Brand	Marui Keiki (丸井計器)
Model	SP-401
Type	Bevel Protractor — Circular Dial (Single Protractor)
Measuring Range	0°–360° (full circle via blade rotation)
Graduation / Resolution	As indicated on dial scale (refer to dial face — markings visible at 30° increments on arc scale; verify against manual for fine graduation value)
Readout Type	Circular dial with pointer — direct reading
Blade Material	Hardened and ground stainless steel
Stock / Body Material	Hardened and ground stainless steel
Clamp	Yes — blade lock clamp included
Country of Origin	Japan
Condition	Used — Good Condition (pre-owned, fully functional, light bench patina)
Included Items	Protractor, original wooden box, bilingual instruction sheet
Not Included	Height gauge holder accessory (sold separately per manual)

Keywords

Marui Keiki SP-401, Marui Keiki bevel protractor, Japanese bevel protractor, dial protractor precision, SP-401 protractor, angle gauge toolroom, precision protractor machinist, bevel protractor 360 degree, angle measurement surface plate, toolroom protractor Australia, machinist angle tool, compound angle setting mill lathe

Protractors

Marui Keiki SP-401 Single Protractor