Inside Outside Caliper

Quick Answer: The Inside Outside Caliper is a transfer-measuring tool with hinged legs: outward-turned tips for internal sizes such as bores and slots, and inward-curved legs for external sizes such as shafts and plates. An inside outside caliper has no scale of its own; the size it picks up is read against a steel rule.

Measuring by Feel and Transfer

Before sliding calipers became common, turners and fitters sized work with plain calipers, and these are still taught in workshop practice because they train a sense of touch. The legs are opened until the tips just drag across the work, the setting is held, and the distance between the tips is then read on a rule or compared directly with another part.

Outside legs bow inward so they can pass over a round bar and meet on its widest diameter. Inside legs turn outward at the tips to reach the walls of a bore. The listing describes the item as covering both forms; whether it is supplied as two separate calipers or another arrangement, and whether the legs use a spring and adjusting nut or a firm friction joint, should be confirmed when ordering.

Results depend on the user’s feel and on how carefully the setting is read, so plain calipers suit roughing cuts, turning to size in stages and checking that a shaft will enter a bore. For a recorded value, the vernier caliper or the micrometer screw gauge gives a direct reading.

Specifications

Care & Handling

• Always stop the machine before offering calipers to work held in a lathe.
• Set the legs to a light drag, not a forced fit; springing the legs gives a false size.
• Keep the tips clean and free of burrs, since a damaged tip changes the transferred size.
• Store the calipers closed or in a rack so the legs are not bent.

Workshop Uses

• Checking a turned diameter between cuts, with the lathe stopped
• Transferring the bore of a bush to a shaft that must fit it
• Fitting and turning exercises that build measurement by feel in ITI and polytechnic courses
• Measuring inside a recess where the jaws of a sliding caliper cannot reach

Why Choose LabEquip

Workshop instructors in ITIs, polytechnics and engineering colleges buy plain calipers for fitting and turning practice alongside rules and sliding calipers. LabEquip lists this item in the General Lab Products range; tell us the form and leg length you need through the contact page.

Frequently Asked Questions

How is an inside outside caliper read?

It has no scale. After setting the legs on the work, hold one tip against the end of a steel rule and read where the other tip falls, or offer the calipers to a second part to compare the two sizes directly.

What is the difference between inside and outside legs?

Outside legs curve inward so the tips can close over a bar or plate. Inside legs point outward at the tips so they can open against the walls of a hole or slot. Each form is used only for its own kind of measurement.

How do I know the setting is right?

The tips should just touch and drag lightly as they pass over the widest part of a shaft or across a bore. If they have to be forced, the setting is too small; if there is no drag at all, it is too large.

Can plain calipers replace a vernier caliper?

No. They transfer a size but do not read it, so their accuracy is limited by the rule and the user’s feel. They are useful for quick comparisons and for places a sliding caliper cannot reach.

What is a spring caliper?

It is a plain caliper whose legs are held apart by a bow spring and set with a threaded nut, so the setting holds firmly and can be adjusted finely. Firm-joint calipers instead rely on friction at the pivot and are set by tapping the legs.

Is it safe to measure work while it turns on a lathe?

No. Stop the lathe before measuring. Calipers caught by rotating work can be dragged from the hand and cause injury, and a moving surface also gives an unreliable setting.

Last Updated: September 2026