Honing

Stones turn and stroke inside a bore

Honing finishes the inside of a hole by scrubbing it with abrasive stones that turn and slide up and down at the same time. It gives an engine cylinder its round, straight bore and the fine crisscross pattern that holds oil for the piston rings. Hydraulic cylinders, gear bores and fuel-injection parts are honed too.

Photo: Winsletsnyder, CC0, via Wikimedia Commons

honing a cylinder bore · section view and the unrolled cross-hatchCircled numbers match the steps below; the right panel shows the bore wall as if it were cut open and laid flat.

How it works

Insert the hone. The honing tool slides into a bore that was already drilled, bored or ground close to size. Its shaft often floats slightly, so the tool follows the bore's own axis.
Expand the stones. A cone or wedge inside the tool pushes several long abrasive stones (sticks of bonded grit) outward against the wall with steady, gentle pressure.
Turn and stroke together. The tool rotates while moving up and down along the bore. Because the stones are long, they rub the high spots first and wear them away, so the bore becomes rounder and straighter.
Form the cross-hatch. Every grain draws a slanted scratch, and scratches from up-strokes cross those from down-strokes. The angle, often around 45°, is set by the ratio of stroking speed to turning speed.
Flush and measure. Honing oil washes away chips and worn grit. A gauge checks the diameter, and honing stops at size, typically after removing a few hundredths of a millimeter.

Key facts

Materials: Cast iron, steels (hardened too), aluminum bores with liners or coatings, bronze, ceramics
Typical parts: Engine cylinders, hydraulic cylinders, connecting-rod and gear bores, valve and injector bodies
Part size: Bores from a few millimeters to around 1 m across; hydraulic tubes can be several meters long
Precision: Roundness and straightness within a few micrometers; size typically held to about ±0.005 mm
Surface finish: A controlled cross-hatch; roughness typically Ra 0.1–0.8 µm, depending on the grit
Batch size it suits: From a single engine rebuild to automated lines honing millions of cylinders
Tooling and machine cost: Stones and hone heads cost tens to hundreds of dollars; production honing machines are a major investment
Speed: Typically under a minute per bore in production; longer for big bores

Strengths and limits

Strengths

Corrects roundness, straightness and taper left by earlier steps
Leaves an oil-holding cross-hatch that helps piston rings seal and wear in
Works on hardened steel and cast iron
Low forces and temperatures, so no burned or stressed surface layer

Limits

Removes little material, so the bore must already be close to size
Cannot move a bore's position: the floating tool follows the existing hole
Mostly limited to round bores (some machines also do outside diameters or flats)
Stones wear, and their grit must suit the material and the finish wanted

Where it's used

AutomotiveEngine cylinder bores, connecting-rod and gear bores
HydraulicsCylinder tubes, valve bodies, pump housings
Fuel systemsInjector and fuel-pump bores
AerospaceLanding-gear cylinders, actuator bores
Engine rebuildingRe-honing worn cylinders before fitting new piston rings

How to recognize it

A fine crisscross of straight scratches inside a bore, at a regular angle.
A satin, even grey finish in a cylinder rather than a mirror shine.
On a worn engine, the cross-hatch is polished away where the rings rub most.
Honed hydraulic tubes feel smooth but show faint crossing lines in raking light.