Fits and tolerances: Clearance, transition, and interference

This system allows components produced at different times or locations to assemble correctly without custom hand-fitting. By specifying a standardized fit, engineers control whether parts will slide freely, lock together by friction, or align with high precision.

The basics: How the limits and fit system works

Standardized alphanumeric codes, such as 40H7/g6, are used by engineers to define the
allowable deviation from the nominal size. This code provides all necessary information for
the machinist.

In this code the number (e.g., 40) represents the nominal size in millimeters. The capital
letter (H) represents the tolerance zone for the hole, and the lowercase letter (g) defines the
zone for the shaft. The following number (7 or 6) denotes the International Tolerance (IT)
grade. This specifies the ‘tightness’ of the tolerance zone. A standard engineering choice is
a H7 hole, as it can be produced using high-quality reamers.

The three types of engineering fits

The relationship between the hole and shaft tolerances determines the mechanical behavior
of the assembly. There are three primary categories, each serving a distinct functional
purpose.

Clearance fit: Designed for movement

A clearance fit ensures that the shaft diameter is always smaller than the hole diameter. It
leaves a physical gap between the mating surfaces. This gap is essential for components
that must move relative to each other without seizing. Within this category, engineers
differentiate between degrees of freedom based on the intended motion.
Running fits provide enough space for a stable lubrication film to form between the surfaces.

This is critical for parts in continuous motion, such as rotating shafts and engine pistons,
where heat dissipation and friction reduction are paramount. In contrast, sliding fits utilize
much tighter tolerances with minimal clearance.

These fits are not intended for high-speed rotation but rather for linear precision. They are
commonly specified for components that must slide accurately into position, such as a
tailstock on a lathe, where maintaining a precise location is more important than free
rotation.

Transition fit: Designed for precision location

In a transition fit, the tolerance zones of the hole and the shaft overlap. Depending on the
actual manufactured dimensions within the allowed range, you end up with either a slight
clearance or a slight interference. These fits are not meant for continuous movement but for
accurate positioning.

Assembly usually requires a rubber mallet or a light manual press. Engineers specify
transition fits for aligning pins, gear hubs on shafts, and other components that must be
dismantled for maintenance.

Interference fit: Designed for friction-locked assembly

Also known as a ‘press fit’ or ‘shrink fit’, an interference fit ensures the shaft is always larger
than the hole. This creates a permanent, friction-locked connection where the parts act as a
single unit.

Because the metal must deform slightly or be thermally expanded/contracted to assemble,
these fits can transmit torque without the need for keys or fasteners. Common applications
include pressing bearings into housings or bushings into control arms.

Hole-basis vs. shaft-basis: Which to choose?

Most machine shops operate on a hole-basis system. Standard cutting tools, such as drills
and reamers, produce fixed-diameter holes that are difficult to ‘tune’ to a custom dimension.
It is far more cost-effective to produce a standard H7 hole and then adjust the shaft diameter
on a CNC lathe to achieve the desired fit. Shaft diameters are easily modified with high
precision, whereas creating a non-standard hole often requires expensive custom tooling or
slower EDM (Electrical Discharge Machining) processes.

The cost of precision

Tolerances aren’t free. As the IT grade decreases (meaning the tolerance becomes tighter),
the manufacturing cost increases exponentially. Moving from a standard milling tolerance
(IT9) to a precision grinding or honing tolerance (IT5) can easily triple the production cost per
unit.

Tight tolerances require more frequent tool inspections, slower feed rates, and specialized
metrology equipment. Engineers should only specify the tightest fits where functionally
necessary to prevent over-engineering the part.

Design checklist for specifying fits

Before finalizing a technical drawing, engineers should ensure that the selected fit reflects
real manufacturing constraints. This helps avoid unnecessary costs, assembly issues, or
premature failures.

Start by defining the mechanical function of the assembly. Does the part need free rotation
with a clearance fit, accurate location with a transition fit, or a permanent friction-based lock
with an interference fit?

Once the function is defined, choose the appropriate tolerance standard. In most cases, a
hole-basis system is preferred because it allows the machine shop to use standard reamers
and off-the-shelf tooling, reducing the need for custom tools and lowering production costs.

Once you have chosen a basis, choose one of the standard combinations of industry
standards. Pairings such as H7/g6 are a good choice for precise-running applications, and
H7/p6 has historically been considered the standard for press fits.

Lastly, you should ensure that what was decided upon for assembling the parts fits well with
what can be done on the shop floor. An example here could be a high-interference fit, which
would require a hydraulic press to put the shaft in the housing, or a high-temperature heating
process such as liquid nitrogen to shrink the shaft or an oven to expand the housing.

By confirming these assembly requirements at the outset, you will ensure the design is
usable and provides the most cost effective and repeatable manufacturing method.
Worried about over-engineering your tolerances? Avoid unnecessary costs by getting a
professional review. Upload your CAD file for a quote.