Table of Contents
TopicsTypes of Engineering Fits: Clearance, Interference & Transition Explained
Learn how engineering fits work—clearance, transition, and interference fits explained with real-world examples and tolerance charts.
When designing mechanical components, achieving the right fit between mating parts is crucial for ensuring functionality, durability, and performance. Whether it’s a precision-machined shaft fitting into a bearing or a fastener securing two components together, the type of fit chosen can determine the ease of assembly, load distribution, and even the lifespan of a system.
This is where industry standards for Preferred Limits and Fits for Cylindrical Parts, plays a key role. For example:
ISO 286-1:2010 – Geometrical Product Specifications (GPS) – ISO System of Limits and Fits
Standards provide engineers with a systematic approach to defining clearance, interference, and transition fits, helping manufacturers achieve optimal tolerances and allowances.
Why is fit an essential consideration in mechanical design? Selecting the right engineering fit ensures that mechanical parts function as intended—whether they need smooth movement (clearance fit), tight locking (interference fit), or a precise balance of both (transition fit). Without proper fit selection, parts may experience excessive wear, misalignment, or even catastrophic failure.
In this guide, we’ll break down the different types of fits, explain how standards classify them, and help you understand how to choose the right fit for your engineering applications. Whether you're a mechanical designer, machinist, or engineering student, this blog will give you the essential knowledge to ensure your components fit just right.
When designing mechanical components, achieving the right fit between mating parts is crucial for ensuring functionality, durability, and performance. Whether it’s a precision-machined shaft fitting into a bearing or a fastener securing two components together, the type of fit chosen can determine the ease of assembly, load distribution, and even the lifespan of a system.
This is where industry standards for Preferred Limits and Fits for Cylindrical Parts, plays a key role. For example:
ISO 286-1:2010 – Geometrical Product Specifications (GPS) – ISO System of Limits and Fits
Standards provide engineers with a systematic approach to defining clearance, interference, and transition fits, helping manufacturers achieve optimal tolerances and allowances.
Why is fit an essential consideration in mechanical design? Selecting the right engineering fit ensures that mechanical parts function as intended—whether they need smooth movement (clearance fit), tight locking (interference fit), or a precise balance of both (transition fit). Without proper fit selection, parts may experience excessive wear, misalignment, or even catastrophic failure.
In this guide, we’ll break down the different types of fits, explain how standards classify them, and help you understand how to choose the right fit for your engineering applications. Whether you're a mechanical designer, machinist, or engineering student, this blog will give you the essential knowledge to ensure your components fit just right.
Be Smart, Get Good Grades
A fit grade is a numerical designation that represents the tolerance level applied to a fit. The lower the number, the tighter and more precise the fit, while higher numbers allow for greater clearance or interference.
Fit grades typically range from Grade 3 (high precision) to Grade 13 (looser tolerances), and they apply to both holes and shafts. The fit grade system ensures that manufacturing processes remain standardized while providing engineers with the flexibility to choose an appropriate level of tolerance based on application needs.
References for tolerances can assist in the decision.
Fit grades typically range from Grade 3 (high precision) to Grade 13 (looser tolerances), and they apply to both holes and shafts. The fit grade system ensures that manufacturing processes remain standardized while providing engineers with the flexibility to choose an appropriate level of tolerance based on application needs.
References for tolerances can assist in the decision.
| Grade | Description | Use |
|---|---|---|
| Grade 3-5 | Tight tolerances | Used for high-precision applications, such as aerospace components and fine instrumentation. |
| Grade 6-7 | Medium precision | Common in general engineering where a balance between precision and manufacturability is required. |
| Grade 8-10 | Standard commercial fits | Used in general machine assembly, automotive parts, and industrial machinery. |
| Grade 11-13 | Loose tolerances | Suitable for applications where ease of assembly is more critical than precision, such as construction equipment and agricultural machinery. |
Grades are not the whole story, fitting in matters too!
In mechanical engineering, the relationship between a shaft and a hole is defined by the type of fit used. ASME B4.1 classifies these fits into several categories, depending on whether the components are meant to move freely, fit snugly, or be permanently joined through interference.
Let’s break down the key fit types, their applications, and examples of how they work in real-world engineering.
Let’s break down the key fit types, their applications, and examples of how they work in real-world engineering.
RC – Running or Sliding Clearance Fit
Definition:
RC fits provide a controlled amount of clearance between a shaft and hole, allowing for free movement while minimizing wobbling or excessive play. These fits are used where components need to slide, rotate, or operate with minimal friction.
Common Applications:
• Bearings, bearing bush and shafts in rotating machinery (motors, pumps, fans).
• Guide rails where smooth linear motion is required (sliding doors, CNC machine tools).
• Precision sliding parts in telescopic assemblies.
• Close running fits are often seen in precision machinery where tight yet smooth motion is needed, such as high speed spindles.
• Loose running fits are used when generous clearance is needed to accommodate lubrication, contamination, temperature expansion, and more.
Example:
Imagine a fan blade mounted on a motor shaft. The blade must have a free running fit to rotate without sticking. An RC sliding fit ensures the right amount of clearance for smooth motion while keeping wear minimal. Using references from major manufacturers will provide a good starting point when using common components like bearings.
RC fits provide a controlled amount of clearance between a shaft and hole, allowing for free movement while minimizing wobbling or excessive play. These fits are used where components need to slide, rotate, or operate with minimal friction.
Common Applications:
• Bearings, bearing bush and shafts in rotating machinery (motors, pumps, fans).
• Guide rails where smooth linear motion is required (sliding doors, CNC machine tools).
• Precision sliding parts in telescopic assemblies.
• Close running fits are often seen in precision machinery where tight yet smooth motion is needed, such as high speed spindles.
• Loose running fits are used when generous clearance is needed to accommodate lubrication, contamination, temperature expansion, and more.
Example:
Imagine a fan blade mounted on a motor shaft. The blade must have a free running fit to rotate without sticking. An RC sliding fit ensures the right amount of clearance for smooth motion while keeping wear minimal. Using references from major manufacturers will provide a good starting point when using common components like bearings.
LC – Locational Clearance Fit
Definition:
LC fits provide a small clearance but are intended for parts that stay in place after assembly rather than move freely. This type of fit allows for easy assembly but keeps the components loosely positioned.
Common Applications:
• Dowel pins and alignment holes in machine tools.
• Mounting brackets or covers that require easy installation but don’t need a tight fit.
• Components that require occasional disassembly for maintenance.
Example:
Consider an engine block with alignment dowels for mounting a cylinder head. The dowels guide the head into the correct position while allowing some clearance to compensate for thermal expansion and assembly tolerances.
LC fits provide a small clearance but are intended for parts that stay in place after assembly rather than move freely. This type of fit allows for easy assembly but keeps the components loosely positioned.
Common Applications:
• Dowel pins and alignment holes in machine tools.
• Mounting brackets or covers that require easy installation but don’t need a tight fit.
• Components that require occasional disassembly for maintenance.
Example:
Consider an engine block with alignment dowels for mounting a cylinder head. The dowels guide the head into the correct position while allowing some clearance to compensate for thermal expansion and assembly tolerances.
LT – Location Transition Fit (Clearance or Interference Fit)
Definition:
LT fits are a middle ground in the accuracy of location between clearance and interference. Depending on the actual tolerances of the parts, the fit may have a small clearance or a slight interference. This is useful when a part needs accurate location but doesn’t require extreme force for assembly.
Common Applications:
• Gear hubs on shafts, where some may slide on easily, and others may require light pressing where they may be assembled and disassembled.
• Rotors and stators in electric motors, where a balance of fit and friction is needed.
• Sleeve bearings, where the fit may be tight but still allows for controlled movement.
Example:
A pulley mounted on a keyed shaft in an industrial conveyor system. Some pulleys may slide on with minimal effort, while others require gentle tapping with a mallet to ensure a secure position. The fit prevents excessive movement but allows for occasional disassembly.
LT fits are a middle ground in the accuracy of location between clearance and interference. Depending on the actual tolerances of the parts, the fit may have a small clearance or a slight interference. This is useful when a part needs accurate location but doesn’t require extreme force for assembly.
Common Applications:
• Gear hubs on shafts, where some may slide on easily, and others may require light pressing where they may be assembled and disassembled.
• Rotors and stators in electric motors, where a balance of fit and friction is needed.
• Sleeve bearings, where the fit may be tight but still allows for controlled movement.
Example:
A pulley mounted on a keyed shaft in an industrial conveyor system. Some pulleys may slide on with minimal effort, while others require gentle tapping with a mallet to ensure a secure position. The fit prevents excessive movement but allows for occasional disassembly.
LN – Locational Interference Fit
Definition:
LN fits involve some level of interference, meaning the shaft is slightly larger than the hole, requiring force or thermal expansion/contraction for assembly. These fits ensure precise positioning but do not generate extremely high holding forces like FN fits.
Common Applications:
• Gears on shafts that need a secure fit without excessive force.
• High-precision aerospace and automotive components that require accurate positioning.
• Bearings in housings, where slight interference prevents unwanted movement.
Example:
A wheel hub fitted onto an axle in a train’s suspension system. The interference fit keeps the hub from shifting under extreme loads but allows it to be removed with a specialized press when needed.
LN fits involve some level of interference, meaning the shaft is slightly larger than the hole, requiring force or thermal expansion/contraction for assembly. These fits ensure precise positioning but do not generate extremely high holding forces like FN fits.
Common Applications:
• Gears on shafts that need a secure fit without excessive force.
• High-precision aerospace and automotive components that require accurate positioning.
• Bearings in housings, where slight interference prevents unwanted movement.
Example:
A wheel hub fitted onto an axle in a train’s suspension system. The interference fit keeps the hub from shifting under extreme loads but allows it to be removed with a specialized press when needed.
FN – Force or Shrink Fit
Definition:
FN fits are true interference fits, where the shaft is larger than the hole and must be forcefully pressed in or thermally expanded/shrunk for assembly. These fits create a permanent bond, often replacing mechanical fasteners like bolts or keys, and often have high accuracy.
Common Applications:
• Flywheels on crankshafts in high-performance engines.
• Press-fitted structural parts in heavy machinery.
• Railroad wheels mounted onto axles, which require immense holding force.
Example:
A metal bushing inserted into an aircraft landing gear assembly. The bushing is cooled using liquid nitrogen to temporarily shrink it, allowing it to fit inside a bore. As it returns to normal temperature, it expands and locks tightly in place, ensuring extreme durability and strength.
Many press fit applications only have 20-40% metal to metal contact - so sometimes compounds are used to increase the surface area for the fit. Now that we understand fit grades, let’s explore how different types of fits function in mechanical design.
FN fits are true interference fits, where the shaft is larger than the hole and must be forcefully pressed in or thermally expanded/shrunk for assembly. These fits create a permanent bond, often replacing mechanical fasteners like bolts or keys, and often have high accuracy.
Common Applications:
• Flywheels on crankshafts in high-performance engines.
• Press-fitted structural parts in heavy machinery.
• Railroad wheels mounted onto axles, which require immense holding force.
Example:
A metal bushing inserted into an aircraft landing gear assembly. The bushing is cooled using liquid nitrogen to temporarily shrink it, allowing it to fit inside a bore. As it returns to normal temperature, it expands and locks tightly in place, ensuring extreme durability and strength.
Many press fit applications only have 20-40% metal to metal contact - so sometimes compounds are used to increase the surface area for the fit. Now that we understand fit grades, let’s explore how different types of fits function in mechanical design.
How Grades and Fits Work Together
Understanding both Grades and Fits is essential when specifying the relationship between two mating parts. Fit classifications (such as RC, LC, LT, LN, and FN) define the general nature of the fit, while tolerance grades (such as Grade 4, 6, or 10) determine how much variation is allowed in the dimensions of those parts.
For example, if you are press-fitting a shaft into a hole, you might specify the fit as FN4. Here’s what that means:
• "FN" (Force Fit) indicates that the shaft will always be larger than the hole, requiring force or thermal expansion to assemble.
• The number "4" (Grade 4) defines the tolerance range, meaning how much the dimensions of the shaft and hole are allowed to vary while still maintaining a secure press fit.
If you were to change the tolerance grade, say from FN4 to FN7, the interference level would increase, making the press fit tighter and more secure—but also more challenging to assemble. Conversely, an FN2 fit would allow for less interference, making it easier to assemble but potentially less secure under heavy loads.
For example, if you are press-fitting a shaft into a hole, you might specify the fit as FN4. Here’s what that means:
• "FN" (Force Fit) indicates that the shaft will always be larger than the hole, requiring force or thermal expansion to assemble.
• The number "4" (Grade 4) defines the tolerance range, meaning how much the dimensions of the shaft and hole are allowed to vary while still maintaining a secure press fit.
If you were to change the tolerance grade, say from FN4 to FN7, the interference level would increase, making the press fit tighter and more secure—but also more challenging to assemble. Conversely, an FN2 fit would allow for less interference, making it easier to assemble but potentially less secure under heavy loads.
Finding Your Fit
As a result of copyright restrictions, we are unable to publish full tolerance charts for engineering fits. However, you can find detailed fit and tolerance data using resources such as the AmesWeb ANSI Limits and Fits Calculator:
https://amesweb.info/fits-tolerances/ansi-limits-fits-calculator.aspx
Not sure which fit to choose? This flowchart provides a simple way to determine the best fit type based on movement requirements, precision needs, and ease of disassembly.
https://amesweb.info/fits-tolerances/ansi-limits-fits-calculator.aspx
Not sure which fit to choose? This flowchart provides a simple way to determine the best fit type based on movement requirements, precision needs, and ease of disassembly.
Introduction to Fit Prefixes (H7, g6, etc.)
When specifying how two parts should fit together, engineers use fit prefixes like H7, g6, P6, and F7 to define the precise tolerances for holes and shafts. These prefixes come from standardized fit systems, such as ISO 286 and ANSI B4.1, and help ensure consistency in manufacturing.
Each fit prefix consists of a letter and a number:
• The letter (H, g, F, etc.) defines whether the tolerance applies to a hole (uppercase) or a shaft (lowercase).
• The number (e.g., 6, 7, 8) represents the tolerance grade, where lower numbers indicate tighter tolerances and higher numbers allow for more variation.
Each fit prefix consists of a letter and a number:
• The letter (H, g, F, etc.) defines whether the tolerance applies to a hole (uppercase) or a shaft (lowercase).
• The number (e.g., 6, 7, 8) represents the tolerance grade, where lower numbers indicate tighter tolerances and higher numbers allow for more variation.
Hole-Basis system vs. Shaft-Basis system
Fit prefixes are commonly used in two different systems:
1. Hole-Basis Fits (e.g., H7/g6) – The hole is kept at a standard size, while the shaft is adjusted to achieve the desired fit. This is the most common system because standard hole sizes are easier to machine.
2. Shaft-Basis Fits (e.g., g6/H7) – The shaft size is fixed, and the hole is machined to create the correct fit. This is less common but useful when pre-made shafts must be used.
1. Hole-Basis Fits (e.g., H7/g6) – The hole is kept at a standard size, while the shaft is adjusted to achieve the desired fit. This is the most common system because standard hole sizes are easier to machine.
2. Shaft-Basis Fits (e.g., g6/H7) – The shaft size is fixed, and the hole is machined to create the correct fit. This is less common but useful when pre-made shafts must be used.
Example: H7/g6 Fit for a 25 mm Diameter Shaft & Hole
When using standards such as ASME B4.1, tables with these prefixes define fits. When looking up a specific fit, tables may produce information such as this:
| Feature | Fit Prefix | Tolerance (mm) | Min Size (mm) | Max Size (mm) |
|---|---|---|---|---|
| Hole | Tight H7 | +0.021 / 0.000 | 25.000 | 25.021 |
| Shaft | g6 | -0.005 / -0.014 | 24.986 | 24.995 |
-H7 means the hole is fixed and has a slight oversize tolerance.
- g6 denotes the shaft is slightly smaller, ensuring a clearance fit.
By using fit prefixes, engineers can precisely control the interaction between mating parts, ensuring proper function whether the parts need to slide, align, or press together permanently.
- g6 denotes the shaft is slightly smaller, ensuring a clearance fit.
By using fit prefixes, engineers can precisely control the interaction between mating parts, ensuring proper function whether the parts need to slide, align, or press together permanently.
When to Use Fit Classes, Grades, and Prefixes
Each component serves a specific role in defining how two parts fit together, but they aren’t separate choices—they work together in a structured way:
1. Fit Classes (RC, LC, LT, LN, FN) → Define function (clearance, transition, or interference)
2. Fit Grades & Tolerances (3–13) → Specify the level of precision for that fit class
3. Fit Prefixes (H7, g6, etc.) → Convert fit selection into exact machining dimensions
1. Fit Classes (RC, LC, LT, LN, FN) → Define function (clearance, transition, or interference)
2. Fit Grades & Tolerances (3–13) → Specify the level of precision for that fit class
3. Fit Prefixes (H7, g6, etc.) → Convert fit selection into exact machining dimensions
How They Work Together: A Practical Example
Imagine you're designing a press-fit shaft into a housing.
1. Select a Fit Class:
The parts should be permanently joined, so you choose FN (Force Fit).
2. Select a Fit Grade & Tolerances:
You need a moderate level of interference, so you choose FN4. The FN4 specification includes tolerances that ensure proper press-fitting without excessive force.
3. Specify Fit Prefixes in Engineering Drawings:
The hole might be H7 and the shaft P6, based on standard tables. This gives machinists exact tolerance limits to follow.
When to Use Each Component in the Design Process
| Step in Design Process | Fit Class? | Fit Grade & Tolerance | Fit Prefix |
|---|---|---|---|
| Early Conceptual Design: Should the parts move, align, or lock together? | Yes | No | No |
| Choosing the required precision level | Yes | Yes | No |
| Finalizing tolerances for manufacturing | No | Yes | Yes |
| Specifying exact dimensions for CNC machining | No | Yes | Yes |
Key Takeaways
• Fit grades & tolerances are a single option that defines precision within a chosen fit class.
• Fit classes (RC, LC, FN, etc.) are broad categories that define function.
• Fit prefixes (H7, g6, etc.) convert all this into manufacturing-ready tolerances.
By using them together, you ensure that your parts function correctly, fit reliably, and can be produced efficiently.
• Fit classes (RC, LC, FN, etc.) are broad categories that define function.
• Fit prefixes (H7, g6, etc.) convert all this into manufacturing-ready tolerances.
By using them together, you ensure that your parts function correctly, fit reliably, and can be produced efficiently.
The Science of Correct Fits
Selecting the right fit is a fundamental skill in mechanical design, bridging the gap between theoretical engineering principles and practical manufacturing realities. Whether you're designing precision aerospace components, automotive assemblies, or heavy machinery, understanding how fit classes, tolerance grades, and fit prefixes work together ensures your components function correctly, last longer, and are manufacturable within cost and process constraints.
By following a structured approach to fit selection:
✅ Use fit classes (RC, LC, LT, LN, FN) to define the functional relationship between parts.
✅ Choose the appropriate fit grade (3–13) to control precision and manufacturability.
✅ Apply fit prefixes (H7, g6, etc.) to convert your design choices into exact tolerances for machining and assembly.
These principles help prevent costly manufacturing errors, reduce wear and tear, and ensure optimal performance in real-world applications. Whether you’re designing for loose clearance fits in industrial machinery or tight interference fits in aerospace components, mastering fits and tolerances will make you a more effective engineer, machinist, maker or product designer.
Engineering is all about precision and problem-solving—and choosing the right fit is where those two meet. Now, you have the knowledge to make informed fit selections that improve product quality, assembly efficiency, and overall performance.
By following a structured approach to fit selection:
✅ Use fit classes (RC, LC, LT, LN, FN) to define the functional relationship between parts.
✅ Choose the appropriate fit grade (3–13) to control precision and manufacturability.
✅ Apply fit prefixes (H7, g6, etc.) to convert your design choices into exact tolerances for machining and assembly.
These principles help prevent costly manufacturing errors, reduce wear and tear, and ensure optimal performance in real-world applications. Whether you’re designing for loose clearance fits in industrial machinery or tight interference fits in aerospace components, mastering fits and tolerances will make you a more effective engineer, machinist, maker or product designer.
Engineering is all about precision and problem-solving—and choosing the right fit is where those two meet. Now, you have the knowledge to make informed fit selections that improve product quality, assembly efficiency, and overall performance.
