Plug gauge tolerance is selected by starting from the part’s hole tolerance limits, then applying a suitable gage maker tolerance to the GO and NO-GO members. In a typical cylindrical plug gauge system, the GO plug checks the minimum acceptable hole size and should enter the hole, while the NO-GO plug checks the maximum acceptable hole size and should not enter under the defined inspection condition.
A plug gauge, also called a plug gage or pin plug gage, is a fixed-limit inspection tool used to check whether a hole is within tolerance. It does not measure the exact hole diameter like a bore gauge, air gauge, or CMM. Instead, it provides a practical pass/fail judgment for production inspection.
For CNC machined parts, hydraulic components, bushings, sleeves, precision hardware, molded parts, and assembly holes, correct plug gage size selection is critical. If the GO and NO-GO sizes are selected incorrectly, the inspection result may reject acceptable parts or accept out-of-tolerance holes.
MISHUTE supplies precision gauges for hole inspection applications, including pin gauges and related fixed-limit inspection tools. You can review related products on the MISHUTE pin gauges page.
What Is Plug Gauge Tolerance?
Plug gauge tolerance refers to the allowable manufacturing variation of the plug gauge itself. It is different from the part tolerance.
A part drawing defines the acceptable size range of the hole. The plug gauge must then be made with its own controlled tolerance so that it can reliably inspect that hole.
| Term | Meaning |
| Part tolerance | The allowed size variation of the hole on the workpiece |
| GO plug gauge | Checks whether the hole is large enough |
| NO-GO plug gauge | Checks whether the hole is not too large |
| Gage maker tolerance | The allowed manufacturing tolerance of the gauge |
| Wear allowance | Additional consideration for wear during repeated use |
| Decision rule | The inspection rule used to judge pass or fail |
Part tolerance controls the workpiece. Gage maker tolerance controls the inspection gauge.
This distinction is important. A plug gauge is not made exactly at the nominal size without tolerance. It has its own tolerance band, and that tolerance must be selected carefully.
How GO and NO-GO Plug Gauges Work
A Go/No-Go plug gauge checks a hole against two limits.
The GO member checks the lower limit of the hole. If the GO plug does not enter, the hole may be too small.
The NO-GO member checks the upper limit of the hole. If the NO-GO plug enters beyond the allowed inspection condition, the hole may be too large.
| Gauge Member | Function | Expected Result |
| GO plug gauge | Checks minimum acceptable hole size | Should enter |
| NO-GO plug gauge | Checks maximum acceptable hole size | Should not enter |
| Pass condition | GO enters and NO-GO does not enter | Hole is acceptable |
| Fail condition 1 | GO does not enter | Hole may be undersize |
| Fail condition 2 | NO-GO enters | Hole may be oversize |
For hole inspection, the GO plug should pass through the required hole length, while the NO-GO plug should not pass under the defined inspection rule.
This makes Go/No-Go inspection fast, simple, and practical for production environments.
GO Plug Gauge Tolerance Direction
For a cylindrical plug gauge used to inspect a hole, the GO plug member is commonly controlled toward the plus side.
This is because the GO plug is checking whether the hole is at least large enough. The GO member is related to the minimum hole limit.
For example, if the hole lower limit is 10.000 mm, the GO plug is based around that minimum limit. Its gauge tolerance is normally applied in a way that prevents accepting holes that are smaller than the intended minimum.
In practical terms:
- GO plug checks the minimum hole limit.
- GO plug should enter the hole.
- GO plug tolerance is commonly directed to the plus side.
- If the GO plug does not enter, the hole may be too small.
The GO plug gauge is generally based on the minimum hole limit and is commonly given plus-side gauge tolerance.
However, the exact value should follow the applicable standard, customer requirement, or gage maker’s tolerance chart.
NO-GO Plug Gauge Tolerance Direction
The NO-GO plug member checks whether the hole is too large. It is related to the maximum hole limit.
For a cylindrical plug gauge used on a hole, the NO-GO plug member is commonly controlled toward the minus side. This helps ensure that the NO-GO member properly represents the maximum acceptable hole size without allowing too much product tolerance to be consumed by the gauge.
In practical terms:
- NO-GO plug checks the maximum hole limit.
- NO-GO plug should not enter the hole under the defined condition.
- NO-GO plug tolerance is commonly directed to the minus side.
- If the NO-GO plug enters, the hole may be too large.
The NO-GO plug gauge is generally based on the maximum hole limit and is commonly given minus-side gauge tolerance.
For critical applications, the allowed NO-GO entry depth, force, and inspection method should be defined clearly.
The 10% Rule for Gage Maker Tolerance
A common starting point in Go/No-Go gauge selection is the 10% rule. This rule suggests allocating a small portion of the total product tolerance to the gauges.
For example, if the total hole tolerance is 0.020 mm, a common starting point would be to allocate about 10% of that total tolerance to the gauge system. This does not mean every application must use exactly this rule. It means the rule is often used as a practical guideline when selecting gage maker tolerance.
| Item | Example |
| Hole lower limit | 9.990 mm |
| Hole upper limit | 10.010 mm |
| Total product tolerance | 0.020 mm |
| Suggested gauge tolerance basis | A small portion of product tolerance |
| Final selection | Depends on standard, class, wear, and customer requirement |
The 10% rule is useful because it prevents the gauge tolerance from consuming too much of the part tolerance. If gauge tolerance is too large, inspection results near the tolerance limits may become unreliable.
Gage maker tolerance should be small enough to support reliable inspection, but practical enough for manufacturing, cost, and calibration control.
Example: Selecting GO and NO-GO Plug Gauge Sizes
The following is a simplified example for explanation only. Actual gauge design should follow the applicable standard, internal quality procedure, and customer requirement.
Suppose the drawing specifies:
| Drawing Requirement | Example |
| Nominal hole size | 12.000 mm |
| Lower limit | 11.990 mm |
| Upper limit | 12.010 mm |
| Total tolerance | 0.020 mm |
| Inspection method | Go/No-Go plug gauge |
The selection logic would be:
- The GO plug is related to the lower hole limit: 11.990 mm.
- The NO-GO plug is related to the upper hole limit: 12.010 mm.
- Gage maker tolerance is selected based on product tolerance, gauge class, and standard.
- GO tolerance direction is commonly plus.
- NO-GO tolerance direction is commonly minus.
- Wear allowance and calibration requirements should be considered.
- Final gauge size should be confirmed before manufacturing.
| Gauge Member | Related Hole Limit | Common Tolerance Direction | Inspection Result |
| GO plug | Lower limit | Plus side | Should enter |
| NO-GO plug | Upper limit | Minus side | Should not enter |
This example shows why simply ordering a “12 mm plug gauge” is not enough. The supplier needs the tolerance limits and inspection requirement.
Plug Gauge Tolerance vs Pin Gauge Tolerance
Plug gauges and pin gauges are closely related in hole inspection. In some industries, standard pin gages are also used as pin/plug gages for Go/No-Go inspection. The terms may overlap depending on the supplier and application.
| Item | Plug Gauge | Pin Gauge |
| Main purpose | Fixed-limit hole inspection | Hole, slot, gap, and fit checking |
| Form | Cylindrical member, often with handle | Individual precision pin or set |
| Go/No-Go use | Very common | Very common |
| Size selection | Based on hole limits | Based on hole limits or fit checking |
| Typical purchase | Custom or standard plug gage | Pin gage set or individual pins |
| Inspection style | Pass/fail | Pass/fail or approximate size |
If your team checks many different hole sizes, a pin gage set may be useful. If your team checks one repeated production hole, a dedicated GO/NO-GO plug gauge may be more efficient.
MISHUTE provides pin gauges for precision hole inspection and related dimensional checking applications. You can view available options through the pin gauges category.
Key Factors That Affect Plug Gage Size Selection
1. Hole Tolerance
The tighter the hole tolerance, the more carefully the gauge tolerance must be selected. A wide part tolerance may allow a standard gauge class. A tight tolerance may require a higher-precision gauge.
2. Inspection Purpose
If the gauge is used only for process checking, requirements may be different from final customer acceptance inspection. Always clarify whether the gauge is used for production control, final inspection, calibration, or customer-required acceptance.
3. Hole Type
Through holes and blind holes may require different gauge designs.
| Hole Type | Gauge Consideration |
| Through hole | GO member may need to pass through the required depth |
| Blind hole | Gauge length, chamfer, radius, and depth feature may matter |
| Counterbored hole | Gauge design may need clearance |
| Deep hole | Straightness and insertion depth become important |
| Small hole | Handling and tolerance class become more critical |
4. Material and Wear
If the gauge will be used frequently, wear resistance matters. Hardened steel may be enough for general use. Tungsten carbide may be considered for high-frequency inspection. Ceramic may be useful for corrosion-sensitive or non-magnetic applications.
5. Certificate Requirement
For ISO systems, audits, and customer-controlled inspection, calibration certificates may be required. Confirm whether the certificate must include measured values, uncertainty, traceability, and individual gauge identification.
6. Operator Method
Go/No-Go inspection depends on consistent use. The same gauge can produce inconsistent decisions if operators apply different force, angle, or insertion depth.
Common Mistakes in Plug Gauge Tolerance Selection
Mistake 1: Ordering Only by Nominal Size
A nominal size is not enough. The supplier needs the upper and lower hole limits to define GO and NO-GO sizes.
Mistake 2: Ignoring Gage Maker Tolerance
A plug gauge also has manufacturing tolerance. Ignoring it can lead to false acceptance or false rejection near the tolerance limits.
Mistake 3: Using the Wrong Tolerance Direction
For cylindrical plug gages, GO and NO-GO members typically use different tolerance directions. Confusing them can affect inspection reliability.
Mistake 4: Not Defining NO-GO Entry Conditions
The NO-GO plug may create debate if the inspection method is not clear. The procedure should define how far the NO-GO member may enter, how much force is allowed, and how the result is judged.
Mistake 5: Ignoring Wear Allowance
A frequently used gauge will wear. If wear is not considered, the gauge may drift from its intended inspection function over time.
Mistake 6: Using One Gauge for All Environments
A gauge used in a controlled metrology room and a gauge used on a production line face different risks. Material, grade, and calibration interval should match the environment.
How to Choose Between Standard and Custom Plug Gauges
A standard plug gauge or pin/plug gage may be suitable when the hole size and tolerance match common available sizes. A custom plug gauge is more suitable when the part drawing has special requirements.
| Option | Suitable For | Limitation |
| Standard pin/plug gage | Common hole sizes and general inspection | May not match special tolerances |
| Individual GO/NO-GO gages | Dedicated production hole checking | Less flexible for many sizes |
| Full pin gage set | Many hole sizes, slots, and gaps | Slower than dedicated gauges for repeated parts |
| Custom plug gauge | Special tolerance, length, material, or handle | Requires clear drawing information |
| Carbide plug gauge | High-frequency inspection | Higher initial cost |
| Ceramic plug gauge | Corrosion-sensitive or non-magnetic applications | Not needed for every application |
For repeated production, a dedicated GO/NO-GO plug gauge can reduce operator confusion and inspection time. For engineering inspection or small-batch work, a pin gage set may provide more flexibility.
What Information Should You Send Before Ordering?
To help the supplier select or quote the correct plug gauge tolerance, provide complete technical information.
Prepare:
- Part drawing
- Nominal hole size
- Upper and lower tolerance limits
- Metric or inch requirement
- GO and NO-GO inspection requirement
- Through hole or blind hole
- Required insertion depth
- Gauge material preference
- Quantity
- Handle or marking requirement
- Certificate requirement
- Inspection standard if specified
- Expected use frequency
- Production or calibration environment
If the application involves a pin/plug gage set, also confirm the diameter range, step increment, tolerance class, plus/minus direction, and packaging requirements.
You can start from the MISHUTE website or review related hole inspection products through the MISHUTE pin gauges page.
How to Choose a Supplier for Plug Gauge Tolerance Projects
A suitable supplier should not only sell a gauge by nominal size. They should understand tolerance direction, gage maker tolerance, material selection, calibration needs, and practical inspection conditions.
| Supplier Capability | Why It Matters |
| GO/NO-GO tolerance support | Helps avoid wrong gauge direction |
| Standard and custom gauge options | Supports both common and special holes |
| Pin/plug gage experience | Useful for hole inspection applications |
| Material options | Helps match wear, corrosion, and cost |
| Certificate support | Supports audits and traceability |
| Drawing review | Reduces ordering mistakes |
| Clear marking and packaging | Reduces operator misuse |
For buyers, the most important point is clear communication. A good inquiry should include the part drawing and tolerance limits, not just the nominal diameter.
FAQ
What is plug gauge tolerance?
Plug gauge tolerance is the allowable manufacturing variation of the plug gauge itself. It is different from the part tolerance and must be selected carefully to support reliable Go/No-Go inspection.
How do you select GO and NO-GO plug gauge sizes?
GO and NO-GO plug gauge sizes are selected from the hole’s lower and upper tolerance limits. The GO plug checks the minimum hole size, while the NO-GO plug checks the maximum hole size.
What is gage maker tolerance?
Gage maker tolerance is the tolerance allowed when manufacturing the gauge. It controls how much the actual gauge size may vary from its intended gauge size.
What is the 10% rule for Go/No-Go gauges?
The 10% rule is a commonly used guideline that allocates a small portion of the total product tolerance to the gauges. It is often used as a starting point for selecting Go/No-Go gage tolerance.
Should the GO plug gauge have plus tolerance?
For cylindrical plug gauges used to inspect holes, the GO member is commonly controlled toward the plus side because it checks the minimum acceptable hole size.
Should the NO-GO plug gauge have minus tolerance?
For cylindrical plug gauges used to inspect holes, the NO-GO member is commonly controlled toward the minus side because it checks the maximum acceptable hole size.
Is a plug gauge the same as a pin gauge?
They are related but not always identical. A plug gauge is often a fixed-limit gauge for hole acceptance, while pin gauges may be used for hole diameter, slot width, gaps, and Go/No-Go checks. In many applications, pin/plug gages overlap.
Can I order a custom plug gauge based on my drawing?
Yes. Custom plug gauges can be made based on hole size, upper and lower tolerance, gauge tolerance, material, length, handle, marking, and certificate requirements.
Conclusion
Plug gauge tolerance selection starts with the part drawing. The GO plug is based on the minimum acceptable hole limit and should enter the hole. The NO-GO plug is based on the maximum acceptable hole limit and should not enter under the defined inspection condition. Gage maker tolerance, tolerance direction, wear allowance, certificate requirements, and inspection method all affect the final plug gage size.
For general hole inspection, standard pin/plug gages or pin gage sets may be suitable. For repeated production parts or special tolerances, custom GO and NO-GO plug gauges are often more practical.
If your project requires pin gauges, pin/plug gages, or related hole inspection gauges, MISHUTE can help evaluate the right size range, tolerance class, material, and certificate requirement. Visit the MISHUTE pin gauges category to learn more.
