Go/No-Go Gauges

Part of Precision Measurement

The go/no-go gauge is the fastest and most reliable way to inspect parts to tolerance — eliminating measurement reading errors and enabling unskilled workers to check precision parts.

Why This Matters

The go/no-go gauge is one of the most powerful concepts in precision manufacturing. Instead of measuring a dimension and comparing it to a specification, you simply try two gauges: if the “go” side passes (the part is not too large), and the “no-go” side doesn’t pass (the part is not too small), the part is within tolerance. No reading, no arithmetic, no training in measurement technique. Pass/fail. Green/red.

This simplicity has profound implications for a rebuilding community. With go/no-go gauges, a craftsperson who has been working for three months can reliably check parts to the same tolerance as someone with years of measuring experience. Production consistency can be maintained without instrument expertise. Parts made in different workshops, by different people, can be assembled together because they all passed the same go/no-go gauge.

This is the beginning of interchangeable parts manufacturing — one of the most transformative concepts in industrial history. Before go/no-go gauges, every assembly was custom fitted. After them, parts can be made in batches and combined in any order.

The Core Concept

Every dimension on a part has a tolerance: an acceptable range between minimum and maximum. A shaft designed to fit a 20mm hole might have a specification of 19.8mm to 19.9mm — any diameter in this range produces an acceptable fit. Below 19.8mm, the shaft is too loose; above 19.9mm, it won’t fit.

The go/no-go gauge for this shaft has two ends:

• Go end: an opening of exactly 19.9mm. If the shaft passes through this opening, the shaft is 19.9mm or smaller — it’s not too large.
• No-go end: an opening of exactly 19.8mm. If the shaft does NOT pass through this opening, the shaft is 19.8mm or larger — it’s not too small.

A shaft that passes go but doesn’t pass no-go is within specification. A shaft that doesn’t pass go is too large (scrap or re-work). A shaft that passes no-go is too small (scrap).

The gauge embodies the tolerance limits as physical dimensions, replacing measurement with a mechanical test.

Designing a Go/No-Go Gauge

Before making a gauge, determine the tolerance:

What is the minimum acceptable dimension? This becomes the no-go limit — the gauge that rejects parts that are too small.

What is the maximum acceptable dimension? This becomes the go limit — the gauge that rejects parts that are too large.

For a fit, the tolerance depends on the function:

• Clearance fit (shaft must slide freely in hole): shaft maximum diameter = hole minimum diameter − 0.1 to 0.5mm (depending on desired clearance)
• Transition fit (shaft may or may not be press-fit): shaft maximum ≈ hole minimum
• Interference fit (shaft must be press-fitted): shaft minimum > hole maximum by 0.05 to 0.2mm

For most practical applications without precise engineering specifications, use a clearance of 0.5mm between the go and no-go limits. This gives a 0.5mm tolerance band — achievable by a competent craftsperson without precision measuring instruments.

Example: a wheel hub bore intended to run on a 25mm shaft

• Desired clearance: 0.3mm (shaft turns freely without slop)
• Go gauge: 25.3mm opening (rejects bores smaller than 25.3mm)
• No-go gauge: 25.8mm opening (rejects bores larger than 25.8mm)
• Acceptable bore range: 25.3mm to 25.8mm

Making a Plug Gauge (for Internal Dimensions)

A plug gauge checks a hole or bore — the go end is inserted into the hole; the no-go end should not enter.

Materials: A cylinder of hard metal (drill rod, or hardened and tempered mild steel) turned or filed to close dimensions.

Steps:

1. Turn or file a short cylinder (the go plug) to the go diameter. Check with calipers. The diameter must be accurate to ±0.05mm.
2. Turn or file a second short cylinder (the no-go plug) to the no-go diameter — 0.5mm smaller.
3. Mount both plugs on opposite ends of a handle (a wooden or metal bar). Traditional double-ended plug gauges have the go end longer than the no-go end, so the difference is immediately visible.
4. Harden the plugs: heat to cherry red (800°C) and quench in water. Temper by reheating to straw-gold color (220°C) and air-cooling. Hardened plugs resist wear much better than soft metal.
5. After hardening, check dimensions again — hardening can cause small dimensional changes. File and recheck if needed.

Using the plug gauge: Insert the go end into the bore. It should enter fully with light hand pressure. If it won’t enter, the bore is undersize. Then try the no-go end — it should NOT enter. If it does, the bore is oversize. A bore that accepts go but rejects no-go is within tolerance.

Making a Ring Gauge (for External Dimensions)

A ring gauge checks a shaft or pin — the go end should slide over the shaft; the no-go should not.

Making a ring gauge from metal:

1. Drill a hole in a metal block to the go diameter. Use a drill slightly undersize, then file or ream to the exact dimension. Check with the appropriately-sized bar stock or with inside calipers.
2. Drill a second hole for the no-go diameter (0.5mm smaller than the go hole).
3. Both holes can be in the same block — one at each end.
4. Harden as for plug gauges.

Making a ring gauge from wood (lower precision, adequate for larger tolerances): Wood ring gauges work for dimensions where ±0.5mm tolerance is adequate. Drill holes slightly undersize and carefully file or sand to the target dimension. Wood gauges must be rechecked frequently as they change dimensions with humidity.

Snap Gauges (Combined Go/No-Go in One)

For flat dimensions (thickness, width), a snap gauge checks both limits in one operation. It has a fixed anvil and two adjustable contacts: one at the go limit (farther apart) and one at the no-go limit (closer together).

To check a part: slide it into the snap gauge from the go end. If it enters the go opening but stops before reaching the no-go opening, it is within tolerance. If it doesn’t enter the go opening, it’s too large. If it passes the no-go opening, it’s too small.

The snap gauge can be made from two U-shaped metal frames mounted concentrically. The outer frame is the go gauge; the inner is the no-go gauge.

Organizing a Gauge Library

As the community builds more precision tools, organize gauges systematically:

Gauge	Go Dim.	No-Go Dim.	Application
Wheel axle plug	25.3mm	25.8mm	Wheel hub bores
Bolt body ring	9.9mm	10.1mm	Standard 10mm bolt shanks
Bearing bore plug	30.2mm	30.7mm	Standard bearing fits

Keep all gauges together in a protected case. Label each gauge with its go and no-go dimensions clearly. Calibrate against the community’s primary standard every 6 months. Replace worn gauges — a gauge worn even 0.2mm beyond its intended dimension passes parts that would previously have been rejected, silently degrading quality.