Research / editorial guide. We reviewed manufacturer pages and a manufacturer catalog on October 12, 2026. We have not tested these tools, measured a repaired hole, or inspected a delivered calibration certificate. As an Amazon Associate I earn from qualifying purchases. This guide has no Amazon product links. See our affiliate disclosure.
Start with the job the tool must do
If the thread size is still unknown, use our pitch gauge and thread checker comparison first. This guide starts after identification, when you need to specify an inspection purchase.
| Your job | Tool category to investigate | What to confirm before ordering |
|---|---|---|
| Sort an unknown bolt or nut by nominal size | Thread checker | Does the exact model include the size and pitch you need? |
| Inspect a specified internal product thread | Working thread plug gauge | Match the drawing, thread class, gauge tolerance, and inspection procedure. |
| Inspect a specified external product thread | Working thread ring gauge | Confirm the external-thread requirement and the appropriate ring arrangement. |
| Set or check an adjustable thread ring gauge | Thread setting plug gauge | Confirm the ring and setting procedure; a setting plug is not the working plug for your threaded hole. |
Vermont describes rings as tools for external threads and setting plugs as tools for checking and setting adjustable rings. Similar-looking names do not make these tools interchangeable. Manufacturer explanation.
An M8 example: size coverage and inspection specifications
S&W lists SWTC-M21 with both M8 × 1.0 and M8 × 1.25 among its metric sizes. That is useful coverage information for an identification purchase. The model page we checked does not establish that this checker substitutes for a working GO/NO-GO gauge selected to your drawing. S&W model page.
Vermont's working-plug page lists 6H under metric class of fit and Class X under gauge tolerance. Keep those in separate fields: 6H describes the internal-thread requirement represented by this example; Class X describes the gauge tolerance. Neither label is a rating of repair strength. Vermont identifies ASME B1.16M for its M-series metric thread gauges; a standard number alone is not a complete inspection procedure. Working-plug specifications and thread-gauge overview.
Use the following only as a catalog procurement example for a drawing that actually calls for M8 × 1.25, 6H. We checked the 2025 catalog file labeled R7, printed page 69 (PDF page 71). Its age and online availability do not guarantee that an order today has the same specification or contents. Confirm the current item with the supplier.
| Vermont catalog item | Part number | Ordering distinction |
|---|---|---|
| Taperlock working plug, GO member | 302120510 | One listed gauging member; do not assume a complete two-ended assembly. |
| Taperlock working plug, NO-GO member | 302120520 | A different member from GO. |
| Taperlock working plug, GO/NO-GO set | 302120530 | The set column, rather than either single-member column. Confirm supplied handle and contents. |
| Reversible working plug, GO/NO-GO set | 312120530 | A different design and order number; confirm its arrangement suits the job. |
| Setting plug, 6g, GO/NO-GO set | 372120530 | An adjacent catalog row for a different job, not the 6H working plug. |
Original catalog, printed page 69. We omit catalog prices and dimensional limits: this table identifies orders, not current offers or acceptance criteria.
A certificate is another item to specify
Vermont says its working plugs include a Certificate of Accuracy and lists calibration certificates, with or without ISO 17025 accreditation, as separate options. Do not assume the included document is the calibration report your customer or quality system requires. Working-plug documentation.
Its calibration-services page also distinguishes work within its accreditation scope from work offered with a non-accredited certificate. Ask for the applicable scope and a sample of the document you intend to order. We did not independently verify the laboratory's current accreditation scope or a certificate for either M8 set. Calibration services.
Fill out this order worksheet
The entries below are questions to resolve with the drawing owner, quality team, or supplier. They are not additional specifications for every M8 hole.
| Record | Write down or obtain |
|---|---|
| Part requirement | Drawing number and revision; internal or external thread; complete thread callout and hand. |
| Inspection stage | Original product thread, insert-receiving thread, or finished thread after repair. |
| Gauge identity | Manufacturer, exact part number, working or setting function, design, and GO/NO-GO components. |
| Two different classes | The part's specified thread class and the gauge's specified tolerance, recorded separately. |
| Supplied contents | Single member or set; handle, markings, and any accessories actually included. |
| Documents | Included document, required calibration report, gauge identification, results, and applicable accreditation scope. |
| Procedure and access | The approved inspection method and whether the selected gauge can reach the required feature. |
For example, a supplier request could read:
Our drawing calls for an internal M8 × 1.25, 6H product thread. Please confirm the current specification and contents of 302120530, including both members and the handle. Identify the gauge tolerance, thread hand, included certificate, and any separately ordered calibration service. Please provide the applicable inspection documentation and a sample report. We also need confirmation that this design suits the feature and inspection stage shown in our drawing.
This is an editorial request template, not a determination that the set fits your part. Leave 6H out of your request if your drawing specifies something else; copy the actual requirement instead.
Stop when the inspection requirement is missing
Do not buy a gauge solely because a matching bolt threads into the hole. If the drawing, applicable procedure, or repair stage is unclear, get that resolved first. We do not provide a universal number of allowable NO-GO turns or a hand-force rule; use the controlling inspection documentation.
For insert repairs, do not transfer the final screw-thread callout to the receiving thread without the insert-system manufacturer's confirmation. Our M8 coil versus solid insert guide explains why the repair system matters; our blind-hole checklist separates depth and tool-access questions.
A matching gauge result alone does not establish material condition, pullout strength, an approved tightening torque, or the safety of a repair. Safety-critical parts need the equipment manufacturer's procedure or a qualified professional's assessment. Buy against a documented requirement, and keep the gauge order and its documents together.