6H/6g VS 5g6g Thread Fit: Fastener Procurement Guide

1. Basic Definition of Metric Thread Tolerance (ISO 965)

1.1 What do tolerance codes stand for

Uppercase letters represent internal threads, lowercase letters represent external threads. Numbers indicate tolerance grade. Smaller grade number means tighter tolerance bandwidth.

• 6H: Standard internal thread tolerance, zero fundamental deviation, widely used for nuts and tapped holes, typical for DIN934 hex nuts.

• 6g: General external thread tolerance, reserved clearance for normal electroplating, commonly adopted by DIN933 hex head bolts.

• 5g6g: Dual-tolerance external thread, pitch diameter controlled to grade 5, major diameter controlled to grade 6. Overall fit is tighter than 6g.

1.2 Core difference between 6H/6g and 6H/5g6g

2. Root Cause of Gauge Inspection Failure

2.1 Misreading drawing tolerance mark

Buyers treat 5g6g as an upgraded version of 6g and directly replace it in general purchase orders. The drawing requires 6H/6g while the supplier delivers 5g6g, or vice versa. Even if the thread looks visually identical, the ring gauge or plug gauge will fail during inspection.

2.2 Plating thickness consumes thread clearance

Zinc plating, zinc flake and other surface treatments add material on thread flank. 6g has extra built-in allowance for coating absorption. 5g6g tolerance zone is narrower. After plating, the pitch diameter quickly exceeds upper limit, GO gauge cannot be screwed in, and the whole batch is rejected by third-party inspectors or client site QC. This problem frequently occurs for matched sets of DIN933 bolts and DIN934 nuts.

2.3 Inconsistent inspection gauge

If the supplier uses 6g ring gauge for production inspection but the overseas inspector uses 5g6g gauge, qualified goods from factory will be judged non-conforming at destination port. Gauge class must match the tolerance specified on purchase order.

3. Standard Procurement & Specification Writing Guide

3.1 Clear tolerance notation on PO and drawing

Correct marking example: M12×1.75-6H/6g or M12×1.75-6H/5g6g. Tolerance class must be written completely, not simplified. When ordering paired fasteners, fill in full tolerance marks for DIN933 bolts and DIN934 nuts in your inquiry and purchase order.

3.2 Special note for plated fasteners

If external threads will be electroplated, confirm the reserved allowance before plating. 5g6g is not recommended for thick coating jobs. For heavy plating requirement, select 6e or 6f tolerance class.

3.3 Gauge calibration requirement

Request the supplier to provide gauge calibration certificate. The GO/NO-GO ring gauge and plug gauge must correspond exactly to the tolerance class in the contract.

4. Common Procurement Mistakes

• Mistake 1: Assume 5g6g is higher quality and use it for all general fastener orders.

• Mistake 2: Only mark thread size without tolerance class on inquiry sheet.

• Mistake 3: Ignore coating influence, specify 5g6g then carry out conventional zinc plating.

• Mistake 4: Factory inspection gauge and customer incoming gauge adopt different tolerance standards.

5. FAQ

Q1: Can 6H/5g6g replace 6H/6g directly? A1: Not recommended. 5g6g has tighter pitch diameter tolerance. If there is plating, replacement will greatly increase non-conformance risk. Only use it when the drawing clearly requires precision fit.

Q2: If goods already failed gauge test after plating, is rework feasible? A2: Rework by chasing thread is possible, but it will remove coating and affect corrosion resistance. Most overseas projects refuse reworked threads for critical connections.

Q3: Does 5g6g mean stronger bolt? A3: No. Thread tolerance only controls dimensional fit, and it has no relation with mechanical strength grade such as 8.8, 10.9 or A2-70.