The nominal wall thickness of a 25mm hollow anchor bolt is typically 5mm (often marked as Φ25×5), which is the most common configuration in railway and national standard engineering projects.
The national standard GB 50086-2015 requires that the wall thickness of hollow grouting anchor bolts should not be less than 5.0mm. This means that 5.0mm is the minimum requirement, and the actual product wall thickness can be higher.

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Performance Grades of Stainless Steel Bolts
The performance grade 8.8 for stainless steel bolts refers to a tensile strength limit of 800 MPa and a yield strength limit of 640 MPa.
The performance grades for stainless steel bolts and studs are divided into 10 grades: from 3.6 to 12.9. The figure before the decimal point represents 1/100 of the material’s tensile strength limit, whilst the figure after the decimal point represents 10 times the ratio of the material’s yield strength to its tensile strength limit.

Performance Grades of Stainless Steel Nuts
Nuts are classified into seven performance grades, ranging from 4 to 12, with the number roughly representing one-hundredth of the minimum stress that a stainless steel nut is guaranteed to withstand. For standardised imperial threads, there are three grades for external threads: 1A, 2A and 3A, and three grades for internal threads: 1B, 2B and 3B; all of these are loose fits. The higher the grade number, the tighter the fit.
1. Grades 1A and 1B are very loose tolerance grades, suitable for tolerance fits in both external and internal threads.
2. Grades 2A and 2B are the most commonly specified thread tolerance grades for the imperial series of mechanical stainless steel fasteners.
3. Grades 3A and 3B produce the tightest fit when screwed together; they are suitable for stainless steel standard parts with tight tolerances and are used in safety-critical designs.
For metric threads, there are three thread grades for external threads: 4h, 6h and 6g; and three thread grades for internal threads: 5H, 6H and 7H. Thread fits are best combined as H/g, H/h or G/h. For precision-manufactured fasteners such as bolts and stainless steel nuts, the standard recommends a 6H/6g fit.