Is this statement still valid?

I don’t think so; the length is chosen according to standards, but it will definitely show through after tightening. As for how much, I’m not sure

Code for Construction and Acceptance of Industrial Metal Piping Projects GB 50235—97 6.3.8 Flange connections shall use bolts of the same specification, and the installation direction shall be consistent. After the bolts are tightened, they should be in close contact with the flange, with no wedging gaps. When washers are required, no more than one should be used per bolt. The bolts and nuts after tightening should be level. Usually we keep it at 1-3 deductions; let’s take a look at the revisions to the specifications: 6.3.8 This clause is rewritten based on Articles 5.2.10 and 5.2.10 of the original specifications, along with references to the American standard ANSI/ASME B31.3. Change “The exposed length after tightening shall not be greater than 2 times the pitch” to: “The bolt and nut after tightening should be level.” Regarding the exposed length of bolts, relevant Chinese standards specify it to be 2–3 turns; for example, the \"Technical Specifications for Construction and Acceptance of High-Pressure Steel Piping\" require at least 2 turns ； The Interim Technical Specifications for Power Generation Facility Construction and Acceptance (Pipelines Section) stipulate 2– deductions ； The \"Quality Inspection and Evaluation Standards for Building Installation Projects\" (Industrial Pipeline Projects) specify 2–3 deductions ； The \"Code for Construction and Acceptance of Industrial Piping Projects\" (Metal Piping Section) specifies a value not exceeding 2 times the pitch, etc. The American standard ANSI/ASME B31.3 stipulates that: “It is preferable to tighten all bolts completely into the thickness of the nut.” ……As long as the number of threads that are not fully engaged is 1 or less, it can still be considered acceptable.” The exposed length of the bolts has the following issues: (1) The exposed threads are affected by paint, rust, or damage. Disassembly is difficult; cutting is often required during maintenance, resulting in significant waste. (2) The exposed threads have no impact on the strength of the bolt connection, but they increase the workload for installation and removal. (3) The increased total length of the bolts results in more steel being used, thereby increasing the amount of machining required. It increased costs. We consider the provisions of American standards to be reasonable; therefore, it is specified that \"the bolts and nuts after tightening should be level.\"

Years of practical experience suggest that leaving 2 or 3 buttons exposed, or 2 to 3 mm, is optimal. As for the standard, it takes into account the actual conditions of installation.

Generally, 2–3 threads are required, with the value to be kept within 5 mm. We take into account this when selecting the length of the bolts, but at least 2 threads are necessary

The bolt and nut after tightening should be level with each other. During the actual tightening process, could it happen that one end sticks out while the other is recessed, meaning they are not fully engaged?