Tapping Method: Use a telescopic rod to gently tap on different areas of the tile. If the tile produces a hollow or echoing sound, it indicates airspace beneath it, suggesting it may be hollow. If the sound is dull or solid, the tile is more likely to be properly bonded to the surface. So if a tile is hollow, it can lead to several issues over time. The main effect is that the tile may become unstable and prone to cracking or chipping, as the hollow area beneath it means the adhesive has failed

Tap size Drilling chart | Thread Hole Size Specifications - Upmold

Drill Tap Referencing | CNC Precision Machining

Thread Chart (Metric Threads) - Major and Minor Diameters