What size should the bottom hole for M30 be drilled to?
3 Answers
m30 bottom hole size should be determined based on the pitch. Concept of bottom hole: The bottom hole generally refers to the diameter of the pre-drilled hole before tapping internal threads with a tap. It usually refers to the built-in circular hole of the workpiece, which is the bottom or top circular hole, hence called the bottom hole. However, the bottom hole is not necessarily drilled out; it can also be processed by other methods. Calculating the bottom hole size: Since the formula for the bottom hole is thread diameter minus pitch, given that the thread diameter is 30 mm and assuming the pitch is 1.5 mm, substituting into the formula gives the bottom hole size as 30 - 1.5 = 28.5 mm.
When it comes to the drill size for M30 tap holes, I often encounter this issue during car repairs. M30 refers to screws with a thread diameter of approximately 30mm, and the tap hole is the pre-drilled hole for threading. Under standard conditions, the pitch is typically 3.5mm, so a drill size of around 26.5mm is recommended. However, in automotive repair, you need to pay extra attention to the material—softer metals like aluminum may require a 26.2mm drill to reduce burrs, while harder materials like steel may need a 26.8mm drill to prevent thread jamming. I usually measure the bolt first to confirm the pitch, then select the drill bit and proceed slowly to avoid misalignment, which can break tools during tapping. It’s best to consult the vehicle manual, as specific models may have different requirements—for example, chassis holes must be precise to prevent loose screws and safety risks. Always keep a caliper handy to measure accurately before drilling to avoid rework.
I usually work on modified cars and have learned lessons from drilling M30 bottom holes. It's about 26 to 27mm, depending on the thread pitch parameters. I measure the bolt diameter and subtract the thread pitch—for example, a standard pitch of 3.5mm gives a 26.5mm drill hole. In practice, I start with a 26mm small drill bit, drill a little, then test-fit the bolt hole. If it's too tight, I increase the size slightly. Remember to use a lathe or fixed bracket for drilling to avoid shaky hands making the hole crooked—I once drilled by hand, and the screw wouldn’t go in, wasting effort. When modifying, also consider the location—for example, drilling on an engine mount—if the position is off, the whole part could be ruined. Keeping a thread pitch chart or checking data on your phone is easy. I recommend using coated drill bits to reduce friction.
