The Clearance Hole: Why the Bolt Should Not Touch the Top Part

Picture two plates with a threaded hole through both, and a bolt wound into them. As the bolt turns it advances through BOTH sets of threads at the same rate. Nothing moves relative to anything: the gap between the plates never closes. Clamping force comes from stretching the bolt between the head and the threads it is engaged in. That needs the top part to be able to slide along the shank, which it cannot do if it is threaded. Test it once and it is unforgettable. Two plates, one threaded pair of holes, tighten hard, and they will still rattle. Drill the top one out to clearance and they lock together instantly with much less effort.

CLOSE fit is about 0.1 to 0.3 mm over the bolt — an M6 in a 6.4 mm hole. It locates the parts precisely and demands accurate hole positions. NORMAL fit is around 0.5 mm over — M6 in 6.6 mm. This is the everyday default and absorbs ordinary marking-out error. LOOSE fit is a millimetre or more over, for assemblies where the holes cannot be positioned accurately or where thermal movement must be allowed for. Choose deliberately, because the clearance hole is where you decide how much adjustment the assembly will have. Too tight and the parts fight you; too loose and nothing lines up where you intended.

A wood screw needs two holes, not one: a PILOT hole in the lower piece that the thread bites into, and a SHANK hole in the upper piece that the thread passes through freely. It is the identical principle. A screw whose thread grips the top piece jacks the two apart instead of pulling them together, and in wood this leaves a visible gap in the joint that no amount of driving will close. Modern screws with an unthreaded upper shank do the same job by being shaped for it. In hardwood the pilot hole also stops the screw splitting the piece, and near an end grain edge it is not optional at all. Match the pilot to the screw's root diameter — not its outside diameter — and the thread has full material to bite.