The diameter of the pilot hole for a lag bolt should be a bit smaller than that of the bolt itself. Like this stays enough material, so that the threads of the bolt will keep. The right size of pilot hole depends on two main factors: the size of the bolt and the type of the wood material.
Good pilot hole is simple step that very helps in construction and woodworking. It stops splits and ensure solid linkups. Also it saves time and material.
How to choose the right pilot hole size for lag bolts
Split wood do not give stronger tie. Use a drill with size according to your bolt and wood type. Without pilot you can not do that.
General rule says: choose drill of 1/8 to 3/16 inch smaller than the bolt. For 1/4 inch lag screw in oak answers 5/32 inch bit for hardwoods. For 1/4 inch lag screw, 3/16 inch bit could work.
At 3/8 inch lag bolt you use usually 1/4 inch diameter for wood or soft materials. That is 65 (75% of the screw diameter). At 3/8 inch bolt the unthreaded shank have same diameter.
For such holes 3/16 inch bit should work. For 1/4 inch screw 3/16 inch bit is one size less.
The wood type play big role. Most commonly you take 15/64 inch bit. For hard woods answer 5/16 inch.
Correct according to soft, medium or hard wood. In softwoods value to bid more small size. 1/32 inch difference does not matter much in pine, fir or spruce.
At wet wood the diameter can grow in 1/32 to 1/16 inch. Hard woods sometimes require bigger hole. Always start by means of little and expand it, if too much.
HSS drill bits and other cheap options work well for such holes. Big bits must go slowly, to escape heat. Help yourself: erase the sawdust from the drill flute between two strokes.
Like this heat does not stay trapped at the metal. Start initially in slow speed, to do not split the wood.
