Carpentry and Woodworking - Sizing hole for screws

I have a small safe that I'd like to hide in the attic. My plan is to put it under the insulation and bolt it down to the top plate of a wall. The safe is a quality model that came with an assortment of hardware, I would like to use some of the screws, they are small lag bolts with 3/8" heads. My micrometer says the outside diamter of the screw (threads) is 1/4", the shaft is smaller of course.

What size drill bit should I predrill the holes with? Is this screw small enough to sink in without drilling a hole, or should I drill to make sure the top plate does not crack?

Is there any simply formula to choosing the drill size for a particular screw? For example, should it be the diameter of the shaft? Somewhere between the thread diamter and the shft diameter?

Thanks!

You have it....diameter of the shaft is what most people use. Though it sometimes depends on type of wood, length and type of screw, etc...but dia of shaft is rule of thumb, gives max bite to the threads. Use soap or a candle rubbed on the threads to help it drive easier.

Hi, One thing you might think about, is the attic is a poor place to have your valuables in case of a fire. Most small safes have at best a limited time for which they will withstand intense heat. In terms of fire loss, the safest place in the house is the basement. Of course your gold stash will survive just about anywhere :).

Bud

It's an extra gun safe that I had laying around. I figured I'd put a last ditch handgun and some cash up there.

The economy is getting bad, you can't be too prepared for the upcoming crime increase.

Ya, born in Montana and used to think my parents and grand parents were a bit overly negative about our government. Then lived in NJ and had an episode of riots come right to my door step and realized you have to be able to take care of yourself. Suddenly my elders started sounding a whole lot smarter:). Now I'm the elder and much better prepared.

Bud