Hex bolts, hex nuts and the 50% wrench rule

A few days ago, I was helping a friend put together a slide for his kids.  He’s a mechanical engineer and race car driver so this was a relatively simple task for him. 

While we assembled this unwieldy contraption, I noticed the frame was put together entirely with 1/2-inch bolts and nuts.  I told my friend, and he responded by handing me a 1/2-inch open end wrench.  I looked at the wrench then looked at him and said, “you too?”

I explained to him what I’ve explained to countless model builders during my 20 years in the micro hardware business.  A 1/2-inch wrench does not fit a 1/2- inch bolt.  Neither does it fit a 1/2-inch nut.

The 1/2-inch bolt has a 1/2-inch diameter threaded shank.  But the width of the bolt head (flat to flat) is close to or exactly 50% greater than the threaded shank diameter.  Therefore a 1/2-inch bolt requires a 3/4-inch wrench.

This 50% rule applies universally to all hex nuts and bolts regardless of size.  Our 1.0mm hex bolts here at Model Motorcars require the 1.5mm wrench.  Just like the 2.0mm hex bolts require a 3.0mm wrench.  Same applies to the nuts.

Many of you already knew this, I’m sure.  But for some folks it’s an epic revelation.  For others it is an attack on common sense and order in this universe.  But it is what it is.

Bob Breslauer