Head Sprung

Bench Racing

by Ryan King

Have you ever seen these crown-headed bolts?

If you've removed a stock Ford V8 clutch assembly or differential, you probably have.

This is a place bolt.

This bolt is both self-locking and something much more ingenious: it's a shock absorber.

Not only is it tough as nails – well, technically, much tougher – but the head design is such that it deforms to absorb shocks. Which is why Ford uses them on both clutch assemblies and in differentials.

In fact, ARP has their own version of a place bolt and if you've used their flywheel, pressure plate, or ring gear bolts on your Ford, Chevy, or Mopar, you have likely used place bolts.

The way they work is this: When a shock hits the bolt, instead of deforming the shank by stretching it, which causes the bolt to loosen, the head itself compresses, thus absorbing the shock and maintaining the proper load of the fastener.

The way it achieves this is through the design of the head. By counterboring its underside, it deforms to a true bearing surface when the bolt is properly tightened, thus increasing the residual stress in it, creating, in essence, an integral lock washer. As a part of the design, the head is stronger than the shank and when a shock hits the slotted head, it is able to compress similar to a spring – deforming in place of the shank – which allows the shank to maintain its load without elongating further, before the head snaps back into shape, thus retaining correct bolt load through the shock.

According to legendary race car engineer, Carroll Smith, this design was likely originally invented to absorb shock loads in steam engines.

If you found this post fascinating, I highly recommend checking out Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook for a much more in depth look at fasteners and their use in automobiles.

Ryan