The wrist pin links the connecting rod to the piston in your engine. That’s simple enough. But the job of the wrist pin is absolutely critical. Should the pin move laterally, then sudden (and utter) engine destruction is pretty much guaranteed. It’s that simple. Now, there are a number of ways to keep a wrist pin locked within the piston. The most common is some sort of pressed pin arrangement or a floating pin with a lock ring of some sort. There are components out there called pin “buttons”, but for the most part, buttons are only used on supercharged drag race applications (blown alcohol, top fuel, etc.).
Pins that are pressed are likely the most common in passenger car applications. In this situation, the wrist pin is press fit into the small end of the connecting rod. The reciprocating motion necessary in the connecting rod-piston component of the engine occurs between the wrist pin and the wrist pin bore inside the piston. Engines designed with press-fit pins must have the rods and pistons assembled by an engine (or machine) shop using specialized equipment. Essentially, the small end of the connecting rod is heated and the piston is set in place over the rod small end. Next, the pin is pressed into place (through the pin bore in the piston, into the connecting rod small end).
Wrist pins that “float” are regularly found in high performance engines. In this configuration, the pin physically floats within the connecting rod small end. Instead of being pressed in place, the wrist pin is held in place by way of one of three different types of locking devices – Spiral Locs, Round Wire Locks or Snap Rings (most often referred to as “Tru Arcs”). High performance pistons are machined with special wrist pin retainer grooves that accept the locking device (many are machined for double retainers per side). The most effective retainer, and perhaps the most common is likely the Spiral Loc (this is manufactured from a flat coil of hardened steel). Unfortunately, the Spiral Loc is also the most difficult of the wrist pin retainers to install. The photo below gives you a little more advice for installation.