Tech Talk #49 – Wrist Pins and Unintended Consequences

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“As power levels have escalated, many racers are using wrist pins that are just too light.”

I come from a family of teachers, so perhaps I’m genetically programmed to stand in front of a classroom. Maybe that’s the motivation behind the engine building classes that we conduct regularly at Reher-Morrison Racing Engines. What I’ve discovered, however, is that a teacher learns as much from the students as the students learn from the teacher.

When a racer attending one of our seminars asks why we prepare a part a certain way, I have to think about the experiences that led us to adopt a particular technique or choose a specific engine component. A case in point is the lowly wrist pin – one of the most overlooked yet most important components in any motor.

The wrist pin is a crucial link in the chain that connects the power to the pavement. It has to withstand the full force of the cylinder pressure while accelerating several thousand pounds of race car. Yet racers who will gleefully study cylinder head airflow graphs and camshaft profiles for hours seldom give any thought to the wrist pins in their engines. Let’s face it: Wrist pins simply aren’t sexy.

I’ve built racing engines for more than 30 years, and have always subscribed to the belief that lighter is better. But recently I’ve had to reevaluate my thinking in regard to wrist pins. I’ve come to the conclusion that as power levels have escalated, many racers are using wrist pins that are just too light. I’ve also realized that some of the parts that extract more horsepower from an engine also increase the stresses on the wrist pins. Racing isn’t exempt from the rule of unintended consequences: Every solution breeds new problems.

When selecting parts for a racing engine, the general rule is “light is good.” For example, a lightweight crankshaft assembly require less power to accelerate than a heavy crankshaft assembly, and therefore more of the engine’s output can be used to accelerate the race car. But if the parts are so light that they deflect and deform under high loads, then the result is the exact opposite – more friction is created and more power is siphoned off in the form of heat that destroys the pins, pistons, and connecting rods.

In reality, more power equals more cylinder pressure, so we have to select parts that are appropriate to the engine’s power levels. Once you go beyond a set of off-the-shelf pistons, the choices of wrist pin material, diameter and wall thickness become critical.

When a customer orders a set of wrist pins, the first question I ask is, “How powerful is the engine you’re building?” A set of tool steel wrist pins with a .090-inch wall thickness might be fine for a small-block Super Stock engine, but a 1,000-horsepower big-block typically needs pins with .150-inch wall thickness – and the pins must be correspondingly thicker for a nitrous-injected engine. The wrist pins used in blown alcohol and nitro-burning engines illustrate just how strong the pins must be to survive under extreme conditions.

A catastrophic pin failure is an expensive way to learn that the pins are too light. Fortunately there are some early warning signs that indicate the pins are overstressed. Black streaks in the pin bosses or the small ends of the connecting rods are danger signs. In more advanced cases there may be aluminum welded to the pins from the piston pin bosses and rods (in the case of aluminum rods). With steel rods, look for signs of distress in the bronze bushings in the small ends of the rods. These problems are usually the result of wrist pin deflection, insufficient clearance or inadequate lubrication.

Wrist pin problems were rampant in Pro Stock several years ago, and the cure was to apply some very expensive coatings. Spending $800 for coated pins is not a cost-effective solution for most sportsman racers, however; the object is to make as many runs as possible at a reasonable price. Assuming that the wrist pins have adequate strength, it’s possible to head off many problems simply by ensuring that the pins have adequate clearance and lubrication.

The crankcase in an engine with a conventional wet-sump oil pan is awash in oil. If there is a condition that’s heating up the wrist pins or stressing the piston and rod bores, the sheer volume of oil in the crankcase will carry off the excess heat. But a well-designed oil pan with a kick-out, crankshaft scraper and a vacuum pump (or a good dry-sump system) will dramatically reduce the amount of oil in the crankcase. While an efficient oil system reduces windage and increases horsepower, it can also put the wrist pins in jeopardy – another instance of unintended consequences. In fact, the wrist pins in the even-numbered cylinders are often the first to show signs of distress caused by insufficient lubrication because they are on the side of the engine that has much of its lubrication stripped away by the crankshaft scraper and oil pan kickout.

In this situation, my recommendation is to increase the wrist pin clearance. Most engine manuals recommend wrist pin clearances between .0008 and .0010-inch for conventional engines; my advice is to run .0020 to .0022-inch wrist pin clearance in a serious drag racing engine. My perspective on engine clearances is straightforward: When in doubt, a little loose will seldom result in catastrophic failure, but a little too tight will almost always cause problems.

Remember that drag racing engines are usually stone cold when they go down the race track. We start the engine and get to the staging lights as quickly as possible because a cold engine makes more power than a hot one. Consequently drag racers don’t have the luxury of an extended warm up period to bring all of the parts up to operating temperature. In these circumstances, a little extra clearance is a good thing.

I’m an advocate of double Spirolox pin retainers. Yes, they are a pain to install and remove, but that’s exactly what’s needed in a pin retainer – a lock that won’t come out on its own.

Wrist pins aren’t glamorous, but they are absolutely essential to the health and well being of a racing engine. Don’t scrimp on pins; a few extra grams of wrist pin weight can increase your engine’s life expectancy.