5 Steps to Fix Wrist Pin Knock

A funny little noise from the car is one of the issues auto owners and drivers find difficult to resolve. Otherwise called a wrist pin slap, the wrist pin knock can be nauseating especially if it becomes frequent and lingering. Let me concede that it can be difficult to diagnose or locate where the noise is coming from. However, with a careful study of the guide provided in this article, you shouldn’t have a problem fixing a wrist pin knock. Let’s get started.

What is a Wrist Pin Knock and How Severe is it?

Let me start by explaining what a wrist pin and wrist pin knock is. In an auto system, the wrist pin attaches the connecting rod to the piston head. The pin is located in the internal combustion engine, passing through the piston skirt. A bent wrist pin will cause a noise to happen. 

Typically, there are two types of wrist pins: the (semi-) floating pin and the anchored or fixed pin. With the support of a bushing, the fixed wrist pin hinges within the end of the connecting rod. On the other hand, the semi-floating pin connects to the rod through friction, allowing the piston to pivot freely within the piston pin bosses. 

When the wrist pin goes bad, a wrist pin knock occurs. You may notice this happens usually when your car’s revolution per minute (rpm) stays constant or keeps increasing. The knocking sound is audible when your car has hit between 1500-2000 rpm. It is very rare to observe the little noise when you back out of the rpm or the car is stationary. 

When your car piston reverses from the top dead center (TDC) to the bottom dead center (BDC), the wrist pin knock will go with a light double click. A cracking noise comes up only when your car is moving and the engine comes under a load. However, a bad wrist will cause the funny noise to occur even when the car engine isn’t even under any loads. 

This spark knock sound may keep increasing and becomes so loud. At first, it may seem that a valve clutter is responsible but the increasingly deeper level of the sound causes you to worry and perhaps you may not suspect the valve. But should you inspect the valve clutter? Yes. 

How to Fix a Wrist Pin Knock 

The thing to do when you observe the sound coming from the engine is to replace a failing wrist pin or faulty connecting rod. Either of these can cause a knocking noise. Needless to say, the piston locks must sync to the connecting rod at all times. 

But you must first identify if your wrist pin is bent. A bent wrist pin won’t install smoothly or nicely in the piston pin boss. As a symptom, you’ll feel some resistance or hear an audible knock as you try to install the pin in its boss. Alternatively, you may also notice an air gap at the top of the wrist pin. This may disappear on the outside but indicate on the inside.

Now that you’ve established that there’s a bent wrist pin, what should you do? It’s simple. You should remove and replace the bad pin. 

  • Step 1: Park your car on a level ground 
  • Step 2: Start the engine and allow it to idle. The engine will run and the wrist pin knocks
  • Step 3: Pull out or short each plug wire, one after another, using a test light. The noise gets quieter when there’s no load on the piston. There’s a likelihood that the noise gets louder as you pull the plug wire 1 and quieter on numbers 3, 5 or 7. 
  • Step 4: The noise from the wrist pin will naturally disappear as soon as you get to the bad cylinder section. 
  • Step 5: However, if the noise doesn’t go away, you may need to go check the valve. A little valve adjustment may be all you need 

In case there is a loose bolt that holds the fuel pump drive cam I had a similar noise in my 351C engine in my 53 Ford. The bolt that holds the cam gear and fuel pump drive cam, the wrist pin knock will come up. 

Consequently, you may that the pump cam is hitting the timing cover. You shouldn’t allow the noise to linger for too long to avoid the connecting rod from breaking or causing any damage to the engine. In that case, you should tighten the bolts to hold the wrist pin securely in place. Alternatively, you may need to replace the bolts in case they’ve lost their threads. 

The other technique is to run thicker oil in the range of 15WD-50WD. This may lessen the noise at least temporarily. You can also go with 20-grade thinner oil to cushion the wrist pin noise.  

FAQS

Should a Wrist Pin Rotate?

Normally, the wrist pin should rotate between the piston bosses. This way, it can ensure that the connecting rod can maintain a smooth and nice spot from side to side. By design, the piston boss should be at an angle of 90 degrees to the crankshaft and the wrist pin. 

Can a wrist pin knock go away with time?

The severity of a pin knock is no doubt and when you hear the sound, it means that your engine isn’t in good condition. Note that a wrist pin knock isn’t an issue that’ll resolve itself or naturally die off or go away unattended. There are steps to take to fix the wrist pin knock. 

Conclusion

Usually, a wrist pin knock often sounds like a piston slap or lifter noise, especially when the pin has bent and got bad. When this happens, the pin will knock on the water pump’s downstroke and upstroke, producing a lifter-like sound. 

A wrist pin knock could also be similar to some lower-end noises or noise from worn journals. Regardless, you should do everything almost immediately to resolve the issue before it gets worse and cause collateral damage to your car engine.  

  • James Smith

    I'm James Smith, a car enthusiast and a content writer for Daily Car Tips. I have a passion for all things cars, from classic muscle cars to the latest electric vehicles. When I'm not writing about cars, you can usually find me tinkering with my car or checking out the latest automotive trends.

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