How to Fix Rod Knock

There is no easy way to put this – if ignored, rod knock can result into very serious issues with...

There is no easy way to put this – if ignored, rod knock can result into very serious issues with your car engine.

Rod knock occurs when the rod bearing has been partially or completely busted. It can be serious enough to render the vehicle unusable. The bigger issue with rod knocks is that they lead to other cascading effects such as a broken connecting rod and resultant holes at the periphery of the engine block. Drops in oil pressure are also commonly observed.

Not mincing words, a rod knock is the equivalent of a distress signal from your engine. This is not to say that the engine is beyond fixing, but before having to resort to getting a mechanic to work on it, there are certain mitigating measures you can undertake to not worsen the situation. In some cases, you may even be able to fix it.

So what it is that happens for a rod knock to occur? More importantly, is it possible to employ any mitigating measures?

Mechanics Involved in Rod Knock

The rod being discussed about here is the connecting rod and it is that part of the engine which connects the pistons to the crankshaft. At the juncture at which the rod meets the crankshaft, there are bearings which distinguish the rod from the crank journal.  When an engine is manufactured, there are very crisp gap tolerances incorporated into the design. Smooth in texture, these bearings are made of a different material to the rod or the crankshaft. Modern engines utilise bearings made out of lead-free aluminium alloy.

Engine designs incorporate certain gap tolerances, which are very precise distances with an order of magnitude of around 10-3 inches. The purpose behind these gaps varies with their placement. The rod bearing to the crankshaft journal is no exception to this rule. Gap tolerance is incorporated so as to let out a preordained amount of oil from a hole in the crankshaft journal at the junction with the rod bearing. Likewise, the oil film on the rod bearing serves a very important role – it prevents the rod bearing and crankshaft journal from making contact when the pistons kick in during engine operation.

What may happen due to wear and tear is that this specific gap tolerance could get too big. Consequently, the oil no longer films adequately and what then occurs is rod knock.

Why Is It Called A Rod ‘Knock’?

Why ‘knock’? The term knock is used because of the sound that is emitted when the bearing comes into contact with the crankshaft courtesy of skewed lubrication. This occurs during the engine’s power stroke. Really extreme cases of rod knock happen when the rod itself comes into contact with the crankshaft journal. Naturally, this happens at the point wherein the bearing has spun out of place.

rod knock

While the bearing may degrade due to just normal wear and tear, it may also erode due to overheating of the bearing material. This overheating is usually a result of oil starvation since the parts cannot move over each other without generating tremendous amounts of friction and its by-product – heat. At times, even abusive driving could be a contributory factor as it can lead to a rod cap stretch. Overall, rod knock can be caused due to a a standalone or a combination of any of the above mentioned factors.

Guidelines for Fixing Rod Knocks

If diagnosed early enough, rod knocks can be fixed without much ado. What is enumerated below is probably the most cost-effective and easy to follow routine that will help mitigate the damage and if lucky, even fix it. To appraise the extent of damage, you will need:

  • Mechanic’s ramps: Mechanic’s ramps are imperative in getting a good view as well as providing an easy point of access to the engine. They help elevate the car so that any repairs can be carried out in more natural body positions.
  • Oil pans: Since rod knocks are a consequence of oil imbalance, there is a great chance that oil will spill from the engine parts while assessing the damage. It is also crucial to drain the oil before arriving at a fix for the rod knock. An oil pan does a great job of collecting the oil and preventing a mess which nobody likes, right?
  • Oil-filter wrench and a new oil filter: The oil-filter wrench makes the cumbersome process of removing the oil filter and replacing it with a new one very intuitive to execute. Occurrence of rod knock may have led to the oil filter not functioning well. Remember the cascading effects that were being discussed about earlier? The new oil filter will be useful in such a situation.
  • Fresh oil: Oil that has been drained out will have to be replaced.
  • Fuel injection cleaner: While this isn’t necessarily required, it is recommended as a part of the clean-up process. The fuel injector has several deposits generated as by-products of the combustion process. However, if your car has a carburettor, use a carburettor cleaner instead. It can be sprayed directly onto the carburettor itself.

rod knock with pistons

Now that you have the inventory in order, it is time to get down to how to use them to fix the rod knock. It is a fairly straightforward 5-step process:

  • To begin with, drive your vehicle onto the mechanic’s ramps. Make sure that vehicle is in a stable position that offers easy access to the engine and does not let the vehicle slide back down.
  • The next step entails draining the oil out. To do this, position the oil pan under the oil cap. After completely draining the oil from the vehicle, replace the oil cap.
  • Having drained out the oil, the next step is to replace the oil filter. To do so, use the oil-filter wrench. Fit it in at the middle of the oil filter and turn in clockwise direction to remove the oil filter. Fit in the new oil filter into the slot and rotate the wrench anti-clockwise to screw it in.
  • The drained oil would need to be replaced at this point. About five quarts of fresh oil should suffice. Remove the oil pan once done. It is time to inspect the rod bearings now. Replace or tighten the bearings as per your assessment of the damage that has been caused.
  • Next step in the maintenance process is the fuel injection or the carburettor cleaner and using either will prevent grime from entering your engine.

Serious Avatars of Rod Knocking

While these steps will help resolve cases of rod knocking caught early enough, however, it is not a remedy for dealing with more serious stages of the problem. In such situations, it will become necessary to get a mechanic to have a look at it. This could be an expensive affair. However, there are certain parameters you can use to assess the damage caused and the consequent charges of repair. Some easy to inspect signs are the duration for which the noise has been occurring and any visible damage that may have been caused to the engine. More advanced cases will result in metal shavings having been circulated in the engine. Additionally, the more specialised an engine, the greater will be the expense for fixing a serious case of rod knock.

Since a knock rod can trigger a domino effect on the other parts, replacing the connecting rod will also involve replacing additional components of the engine, such as the seals and gaskets. The engine might need to be flushed and cooling lines will have to be replaced. More direct replacement costs could be incurred in replacing the crankshaft, the timing chains and even the piston rods.

Yes, it may require an entire engine overhaul at times.

Concluding Remarks

Often, there are some other noises which may be mistaken as being symptomatic of rod knocks, some of them being piston slap, valve train, piston pin ticking or even detonation. If you believe you have a rod knock with your vehicle, you should always seek a second opinion from a reputable mechanic.  Be especially careful about this, because diagnosing engine noises is a fairly complex thing to do. In fact, misdiagnosis is the norm in this case.

Diagnosing engine noises is actually a probabilistic exercise – it is almost entirely about eliminating all other possibilities and narrowing down to one. Having said that, rod knocks do not occur on a regular basis. As a rule of thumb: these are loudest at higher speeds above 2500 RPMs. For instance, it is easy to mistake it for a piston slap, which occurs due to the malfunctioning of the wrist pin which is literally on the flip side of the rod bearings.

Rod knocks can lead to drastic consequences. You may not be able to drive the car right away. The worst possible thing to do is hope that it fixes itself because it will not. In fact it gets progressively and exponentially worse. A rod knock is indeed an expensive affair to rectify. This makes it doubly important to be sure what you are shelling out your hard earned cash for so that you are adequately educated about the condition and also the costs associated with it.

Sources:

  1. Rod knock – Causes, Information, and Fixes – Mechanic Base
  2. What is engine knock? – howstuffworks