Some like it hot … pistons need it cool

The piston cooling nozzle may be a relatively small part, but it does an extremely important job for the piston: preventing it from overheating and protecting the engine from serious damage.

The combustion heat leads to very high temperatures throughout the cylinder and at the piston crown (reaching over 500°C in steel pistons). And this is where the cooling nozzles come in. During operation, they cool the pistons continuously. How? They inject the engine oil onto the pistons from below, reliably dissipating the accumulated heat. It’s pretty cool actually!

An underestimated little part: the piston cooling nozzle

Cooling nozzles in the engine block

It might look unspectacular, but it’s essential for an engine: the cooling nozzle is a small, curved tube that protrudes into the bottom of the cylinder. Depending on the engine type and output, a modern (turbo) engine usually has one nozzle per cylinder.

The construction of the piston is also important for cooling. Diesel engines often use pistons with a cooling channel, while spray jet cooling is sufficient in petrol engines, so no cooling channel is required. In diesel engines, the stream of oil is sprayed out of the nozzle directly into the piston’s cooling channel. In the annular cooling channel, the oil circulates through the piston, absorbing heat. It then flows through a second opening in the piston back into the oil pan.

Check out the process in this short animation:


Small cause, major damage

Maybe you had the best intentions of repairing the crank mechanism, but ended up with a piston seizure or similar damage? What could have caused this? It could be a damaged cooling nozzle!

To find out, have a look at the following damage scenarios:

  • The piston has scoring and seizing marks on the skirt.
  • The piston crown, ring belt, and top land are melted or scorched.
  • The cylinder liners are discolored in places, or even cracked.

An extremely serious situation: this annoying and expensive kind of damage often occurs when the piston is removed or fitted. Even with good intentions, you can end up making things much worse. If the cooling nozzles aren’t removed before fitting or taking out the piston, they can be inadvertently damaged or even torn off by the connecting rod. As a result, the oil doesn’t get to where it’s supposed to go.

  • Overheated Ferrotherm piston (steel piston crown and aluminum skirt) showing seizing marks, with cylinder liner
  • Cooling nozzle fitted (top) and broken (bottom)
  • If the nozzle is bent, the cooling engine oil doesn’t get to where it’s supposed to go
Noticeable marks on piston skirt indicate cooling nozzle impact

Important to note during installation!

The nozzles should be removed before fitting or removing the pistons

Always remove the cooling nozzles when carrying out any repairs requiring removal and fitting of the piston and connecting rod. It’s crucial to make sure the cooling nozzles are properly aligned and in good working order when fitted; gasket residue and deposits may clog the nozzles and should therefore always be thoroughly removed.

If the piston cooling nozzles are damaged, the piston crown will not be cooled—or insufficiently cooled—during operation after the repair. This leads to the worst-case scenario: overheating of the piston and cylinder, resulting in serious engine damage!

Watch out for clogging!

Failure to observe the oil change interval may result in damage to the cooling oil nozzle. In this case, there’s a risk of polymerization of the engine oil, particularly with biofuels such as rapeseed and soybean oil, which can cause clogging of the cooling oil nozzles.


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


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