√ What is a Blowby Engine, Complete with its Parts

In the world of internal combustion engines, blowby is a term that engine enthusiasts and mechanics often come across.

It refers to the leakage of combustion gases from the combustion chamber past the piston rings and into the crankcase.

This article will delve into the meaning of blowby engine, its common causes, how to fix it, and preventive measures to avoid blowby occurrences. So, let’s explore the world of blowby engines.

What Is Blowby Engine?

What Is Blowby Engine

Blowby engine, also known as an engine blowby, refers to the phenomenon in which combustion gases escape past the piston rings and enter the crankcase of an internal combustion engine.

This occurs during the combustion process when the cylinder pressure exceeds the sealing capability of the piston rings, resulting in gas leakage.

Blowby is commonly found in both automotive and motorcycle engines and can lead to various performance and reliability issues if left unaddressed.

4 Common Causes of Blowby

1. Worn Piston Rings

Worn Piston Rings

Over time, piston rings can wear out, lose their sealing ability, and develop gaps. This allows combustion gases to bypass the rings and enter the crankcase.

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2. Cylinder Wall Wear

Cylinder Wall Wear

Excessive wear on the cylinder walls can result in irregularities that prevent the piston rings from sealing properly, leading to blowby.

3. Clogged or Faulty PCV System

Clogged or Faulty PCV System

The Positive Crankcase Ventilation (PCV) system is responsible for regulating the pressure and removing excess gases from the crankcase. If the PCV system becomes clogged or malfunctions, it can cause increased crankcase pressure and blowby.

4. Over-pressurized Crankcase

Over pressurized Crankcase motocycle

A crankcase that is overfilled with oil or has excessive blowby gases can experience increased pressure. This pressure can force gases past the piston rings and contribute to blowby.

How to Fix Blowby

1. Engine Blowby Tester

Engine Blowby Tester motocycle

An engine blowby tester is a diagnostic tool that measures the amount of blowby gases escaping from the engine. By connecting the tester to the crankcase, it provides valuable information about the severity of blowby and helps in determining the appropriate repair steps.

2. Engine Blowby Flow Meter

Engine Blowby Flow Meter motorcycle

A blowby flow meter is another device used to measure the volume of blowby gases passing through the engine.

It aids in quantifying blowby and can be helpful in diagnosing the underlying issues causing blowby.

3. Piston Ring Replacement

Piston Ring Replacement motorcycle

If worn or damaged piston rings are the root cause of blowby, replacing them with new ones is often necessary.

This repair requires disassembling the engine and carefully installing the new rings to ensure proper sealing.

4. Cylinder Honing or Re-boring

Cylinder Honing or Re boring

In cases where cylinder wall wear is contributing to blowby, honing or re-boring the cylinder walls may be required.

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This process restores the smoothness and correct dimensions of the cylinder walls, allowing the piston rings to create a better seal.

5. PCV System Maintenance

PCV System Maintenance

Regular inspection and maintenance of the PCV system, including cleaning or replacing clogged valves or hoses, can help alleviate blowby issues caused by a faulty or obstructed PCV system.

How to Prevent Blowby Engine

Regular Maintenance Blowby Engine motorcycle

1. Regular Maintenance

Following the manufacturer’s recommended maintenance schedule, which includes oil changes, filter replacements, and routine inspections, can help prevent blowby by keeping the engine in optimal condition.

2. Proper Engine Break-In

During the initial stages of engine operation, it is crucial to follow the manufacturer’s recommended break-in procedure.

This process ensures that the piston rings properly seat against the cylinder walls, promoting effective sealing and reducing the likelihood of blowby.

3. Avoid Overloading the Engine

Subjecting the engine to excessive loads, such as towing beyond its capacity or over-revving, can increase cylinder pressures and contribute to blowby.

Operating the engine within its designed limits helps maintain optimal sealing and prevents blowby occurrences.

4. Quality Lubrication

Using high-quality engine oil that meets the manufacturer’s specifications helps minimize friction between the piston rings and cylinder walls, reducing wear and the likelihood of blowby.

5. Regular Inspection of Piston Rings

Periodically inspecting the condition of the piston rings during maintenance intervals allows for early detection of wear or damage.

Timely replacement of worn rings can prevent blowby from worsening.


Understanding blowby engine, its causes, and the necessary preventive measures is essential for maintaining a healthy and reliable internal combustion engine.

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Regular maintenance, early detection of issues through the use of blowby testers and flow meters, and appropriate repairs can help mitigate blowby problems and ensure optimal engine performance.

By following these guidelines, enthusiasts and vehicle owners can enjoy the benefits of a well-maintained engine that operates efficiently and reliably for years to come.


How Serious is Engine Blowby?

Engine blowby is a serious issue that can have significant consequences for engine performance and longevity.

It leads to power loss, decreased fuel efficiency, increased oil contamination, potential damage to engine components, and higher emissions.

Prompt attention and maintenance are necessary to prevent further damage and ensure optimal engine performance.

What Causes Engine Blowby?

Engine blowby can be caused by worn piston rings, cylinder wall wear, clogged/faulty PCV system, and over-pressurized crankcase.

How Much Blowby is Normal?

A small amount of blowby is considered normal in an engine, but the acceptable level can vary.

Regular monitoring during maintenance is recommended to ensure it remains within an acceptable range.

Originally posted 2023-07-01 19:23:39.

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