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Engine Valve Head Fracture at Stem To Head Radius

Head Fracture - Valve Fracture at Stem to Head Radius - Advanced Engine

Understanding Engine Valve Head Failure

One of the most common engine valve head fractures is the failure at stem to head radius. This is very important to remember because since the least common result of valve failure is the valve itself, you must always make sure you can identify the root cause of a fracture prior to replacing the engine valve (s).

While a small part compared to a whole engine, a valve stem fracture (VSF) will typically results in considerable and costly engine damage. This is especially true for double overhead cam, four valve per cylinder engine designs.
The majority of VSFs are due to one way bending, although two way bending and reversed bending by rotation will also sometime occur. As mentioned earlier, the most common stem fracture appears as a valve head to stem breakage in the region of the junction of the head radius and the stem , see Photo 1.
Head Fracture - Valve Fracture at Stem to Head Radius - Advanced Engine

Photo 1. Typical valve stem fracture at the underhead radius and stem junction. The stem fracture face is
usually in good condition, while the valve head fracture face is typically too damaged to be of use.

Photo 1. Typical valve stem fracture at the under-head radius and stem junction. The stem fracture face is usually in good condition, while the valve head fracture face is typically too damaged to be of use.

 

 

 

 

 

 

 

 

 

What about a Engine Valve Head Fractures At Stem On A Welded 2 Piece Design?

Even when the stem is a welded two piece design, this type of fracture does not occur at the weld, but typically closer down towards the head. The stem is often drawn up into the head and the fracture face remains undamaged, while the valve head typically drops into the cylinder and is too damaged for the fracture face to provide evidence.

 

As a result, we at Advanced Engine can determine the cause of valve stem fracture is rarely a manufacturing or materials fault and nearly always an operational fault.
Some of the this that will cause this type of VSF are all mechanical, including wear and breakage, which have nothing to do with the combustion chamber environment.

 

The Top 3 Most Common Causes for Engine Valve Head Fracture at Stem To Head Radius

  • Excessive spring pressure
  • Piston to valve contact bending valve
  • Excessive seat to guide roll-out.

These breaks at the stem are diagnosed as “Impact Failure” , meaning there is an appearance of an impact as the starting point of the break. This types of breaks are sudden, or “Fatigue Failure” which means that the material got weaker overtime, types  after thousands of cycles, starting out small and eventually breaking.

Other Engine Valve Head Fracture at Stem To Head Radius Causes:

  • Engine overspeed.
  • Incorrect valve timing.
  • Sticking valve stem.