The decompression valve: start your petrol chainsaw with ease

The STIHL decompression valve releases compression to make starting your chainsaw significantly easier.

A man wearing protective equipment working with a STIHL TS 500i cut-off machine on a construction site

What is a decompression valve?

The engine cylinder of a chainsaw contains a mixture of gases. When you begin the starting procedure this mixture needs to compressed in order for ignition to succeed – but excessive pressure requires you to put a lot more effort into starting the engine. When the starter cord is pulled, the piston inside the cylinder moves, compressing the fuel-air mix and increasing the internal pressure. Overcoming this pressure on chainsaws with a larger engine capacity takes considerable effort, but a decompression valve reduces the pressure, making the engine easier to turn over. There's a variety of engines fitted with decompression valve systems that are activated in different ways - we'll talk about them later on!

A STIHL MS 261 C-M petrol chainsaw being started by someone wearing protective clothing

2-stroke engines - a brief overview

Not all combustion engines feature a decompression valve, but all 2-stroke engines work on the same principle: a mixture of fuel and air is drawn through the cylinder and into the combustion chamber, where it is put under significant pressure and then ignited. This process moves the piston, which continues the compression and ignition process while also working with the connecting rod and crankshaft to convert the stroke motion of the piston into a rotation of the crankshaft. This stroke can then drive additional components, e.g. a saw chain on a STIHL chainsaw.

The decompression valve

You may wonder why our chainsaws don't have an electric starter. The reason for this is because the fuel-air mix is compressed to a pressure of several bar, in order to overcome the mass inertia of the crank mechanism. The larger the stroke motion of the engine, the more power needed to do this. To make the starting process faster and easier, we have instead developed a decompression system that is based on a small but effective component: the STIHL decompression valve.

How does the STIHL decompression valve work?

Decompression means easier starting

Opening a decompression valve releases gas to ensure that the peak pressure when starting the engine is reduced. This compression release means less effort is required when starting your chainsaw or other tool. As soon as the engine starts, the decompression valve automatically closes again; compression increases; and the start-up momentum of the cylinder initiates normal combustion operation so you experience full engine performance.

Decompression system types

There are three decompression valve types available, all of which are activated in different ways. Below is an overview of the different systems for compression release.

Close-up of a decompression valve on a STIHL petrol chainsaw

Can decompression valves be retrofitted?

In principle, yes they can. Not all chainsaws are large enough to need a decompression valve, but if your STIHL chainsaw has a blind plug on the cylinder, you can assume it can be used to retrofit a decompression valve. Contact yournearest  STIHL Approved dealer to discuss further. 

Summary: the STIHL decompression valve:

  • The STIHL decompression valve releases gas to reduce peak compression pressure in the cylinder of your chainsaw
  • The STIHL decompression valve makes it easier to start engines that use pull cords
  • Engines in general may be fitted with manual, semi-automatic or automatic decompression valve systems
  • All STIHL decompression valves close automatically after the engine starts
  • Decompression valves can be retrofitted in STIHL tools that are appropriately equipped