What is Valve Seat Recession
If you drive a pre-1986 vehicle, you need to know about VSR or Valve Seat Recession.
Valve Seat Recession is when the valves from an internal combustion engine embed themselves into the cylinder head. This is generally due to a lack of lubrication between the valve and the cylinder head.
What causes VSR?
The lead in petrol pre 1987, greatly improved combustion and increased octane levels. This high-octane fuel enabled cars to run higher compression ratios than cars of today. In addition, the lead oxides that formed during combustion would settle on the valve seats and act as a cushion, protecting the seats from erosion.
When leaded fuel was phased out, car manufacturers lowered compression ratios and started using hardened valve seats that did not require the lubricating properties of lead.
Consequently, when cars designed to run on leaded fuels were run on unleaded fuels, they run the risk of suffering valve seat recession.


How does LPG affect valves and valve seats?
LPG once vaporised is a pure vapour and has no lubricating properties at all. This then passes through the inlet valves and into the cylinder where it is ignited by the spark plugs, combustion takes place and the hot spent gasses exit through the exhaust valves.
These gasses are very hot and contain no lubricants or additives. They very slowly start to burn the valve face and valve seat of the exhaust valve away. This process is called recession. Over time the valve begins to recede in to its seat, subsequently reducing the valve or tappet clearance to zero. Once this has happened it is only a matter of time before the valve head starts to disintegrate causing loss of compression and misfire.