These are CNC made in Europe with high quality material to replace your original inserts. Price is for one piece. 
 
The concave surface of the ball is about 20% larger than the original, which gives a greater possibility of introducing the air flowing into the upper chamber into the swirl where the mixing occurs and is pre-ignited.
 
Inserts made of high quality, high temperature steel (the same steel used for cylinder head valves). They are 30% stronger than stock. These are 1 mm longer than stock so that after correct assembly, the excess pin is ground off. They will only fit the OM605 and OM606 engines.