The metal cage bearing on the other hand is more resistant than the polymer version, but unfortunately it also generates more friction and hence more heat, which results in a shorter life expectancy. The version with polymer plastics cage has higher elasticity and a reduced friction coefficient as compared to the metal ball cage. This is also the type of bearing that we prefer for our racing engines. What is important for these bearings are the right ignition timing and carburettor settings, since these bearings should not be subjected to more than 120°C for a longer time in order to prevent damage to the polymer plastics cage. This is also the reason why the TN9 bearings should not be used in engines with catalytic converters. Please also keep in mind that the half-open cage must be inserted so that it faces the crank webs.