Novarossi has made 4 new reducers with dimension:
Cod. 24453 – Ø 5.3 mm
Cod. 24451 – Ø 5.1 mm
Cod. 24449 – Ø 4.9 mm
Cod. 24447 – Ø 4.7 mm
This new reducers are used on the .12 ON-ROAD engines and .21 OFF-ROAD engines. This setup allows to reduce the fuel consumption.
Source : Novarossi