3.0mm needle seat valve is what comes in the majority of dirt bike carbs ranging from 65cc to 300cc two strokes.
Sprung 3.3mm 6 hole or sprung 3.5mm needle seat valves are used for bigger two stroke dirt bikes like the KX or CR 500 that have substantial vibration.
A 3.6mm 6 hole is used in many gravity fed alcohol applications with oversize bowls.
2.0mm Viton is used in sleds, karts, etc. running gas with high flow pumps.
The 2.5mm steel tip is used for alcohol applications with high flow pumps. It is also used in sled demanding more gas with a high pressure pump
If you are still unsure of which to order, email firstname.lastname@example.org with your application information including the model, fuel, and use.
Each needle seat valve comes with a gasket.