This is off HPI web site and as you see it is the only HPI CB to run with a steel spur.

These Racing Clutch Bells from HPI Racing are made from high strength machined steel and hardened for maximum durability. These Clutch Bells can be used with the stock Savage Spur Gears, and they are a mandatory upgrade when using HPI Steel Spur Gears. Machined holes help keep the weight down for quick acceleration, and the holes also help cool the clutch shoes for consistent performance. Each gear is stamped with the number of teeth for easy identification. Available in 13T, 14T, 15T, 16T, 17T, 18T, 19T and 20T sizes for a wide variety of gear ratios.