Externally it fit perfectly into the airbox boot. I did have to de-burr it though to make it slip in easier. Internally its inlet bore is not machined concentrically nor does it have the bell shape of the 021-216A. It is machined eccentrically with the center being a little high. This means that the bottom of the adapter where the two air jet holes are creates a bit of a lip for the incoming air to overcome. I really do not think it is all that bad as the inlet is still larger than the exit bore. Those trying to attain huge hp gains may notice on the race track, but the airbox boot is quite restrictive I think there are gains to be made there. The exit bore of the 021-213 adapter is the exact same as the 021-216A. I did decide to fix the eccentricity though.

To fix this I centered the 021-213 adapter in a lathe and slowly, starting at 45*, and using 10* increments, machined the inlet bore so that it is concentric to the outside diameter. This created a little more of a bell shape to the inlet. In the end it turned out nicely. There is now almost no lip to be seen and the transition from boot to adapter is almost linear. I think it might even be smoother than the 216. The 216 looks to me as though it creates an circumferential void at the interface which could create vortices which could affect air flow.

To install the carb with the 021-213 adapter attached, I had to remove oil return tank, the rect/reg, pull the subframe back, slip the carb into the airbox boot, slip the engine boot on the engine, and then push it all together and bolt up the subframe. It was pretty easy. I did not have to fight with anything and even with the added length of the 021-213 it easily slipped between the engine and frame. I also have the hydraulic preload adapter on an Ohlins shock which takes up space near the front of the carb.