The top nut of a boreless compressor wheel is solid. We have rich experience in manufacturing this kind of turbine shaft wheel and compressor wheel. We also can convert a casting boreless compressor wheel to a billet one. Thought it is a little complicated to manufacture, boreless billet compressor wheel reduces the risk of hub fatigue in certain applications. A boreless design can last five times as long as a through bore compressor wheel.
Turbine Wheel with Shorter Shaft for Boreless Compressor Wheel Turbocharger
You will see the shaft of the turbine wheel in below picture is shorter than normal turbo turbine shaft and wheel. They are specially for boreless compressor wheel turbocharger.