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The problem you can run into with this "Number" idea is it turbo can be bored out. You could end up doing what i did, running the number on a turbo for sale in need of a rebuild. Buy the turbo and the wheel at the same time. Wheel comes in and you go to put it on the turbo.... and you can fit your finger along the side of the comp housing and turbine fins.

Best way i've found is to remove the top cover and measure the inducer and exducer diameter if you can.