The critical area can be found with the raised cast numbers on the outside of the turbine housing near the inlet. In addition to the outside numbers, some turbine housings have larger indented cast numbers on the inside of the turbine inlet. Hopefully the turbine housing has not rusted to a point where it cannot be read. The critical area is based on the casting not the machining of the turbine housing. A turbine housing casting can be used to make several machined turbine housings to match the wide array of turbine wheels.
Most are marked...12, 14, 16, 18 etc…but this is a generic designation. In actuality, a 12 cm marked turbine housing might actually be 11.2 by design...while others will be 12.4 by design. Hope this helps.