There are variables that must be considered when switching from a fixed pitch to constant speed prop.
It is primarily related to the vintage of cowl that was used when the airplane was built.
Early cowls were sized to use a 4" prop extension with a fixed pitch prop.
Later cowls that worked with a F.P. or constant speed prop used a 2.25" spacer with the fixed pitch or a standard compact hub constant speed.
If you have a cowl that was using a 4" extension with a fixed pitch prop, you will need to do one of three things
- install a newer version cowl
- Use an extended hub style Hartzell prop (generally not approved for aero)
- Use a compact style hub prop with a 1.75" spacer (I am not what options are available for this)
If you currently have a 2.25" spacer on a fixed pitch prop, you should be able top switch to constant speed with no cowl fitment issues.