Nn3 Unitah Mallet
The Nn3 Alliance published a book on all things Nn3 called the "Nn3 Handlbook and it is available from several sources. One of the engines they describe building is the Unitah 2-6-6-2T.
I do not think the Micro Ace 0-6-6-0 is a good starting point as the wheel diameters and...
