What I Recommend Building and Why ?
Building a Single Row
My advice is to build the 9 cylinder radial. At first glance it would seem it is more work than building the 7 cylinder, but actually this is not the case.
The main reason I recommend the 9 cylinder is that 360 degrees is evenly divisible by 9 but not by 7. The result is that the 7 cylinder engines require more difficult setups on a dividing head while the 9 cylinder can be made on a rotary table.
Building a Double Row
Following the completion of a single row engine, the double row engine, either the 14 or 18 cylinder, can be built in the same amount of time it took to build the single row.
This is because the builder is more experienced and all of the tooling is interchangeable between the engines.
Some builders already know they want to build both a single and double row engine. With this in mind, they build both sets of cylinder and cylinder head parts at the same time. In this way when they begin the construction of the double row engine with the majority of the work already completed.
Once the setup is already on the machine, it is an opportunity to make the additional parts.