Well in my methodology sections I usually explain the approach (i.e. how I decided to do it before I did it and why) as well as the experimentation (i.e. how I actually did it after I decided how I was going to do it). You want people in theory at least to see why what you did answered the questions you set out to answer, and to be able to replicate exactly what you did in case they'd like to see it for themselves.
"3.2 Technical data
3.2.1 System specification
3.2.2 system development
3.2.2.1 System design "
Unless the technical data has to do with with parts or procedures you used in actually doing what you did, I'd put it in a separate section, possibly an appendix.
Just use your judgment.
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