I bench tested, set the air gap, and re-installed the ignition coil on a hs45 hedge trimmer.
The test I performed was for secondary coil resistance between the laminates and the spark plug connector. According to various mechanic forums, the resistance should be between 2,200 to 3000 OHMs. Mine was 2,700. You could also test for primary resistance between the laminates and the «little lug» sticking out of the coil, as some mechanics say. Primary resistance should be somewhere between 175 to 250 OHMs, but I cannot verify that number because I do not have the Stihl repair manual.
I set the air gap with thick gauge paper, a book mark. Hardcore do-it-yourselfers and mechanics will want to use a specialty tool made by Stihl, built especially for setting the air gap. See the link below:
Abastecedora Parque y Jardín. Somos una empresa ubicada en Cuernavaca, Morelos, México. Contamos con herramientas y equipos, taller de servicio y refacciones Stihl.