This is probably way old and I don’t know if anyone will still be reading it, but…
Brian, the thhn is a way bad idea! I work at a power equipment store selling stihl products. Even a slight increase past approved string sizes (or length for that matter) can increase the load on the engine dramatically and burn it up very quickly. So I can’t even imagine what thhn would do to it.
You could try using an approved head with composite (plastic) blades, a metal grass blade, or a more durable string. Stihl makes an orange and grey line that’s supposed to be five times as wear resistant as their standard line, but is still rated for use in standard heads.
I was quite surprised when I watched the training video on this, so I figure it’s something most people are probably unaware of. Doesn’t seem like it could make that much difference, but apparently it does. Anyway, just a little technical input
David