Just an addendum to my earlier post.
Understand that these machines were not treated kindly. They are used daily during the season trimming highway right of way. Imagine all of the neat stuff they encountered, bottles, scrap metal, concrete, guardrail etc. We actually would mount circular saw blades to them to cut heavy growth like Acocia plants (tumbleweeds) and sucker growth.
The Stihls were the only ones that would last, even the string heads would last a year at least. The Redmax and Shindaiwa ones would explode if you hit concrete or a bottle.
Not the Stihl.