I like a fixed cutting head for thick line, 31-2 on a fs240 is good, .130 is max for keeping good speed, I have a lot of dry thick grass etc...no spark allowed. went all in on a 561c to run a 31-2 with .170, thats paying for that heavy beast. my 240 with 155 is maxed in tall stuff, I use duro 40-4's mowing low, 4 string .105 is good tierra forming, I always use a speed&feeds 450s for general trimming, now they have a 500!, that will be nicer for fat string.
Last thing, i use 99% of the time round string, i find the sharp edge stuff to break off too soon , and never any serrated types.
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