one more thing..what is the best replacement string for these ,the tough stuff
...??
It kind of depends upon what you are trimming and how you use the machine.
The harder, stronger string tends to stand up better to abrasive wear and long duration weed whacking. It also tends to break off at the feed-holes in the trimmer head, as they get cycled thousands of times per minute at that flex point. Whacking hard against the tall weeds bends the string a lot, and it breaks off, making you disassemble and re-feed.
Softer trimmer line wears out quicker, but tolerates the bending a lot better. Not so good when you are edging a lot of concrete sidewalk, 'cause it just doesn't last as long under high abrasion. Furthermore, the softer line tends to melt inside the trimmer head in the commercial trimmers, purely because it has too much horsepower going through a tiny contact point. Then it quits feeding, and you must disassemble and pull the line out.
Buy what you like, try many variations, keep notes on what works best and where. The little .080 string recommended for your battery powered trimmer would just fly apart if it was attached to my commercial trimmers. You little machine would be bogged down and ineffective with my .105 commercial trimmer line.
The heavier line whacks weeds MUCH more effectively than the thin line. Sometimes we load up .155 trimmer line, but that is only for the seriously overgrown weed patches. Conversely, that heavier line will beat the crap out of your landscape plants, the bark of trees, the landscape timbers, and your wooden post holding the mailbox. I recommend sticking to the .080 thickness trimmer line for most stuff.