The 4140 family of yard tools include three different curved shaft weed whackers as well as "D" handle and bike handle straight shaft trimmers. This goes on to the edger, tiller, and Combi models. They are considered to be part-time home owner tools.
I acquired from Craig's list a few years ago a nice hardly used none running straight FS-55 bike handle bar weed whacker. About six month's later I got a FS-45 with the curved shaft. The FS-45 ran fine but had more sun exposure and use than the 55. These are 27.2 ci two strokes with a regular muffler. This type power head is on its way out for the FS-56 (4125) series or the four strokes.
Stihl FS 45 - Notice the power head is the same as on the FS 55R
All the FS-55 needed was a new carb and some string and then it was just like new. I soon discovered that I didn't like either one. They ran OK and didn't break down but the FS-45 was too small for me and the FS-55 was too bulky for my needs. The FS-55 may have been OK if I had high weeds and used a saw blade but that was not my need.
Stihl FS 55
What got me to discover that I didn't like the Stihls was that I came across a SRM 2100, 21cc Echo straight shaft and then another SRM 2400, 24cc one. These were great for my yard and in fact I also have a hedge cutter, blower, and tiller. The Stihl FS-55R is just like using one of the Echo trimmers.
Looking on Ebay the individual parts to convert either weed whacker to a FS-55R were too expensive and it would be cheaper to buy a FS-55R D-handle type and use what I had for spares? Keeping a eye on Ebay all summer is that you never see a tube & liner for the curved shaft or one piece straight shaft trimmers and if you do, it is better to buy new.
Stihl FS 55R
Well, that's what I did. I bought a complete ratty FS-55R for the price of a gearbox and shield. I used the FS-55R straight tube, handle, FS-55R label, and gear head assembly on my FS-45 and then had at least one nice FS-55R for use around the house and it works good but I suspect the liner in the tube may be a little worn so down the road I may get one of those from the dealer. The liners are inexpensive.
That has created a CAD type addiction situation and I came across a set of covers for a FS45C that were real nice for $19 including shipping and used them on the bike handle FS-55 to make a FS-55R "D" handle out of it. I found a FS-55R shield and D-Handle on the cheap. But the saga goes on. The gear head on the junk FS-55R I bought was from a FS-90 and I like it better than the FS-55 style gear head. So I bought another genuine Stihl used FS-90 gear head to go on the FS-55 that I already had. When buying these parts you have to be patient for a cheaper one to come along. The actual drive shaft in these is a flex one instead of a solid one. I don't know if that is good or bad as far as my needs go? Might try a rigid shaft sometime in the future but not sure which trimmers have rigid shafts? You have to be careful as the Chinese have jumped on the band wagon and sell parts for these too.
Stihl FS 90R - Notice the different gear head from the one on the FS 55
So, soon, I'll have a pair of FS-55R string trimmers.
Stihl is rather secretive about their 4mix trimmers (Which ones are 4mix?). I get the idea that they are great except expensive when new but if one starts to give trouble, you have a can of worms on your hands?