I've been given a Stihl weed wacker. I did some research and the engine is a 31cc. I started disassembling it thinking I was going to get a nice engine for free to convert, but got a nasty surprise. The crankshaft apparently goes through both ends of the engine. The front side drives the cable to the weed wacking spindle like normal, but the crankshaft also comes out the back. This is where the flywheel with magnets and ignition module resides.

My question is....

Is this engine even worth messing with? It looks to me that to have any hope of using it on a plane I will be forced to get rid of the flywheel and ignition module and buy electronic ignition so I can have some place to mount the thing. Even then it looks like it would take some kind of weird motor mount to get it done.

Anybody have some experience with this engine?

Thanks, Jim

Hellow Jim Walker: Post a photo of the engine and we can help you more. Capt,n

Hey Jim, if you cant post a pic just tell me what model Stihl it is and I can look it up at work tomorrow. If it is what I think it is it may be a poor choice. On the bright side I just converted a 27cc current model avail for 129.95. Its the simplest conversion ever!!! I bet if you visit a Stihl dealer he would sell you the engine for even less!

careful with wich end you choose for prop.i had a guy just give me an engine from a fs-52,my math says 18cc.if i put the prop on the flywheel end and make a mount for the back it will be turning cw rotation and is backwards from normal engines.its very light with no cutting yet though.2 rings on piston and screams when it was running.

I don't have a way to get a picture of it at this time. It's a Stihl FS120 weedwacker. I'm even less hopeful after looking at it again. The crankcase has lots of extra mass cast as part of it for the fan shroud and muffler sheild as well as other things that aren't quite clear. It would take tons of machining to get all that excess metal off if it's even possible.

I don't think its the 31cc as i have that one and its identical to the ryobi. Don't give up, get a hacksaw and some files and start cutting, these stihl's are supposed to powerful and last forever.

Hey Jim, the fs120 was only produced for a year or two and if I'm not mistakened is just a toned down version of the fs200 (same crankcase). But let me doublecheck at work tomorrow and I'll post horsepower as well. Either way she was a pretty stout brushcutter, not weed trimmer in her day... Take care,

Tripp

Jim, the fs120 has 35 mm bore with 32 mm stroke and produced about 1.5 ponies at 12,500 rpm. And yes it looks like there is quite a bit of trimming to do as far as weight savings. The fs 200 is the same case with a bigger piston and would probably be more suitable, but free engines work the best

How about dealing with the crankshaft coming out of both ends of the engine? Is it possible to still use it for aircraft with this configuration? That was the first and most important concern. If I have to buy a $150 electronic ignition conversion kit before I even start buying things like motor mount, muffler, prop adapter etc.. it's not worth it.

Jim, You will have to use the output end of the crank to turn ccw for aircraft use. I don't have a fs120 to look at just diagrams. Do you have a fax I could send it to? Honestly, I think another engine would be more suitable[&o]