FS94R and FS90R shaft interchangability?

[javjacob]

I have a Stihl FS94R string trimmer and was wondering if I can change out the cable shaft with a steel shaft from a FS90R? I have seen where other people have talked about it but haven't seen if anyone has actually done it and confirmed it works. Seems if the shafts are the same length they should work?

 

[Ric]

I have a Stihl FS94R string trimmer and was wondering if I can change out the cable shaft with a steel shaft from a FS90R? I have seen where other people have talked about it but haven't seen if anyone has actually done it and confirmed it works. Seems if the shafts are the same length they should work?

I'm not going to say one way or another as to whether it will work or not but I don't think I'd try that. The FS 94 is 2 stroke and 24 cc engine and the 90R is 4 stroke and a 28cc and a lot heavier machine. I'm just guessing here but that cable in the 94 is made to flex so if you happen to hit something you shouldn't have been cutting, it will give so you don't possibly do damage to the engine. The 90 R has a steel shaft so you can use all sorts of attachments, like saw blades and such.

 

[javjacob]

How would the steel shaft damage the motor? Worst case scenario it would just stall the motor if you hit something solid just like it would with the 90. The only benefit of the cable shaft is weight reduction as far as I know. The older version the 80 and 85 were the same trimmer except one had a cable shaft and the other had a steel shaft if I remember correctly. I know some people have put brush blades on FS94's but the cable shaft causes a wobble when you get on and off the gas. I just don't see the cable shaft holding up like a steel shaft would. Maybe I am wrong?

 

[Ric]

How would the steel shaft damage the motor? Worst case scenario it would just stall the motor if you hit something solid just like it would with the 90. The only benefit of the cable shaft is weight reduction as far as I know. The older version the 80 and 85 were the same trimmer except one had a cable shaft and the other had a steel shaft if I remember correctly. I know some people have put brush blades on FS94's but the cable shaft causes a wobble when you get on and off the gas. I just don't see the cable shaft holding up like a steel shaft would. Maybe I am wrong?

Like I said I'm not going to say one way or another as to whether it will work or not. Worst case scenario is it could tear up the motor. I would think the difference between the engines one being a two stroke 24cc and the other a four stroke 28cc could make a big difference. Now I may be wrong in my thinking but that 2 stroke is producing twice the engine speed of the 4 stroke to get the same trimmer head speed and you put a steel shaft that weighs more than what it's capable of turning what will happen? Hey I mean you can do what ever you like it makes no difference to me, it's not my equipment but I personally believe if Stihl could have put a steal shaft in that FS 94 it would be there already. It would have been a great selling point. I'd talk to the Stihl Reps and see what they say. jmo

 

[javjacob]

I will have to do more searching on the issue. The 94 has only been out 2 years now so there isn't a ton of info out there yet. The redline on the 94 is only 9,800 rpm if I remember correctly so it's not much if any more than the 90. The 100 is just a 90 or 110 with a cable shaft for reduced weight. I will just leave it stock unless I find out for sure it works and has no problems though. I have heard other people talk about it but haven't heard of anyone doing it. Seems like someone would have tried it by now though.

 

[upupandaway]

"upgrade" to a solid shaft?? no problem. If the square ends match up, sure why not. I believe some current models have the same motor with solid shaft as one of the differences. My 30year old stihll trimmer motor was also sold with option solid\flex.

 

[javjacob]

"upgrade" to a solid shaft?? no problem. If the square ends match up, sure why not. I believe some current models have the same motor with solid shaft as one of the differences. My 30year old stihll trimmer motor was also sold with option solid\flex.

Thanks for the tip. I was thinking the FS90 shaft would be a direct swap. I know many other models were sold with solid or flex shafts and there is no reason the FS94 wouldn't handle a solid shaft. I'm not in any hurry but if I do this I will do some updates. I might end up buying another FS94R and do the steel shaft in one and keep the other original. I just don't like the overly complicated unreliable heavy 4 stroke motors and the only other 2 stroke option is the FS240R which weighs almost 5 pounds more than the 94. The FS94 has plenty of power IMHO, its got the same power as the FS90.

 

[upupandaway]

fwiw, not sure if it is the HP or indeed flex vs solid shaft, all 4 of my 80's have solid shaft and i need full power to trim grass, yet the echo i have (with the straight but flex) can trim at a much lower rpm - same for old Stihl fs40 (identical head as 80s) I have at my apts.
My theory is that with a solid shaft the speed of the string pulses just like the power cycle vs the flex acts as a buffer so the head spins at a relative steady rate?!?!?!?!?
Never did try my saw head to see if similar in cutting wood.
Anyway....

 

[javjacob]

I will get another FS94R when I do this so 1 is stock. My only other 2 stroke option is the FS240R which is twice as expensive and is very heavy. I am just not a fan of high revving 4 stroke motors so the 90 isn't appealing to me.