[Rooba]

My Stihl FS-300 strimmer yesterday lost drive to the cutting head. The engine seems fine but the head just won't turn.  It is a fairly old machine I bought second-hand a few months ago. Seller claimed it had been regularly serviced. I can't really afford a big repair bill so will have to try to fix it myself.  Any help would be appreciated.
Thanks

[smud6ie]

Was it working OK and did an event take place when it lost drive?
If your lucky the shaft could have slid out from the engine end or the gearbox could have slid down the shaft.either way the inner  drive shaft would have disengaged.
If you fairly lucky the clutch has gone but if your unlucky a gear has gone or the inner drive shaft has gone.
undo the shaft clamp and locating screw from the gearbox end and pull the whole thing off.find a sharpened dowel or something that you can use to take the place of the drive shaft to see if the cutting head will turn normally,if it does ,push the drive shaft to wards the engine, place the machine with the shaft pointing slightly up wards,start at low revs and see if the shaft turns,make sure the shaft is pushed all the way towards the engine.
When you have done that if you could report back members can offer futher advice!
Dave

[Rooba]

The machine did make a kind of grinding noise as I was using it. I immediately shut it down and when I started it again the head wouldn't turn.
I'll follow your instructions and see if I can find the fault.
Thanks for the advice Dave.

[smud6ie]

The machine did make a kind of grinding noise as I was using it. I immediately shut it down and when I started it again the head wouldn't turn.
I'll follow your instructions and see if I can find the fault.
Thanks for the advice Dave.

Concentrate on that head end first!

[Gwiz]

I agree with Dave, I'd certainly look at the gear head first.
It is a fairly common problem, and even more so if it hasn't had a regular squirt of grease in the gears.

[Swed]

http://www.instructionsheets.com/mfg/s/stihl/pdfs/stihl_fs_300.htm

http://www.instructionsheets.com/mfg/s/stihl/pdfs/stihl_fs_300.pdf

p.25(23)-26(24)

Checklist:
p.41(40)-42(41)

Not very much help from the manual...

By your description it does sound like a broken gearhead though...

[Rooba]

Thanks everyone.
I'm afraid my repair efforts failed miserably.
The bolts holding the head on were rusted solid. Despite soaking them overnight with wd40 I couldn't shift them and have succeeded only in wrecking the star-shaped slots in the bolt heads.
Is it worth buying one of those drill-in bolt extractor kits - or is there a simpler way to remove wrecked bolts?
Any advice would be very welcome. Thanks

[Swed]

HD40 will not loosen rusted screws, nut and bolts

You need far stronger, and better stuff.
I use a combination of Omega 636 and 608 oils:
http://www.magnagroup.com/

Then you need a small gas-torch:
Something like this butan burner:
http://www.jula.se/butangasbrannare-213051

If you do not use rust solving and then heat the screw, nut or bolt to just about glowing/orange - there is no way that it will budge... if you still try, you are more likely to wreck the part

Been there - done that. More than once... the most costly time was when I changed the exhaust on my VW Wasser-boxer (type 4 model)... set me back about 500 pounds (1997:ish)... Costly lesson!

Rusted things I simply drop in a barrel with used engine oil - and leave they there "soaking" for "ages" (week, monthes, years) before I attempt to loosen the rusted bolts and nuts. The acid in the used engine oil aids in solving rust, but it takes time, a lot of time...

Fastest way is to use oxygen-acetylene gas welding to heat the screws, nuts and bolts.  
3000'C will do the trick
http://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting

[smud6ie]

Thanks everyone.
I'm afraid my repair efforts failed miserably.
The bolts holding the head on were rusted solid. Despite soaking them overnight with wd40 I couldn't shift them and have succeeded only in wrecking the star-shaped slots in the bolt heads.
Is it worth buying one of those drill-in bolt extractor kits - or is there a simpler way to remove wrecked bolts?
Any advice would be very welcome. Thanks

I am not sure of you skill levels but drilling into such a fine diameter bolt needs quite a lot or a lot of luck!!
Depending on how bad you have damaged the Torx hole in the bolts dictates the next course of action.The steel bolts thread into alloy casing which has grease,seals and bearings so heat in my opinion is not an option.
First make sure you have proper no 25 torx bit.support the head on something solid at the opposite side to the bolt head. Insert the bit into the socket of the bolt and give it repeated light taps with a light hammer.This helps to fully insert the bit and also shocks the bolt in the alloy thread which often helps loosens them,This should have been done when you first found the bolts hard to undo but this knowledge only comes with experience.
If this action fails because the bit will not grip then try cutting a slot across the bolt head with a hacksaw and use a straight bit screwdriver.
If that also fails then if you have access to a MIG welder,a shallow nut can be place over the bolt head and welded onto it and undone when cool ,the quick burst of localised  heat from the welder won't do any damaged.
What is your location?
smud6ie
.

[Gwiz]

I couldn't agree more with smud6ie about the heat. If you apply too much you will find yourself in possession of a pool of molten alloy with some gears sticking to it.
I'd try the cutting a slot method, but I'd then place a small centre punch on the slots edge and give it a sharp tap with a hammer. This will usually shock it around enough to loosen it.

[Rooba]

Thanks for all the advice. I'm off work tomorrow so I'll have another go. Part of the problem was the torx bit I used. This was from a fairly cheap set and it actually twisted in the slot before wrecking it. Useful lesson learned - don't use poor quality tools!
I live near Glasgow, Smud6ie

[smud6ie]

I live near Glasgow, Smud6ie

That rules out any hands on  help then.
smud6ie

[Rooba]

Thanks for your help everyone. I finally managed to get the head off (bought a new torx bit and tapped it into socket with a hammer - great tip). By sticking a screw driver in the head's square drive socket and turning it the shaft revolves at the cutting end but it doesn't turn smoothly and sort of sticks on every revolution. Does this mean the head is shot?  If so, does it mean a new gear head?  I can't see any way to get into the thing. I've just had a look online and a new head costs around £160 -  which sticks in the throat as I only paid £100 for the strimmer. Thanks again for all your help - If any of you are ever in Glasgow I own you a pint!

[smud6ie]

Thanks for your help everyone. I finally managed to get the head off (bought a new torx bit and tapped it into socket with a hammer - great tip). By sticking a screw driver in the head's square drive socket and turning it the shaft revolves at the cutting end but it doesn't turn smoothly and sort of sticks on every revolution. Does this mean the head is shot?  If so, does it mean a new gear head?  I can't see any way to get into the thing. I've just had a look online and a new head costs around £160 -   which sticks in the throat as I only paid £100 for the strimmer. Thanks again for all your help - If any of you are ever in Glasgow I own you a pint!

Thats tough  news and I don't really know what to suggest, the input and output shaft are usually held in the head by circlips in groves which stop the bearings coming out so have to be removed first.the input one is quite deep so a little awkward to get on.

Having said that,we found that getting new gears and ideally bearings along with the work involved was as costly as a new head and is something we no longer do.

How long did the machine run before the disaster,did you have a chance to assess how good the engine is and if good it may be worth getting the new head or may be a local repair shop has a graveyard where you could get a used one.
The other alternative is to put this down to experience,  re auction with the fault declared and buy something newer from a safer source.
The FS300 is a fairly meaty machine,do you really need one as powerful and if not a smaller cheaper one may suffice.
smud6ie

[Rooba]

The machine worked fine for around two months - and was absolutely brillliant. My land is very hilly and some parts are too steep to reach with a mower, meaning I have to use a strimmer. I need something quite powerful - in six years I've killed off two DIY store Chinese-made brushcutters - they simply couldn't cope. I'll keep an eye on the auction sites for a gearhead, but I'll probably have to bite the bullet and buy a new one. Interestingly, there are a few sites selling non-original Stihl parts for a fraction of the price but I suppose these will be inferior Chinese made copies.