Upgrading a strimmer head

I have a basic lawn strimmer - A Mcgregor MET3525 (350 watts, 25cm diameter) It uses a standard two-coil monoline spool, which I would like to upgrade with one of the "Universal" metal heads. In most cases, there is a hole in the case with aligns with a hole in the spindle, so the rotor can be locked to allow the head to be unscrewed by hand. There is no such hole in this model! (I stripped it down to check!)

I contacted the Mcgregor support line, but they didn't know. Can anybody suggest how I remove the existing head?

 

Is that correct, only if it spins cockwise when strimming it would undo itself if it unscrewd anticlockwise.

Wouldn't it?

Never could work this out, but just remember circular saw blade nuts always had left hand tread.

 

Yes, that is the plan, but as I can't stop the motor turning, there is no resistance to unscrew it.

 

A bit different but I used to unscrew the chuck on an old drill years ago by putting the chuck key in and giving it a sharp tap with a hammer in the direction of unscrewing it.
Used to do the same with saw blades sometimes.

If you can either get a spanner on it or a pair of grips it might just loosen it.

 

Just another thought, but usually if these things are supposed to be dismantled there would be something like a flat on the shaft so you can get a spanner on it.