Hi! I’m having difficulty unscrewing grub screw of motor pulley. It is made of very soft metal and the hex thread is gone now. I tried hard pressing hex key in and turning but no effect. Any ideas ? I have round shaft which I gently filed down in one spot.
You can try a flat head screwdriver that just barely does not fir and hammer it in the hex key “head” or hole… of course being carefull not to destroy the motor because it could get expensive pretty quick 
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I used combination of your advise and mole grips - got that bastard off ! Thanks ! 
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You could try a impact screwdriver, failing that time to learn all about busted screw extraction. HF has a cheap screw extraction kit . Chances are it will be easy to fix because your deal with butter soft hardware, and not hardened hardware you find in engines.
I did indeed hammer screwdriver in hoping to get a grip to screw it off. Impact must have loosened screw and was able to pull the pulley off. Lesson learned 
Good for you, but you should upgrade your hardware now, and maybe get some screw extraction tools for the future. A lot of estate parts come with shitty hardware.
Well I got the grub screws off ebay - needed the shortest possible - 3mm. I’ll see if proper hardware retailers in UK have something made of stronger alloy cos I’m not doing this again haha.
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If you have to get a longer one do it and cut it down or grind it then file the cut end smooth. Then use a nut that matches the threads on the screw and work it back and fourth over the cut end to help redefine the threads on then.
It’s can be tendencies work but I do it with a lot of my hardware. Sometimes its just hard to find the right size hardware.
The lesson here is to buy from better sites 
McMaster Carr is a great store, they have almost every length and size of everything, in nearly every material. For example, here’s a 3mm M4 cup point stainless steel setscrew. https://www.mcmaster.com/#92605a009/=18ucbzt
It’s also $3.39 for a bag of 25.
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I’d avoid stainless steel, and get the black hardened steel stuff if possible. Stainless is often brittle, and not nearly as tuff as other alloys.