Hi

I have managed to round the nut that keeps the engine to the chassis, it is a allen screw. How do I undo it ?

If you use a small drill bit with your drill you may be able drill the head of the screw until it breaks off, then remove the engine and carfully use pliers to carefully remove the rest of the screw. I have never done it on my rc cars but have on many other aplications, it should work if you're careful.

You could:

-Cut a slot into the head of the screw and use a flathead screwdriver to remove it
-Threadlock an allen key into the screwhead, let dry, and unscrew
-Use vise grip pliers to grab the head and turn

use a screw extracter

I will try that tell you if it works tomorrow

i have done that several times with the cheap screws on my towerhobbies st-15. Just take a smaller bit then the screw and drill it. After you get into you can pull the engine off the chassis and take pilers and unscrew the screw.

cutting a slot in teh screw requires a dremel usually and i dont have one but that sounds like the best idea...

The slot idea works well, and you can also try tapping the end of the screwdriver before turning. That helps sometimes to shock it into breaking loose a bit. Also, some light oil or WD40 soaked into the thread can help it.

last time i did it i drilled the head of the screw off as said above and used vise grips to get the rest out. works pretty good.