They're probably not 'lock tighted' in - the 501x screws bind and it's a PITA... I've had to spend hours drilling them out of my original chassis... sucks. Believe it or not, grease rather than loctite is recommended for the original screws.
Check:
http://www.wikihow.com/Remove-a-Stripped-Screw
I had to buy a screw extractor set to disassemble my 501x... should hardly be necessary.
Suffice to say I removed and discarded every single Tamiya screw from the set.
Good luck!