501X belt tensioner
 

Just fix in the 3 adjustable belt tensioner and felt that there's a 'click click' feeling when I rotate the spur gear without motor being mounted. Should I adjust the belt bearings touching the belts or leave a 0.5mm gap? Also, the adjustable tensioners are so difficult to adjust especially the bottom one. Any suggestion and ideas? The bottom bearing is very tight with the belt and pulley as the set screw is going clockwise thus rotating the tensioner towards the belt. is it the normal?

Don't have the bearings touching the belt - there should be a tiny gap (0.5-1.0mm). This will stop the belts from skipping on the pulley but will also allow them to move freely.

The best way I found to adjust them is use one of the Tamiya open-ended spanners on the hex part of the tensioner.

How do you actually adjust the bottom one? The space is rather narrow may it be from the front or rear...

Dont you mean 2 rather than 3? I onlyt have 2 on mine.

There is 3 on mine, I have never adjusted the bottom one though, i just set it while building the car with the center transmission out.

My apologies, you were totally right, yes there are three, but the one at the bottom eluded me. I got given the car built so I really need to read the manual.

I dont know if it would be possible to adjust it without taking the center transmission out as its really tucked away under there.

If you remove the ESC (mine is next to the motor) you can adjust it easily, just make sure it doesnt touch the belt. Trial and error, a bit of practice will get you good at it.

The best way is to remove the center assembly.
These are a set and forget item. Be sure toi use thread lock and they should not touch the belt. You want them to stop the belt from skipping. Soo set like others have mentioned. About .5mm gap.
But take the time to set them perfect the first time and you will never need to touch them again.
Unless you want to remove the slipper shaft without removing one of the diffs. That's another discussion. Good luck.

Haaa....so have to remove the whole center transmission again....