Hi, i just wonder if anyone cut one side flat the 5mm motor shaft before installing the coupler, look like its gonna slip i dont do that, is that right?

May people do this . I personally don't because i think that is will weaken the structural integrity of the motor shaft and there for is more likely to break off. I use lock tite when i put mine down. I have never once had a problem with it at any time.
i used the strong lock tite and to get it off I just drop some CA remover down the set screw and it will loosen rite off.
may people will disagree with me but thats fine. every one has their own way to do things.

Taylor Shaw, either lock tite or superglue can also use to lock it right??

lock tite. super glue is a bugger in many ways and is not worth the trouble.
use lock tite.

thanks, i have a question out topic, where i can find fiberglass cloth?? can i use thin fiber glass grille as cloth?

No idea. I buy mine locally. not in Texas

do you have any local hobby shops?

you should find some fiberglass and resin there...

if not there, try a hardware store, they should have some

yeah ? the hardware store fiber glass is horrible stuff.
you want the tin stuff , the stuff that has a tight weave and good structure.

Just for the record, I have put a flat spot on many of my motor shafts and alot of them have come with a flat spot already on them and I have never had one break and I have put them through their trials

i thought i had already replied to this thread but anyway, My 2 cents is always grind a flat.. ive used a drill and drilled a small dimple before also, my 16 cell BL setups will only go about 2 feet before they slip if you don't have a flat on them.. it takes 10 seconds to do it and i think if only done just enough it will stop the coupler from ever slipping, also in most cases it allows the set screw to sit deeper and have a better bite in its thread

Here is what to do.
Yes 1 (one) small flat spot works great. Forget about weakening the shaft to a point of a failure! (not likely)
2 (two) set screws only put pressure on the spots that engage the set screws.
I fly F5B with a 17x18 prop spinning max RPM's 16 cells 4200's on a controller that stops the prop on a Dime. Pulling 200-225 Amps for short 3-5 sec. bursts. No slippage ,AND you can get the prop off when you want to.
Cut a flat spot on the shaft half as wide as the shaft . Loctite the set screw ONLY with blue loctite.
With airplanes we taper the flat spot going deeper toward the motor .This way if the prop gets loose it has to lossen all the way up the taper .Chances are you catch it before the whole prop /spinners comes off.
If your hesitant about grinding on your motor the go to Home Depot and get a Fat nail and practic on it before you touch your motor..
Dyno Don

You don't have to take very much material off the shaft to make a flat spot for the set screw. You can use a regular file rather than a power tool to avoid the risk of taking more metal off than you wanted to. It'll take a little longer but not a big deal.

i found my cloth at auto zone

ditto