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What is this thing for? (Motor parts)


Junior Member
Hi, I'm still a newbie so excuse my ignorance of mechanical terms but I've bought a few electric brushless motors now and I have a few questions about the parts you usually get with them for which I can't find any answers on the internet.

I bought this LOGIC FUSION 3545 850KV and an 11x5.5 prop (which is within the specifications.)

This is what I have after test assembly...


From top left;
  • shrink wrap
  • bullet connectors
  • motor-mount to firewall screws - to hold the motor to the airframe
  • spare motor-mount to motor screw - to hold the black metal plate to the firewall
  • spare prop adaptor to outrunner-body screw - to hold the prop adaptor to the motor
  • thing - I've had similar things supplied with other motors, this one does not fit over the thread only the extra shaft at the back of the motor
  • spare grub screws - for the thing and the shaft to outrunner locking
  • spare circlips - for shaft retention at firewall (back) of shaft
  • prop resizing adaptors

So here's my questions;
What is the thing, and what is it for?
Have I put the washer on the correct side of the prop, between the hex nut and the prop?
Why is there an inch of shaft sticking out the back of the motor and what is it for? (The prop adaptor on the motor is solid.)
Do you normally just leave the inch of shaft sticking through the firewall into a cavity in the airframe? (Seems a bit harsh to cut it off an £40/$80 motor.)

Many thanks.


I'm a care bear...Really?
Prop saver. It's what you attach your prop to the motor using a black rubber band or gasket. The rubber band will pop or snap before the prop breaks..... Well some times.


Junior Member
Don't prop savers have two screws for the rubber band? The thing only has one threaded hole and it's for the grub screw it's already got fitted. (sorry the picture isn't great.)


Elite member
Its a 'safety collar' to go on the shaft at the back. The only thing stopping the prop pulling the centre of the motor out of the front is a little "C" clip. If it does pull out in flight (you are likely to be at full power) chances are it will be lost for good and you need a new motor and prop!
So you put the collar on the rear shaft close to the back of the motor and tighten the grub screw up. Then if the "C" clip does fail the collar will stop it pulling right out.

Ron B

Posted a thousand or more times
thanks Q
i just got a motor similar to this and it had the safety ring with a grub screw and a flat spot on the rear shaft to put it on but I didn't know why.


Junior Member
Thanks quorneng, I've installed it as you suggested.

Am I right that the shaft at the back is just left sticking through the firewall?