The ArduCopter2 frame uses narrow aluminum arms, and so the design calls for motor mounts. The 3DR frame uses 20mm, 3/4" square tube arms, much larger than those on the other frame, and so it offers a different mounting method. Included in the 3DR frame kit, for each arm, are metal screws and a bag of washers/spacers. The 3DR uses a direct mount method. Each arm has two holes spaced about 1.5 cm apart along the center line of the tube. Screws are placed up through the arm holes and connect directly to the bottom of the motors. Only two of the four motor mounting holes are used, but it is plenty solid, and the X-shaped motor mounting brackets included with most brushless motors are not used at all.
The 3DR frame assembly instructions discusses motor mounting, and I encourage you to read all of the instructions carefully, twice before picking up a single tool. Once you are done reading those instructions, I recommend you take the small bag of spacers and ... put them aside. Do not even open them.
"But if I do not use the spacers, don't I run the chance of damaging my motors?" you might ask. The truth is, no. You will not run the chance of damaging your motors, you are virtually guaranteed to damage your motors. The screws are simply too long, "engineering variances" aside, for virtually any motor you are likely to be using. The point of my recommendation, however, is that even with the included spacers, you are still running the risk of damaging your motors. So perhaps I should explain myself?
The 3DR frame is my first "kit" built frame. It is my fourth quad, but the first frame I purchased. When yours arrives, it may well have three or four times the spacers/washers that were included in mine. I purchased one of the earliest 3DR frames available from the DIYDrones store, and it included about 20 spacer/washers. I used all the washers and supplemented some small nuts besides, and I still wasn't sure I was getting the spacing right. Ever eager, I "got by" with fewer of the spacers on one of the arms. And you can see the motor from that arm in the picture above. It is a disaster waiting to happen. It functions around 45% throttle, just enough to get the quad off the ground. Then it stops. I discovered this during routine testing, fortunately, and not during a flight test. I am confident that jDrones shipped me a good motor, so what went wrong? The screw was just long enough that, under load, there was some shorting (think, spark gap transmitter!) inside the motor that damaged part of the stator wiring. The motor will spin up and then will short and fail. Backing the screw out will not help, the motor damage is done. Fortunately for me, my last order from jDrones was a batch of sixteen. What can I say, I am an addict.
The "Quick" Tip
The washers/spacers will be useful, so keep them. For use elsewhere. The problem is that there are just not enough of them, and even if you do have enough of them, it is a pain to count them out (you need at least five per screw, 40 in all, to be safe, and I suspect they are also heavier than this solution.)
Included in your propeller bag (and, if you are using the standard or heavy ArduCopter motors, and the 3DR frame, I really do recommend you use the 10x45 or 12x45 props; unless you know the math better, of course...) are some plastic inserts used, along with the prop adaptor, to fit the propeller to the motor shaft. You can see them in this photo.
unadulterated spacers. You get one of these sets with each propeller.
The second set of spacers, in the middle, shows one item missing. That is the plastic insert that is pressed into the center of the propeller.
Notice on the bottom, two additional spacers are missing along the right hand side. Counted collectively, you will have four spacers of just the right size for use in your props themselves. Then, for each prop, you will have two spacer of very slightly different sizes which will work perfectly for our purposes.
After you have fitted the correct spacer into the propeller, to avoid any mixups, remove these additional spacers, and use them, rather than the included 3DR frame washers, to back the motor screw off so that it cannot damage your motor wiring. Inspect your motors well (we can now speak of "engineering tolerances.") It is possible that you may still need to use an extra spacer or two (rather than a minimum of five per motor.) I did not need to use any of the 3DR-included spacers when I used the propeller inserts, one insert per motor screw.
Total (additional) cost: $0.00. Please let me know which country you are from in the comments and I will happily provide conversions for the additional cost into any currency of your choice. ;) This tip is so simple that you might just be annoyed with me for writing such a lengthy blog about it. But, I suspect, not nearly as annoyed as you would be with four secretly unreliable, damaged motors.
I leave you with a photograph of the final product. I hope that it brings you the same peace of mind and happy motors that I now enjoy.