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Motor mounts: the four dimensions that decide whether it flies straight

September 3, 2026

A motor mount looks like the simplest part on the aircraft. It is a plate with four holes and a hole in the middle.

It is also the part where a machining error turns into heat, vibration and a bearing that fails at 200 hours instead of 2,000.

The four features that matter

Everything else on a motor mount can run to the general tolerance.

1. The bolt circle. Four holes on a pitch circle, usually 16, 19, 25 or 30 mm across for the motors on commercial multirotors. What matters is not the diameter of the circle, it is the position of the holes relative to each other and to the centre bore. Machine the circle and the bore in the same setup and true position of 0.04 mm is routine. Machine them in two setups and you are relying on how well the part was relocated.

2. The mating face. The face the motor bolts down to has to be flat, because the motor flange is stiff and the screws are strong enough to pull the stator into whatever shape the mount is. Flatness of 0.02 mm over 100 mm is a sensible call out. It is also the feature that moves if the back of the plate is pocketed after the face is finished, which is a process problem, not a design one.

3. Shaft and bell clearance. The centre bore has to clear the shaft or the bell without touching it at any point in the rotation, including after the mount deflects under thrust. A clearance that looked fine in CAD can close up when a thin plate bows. Give it real clearance, half a millimetre is usually plenty, and stop treating it as a fit.

4. Stack height. The distance from the mount face to whatever sits under it decides whether the prop clears the arm and whether the screws bottom out. Screw length is the classic field failure. A motor screw that is 1 mm too long bottoms in the stator and destroys the windings on the first flight.

What does not need a tolerance

• The outside profile of the plate.
• Pocket depths, unless a pocket floor is a mating face.
• Lightening holes.
• Chamfers and radii, as long as they are sensible.

Tolerancing these adds inspection time and price and changes nothing about how the motor runs. There is a longer version of this argument in what to put on the drawing.

Why one setup matters so much

On a 5 axis machine, the face, the centre bore and the bolt circle are cut without releasing the part. Every one of those features is then referenced to the same origin, so the relationship between them is as good as the machine, not as good as the fixture.

Split the job across two setups and you introduce relocation error, typically several hundredths of a millimetre, exactly in the relationship you care most about. That is why a mount is one of the parts where 5 axis is cheaper in the end, even at a higher hourly rate.

Material and finish

6061-T6 is the usual answer. The mount is stiffness driven rather than strength driven, it often carries heat away from the motor, and 6061 conducts heat better than 7075 and anodises more evenly. Reach for 7075 only where the mount is also a structural arm end carrying bending.

For finish:

• Clear or black type II anodise for most aircraft.
• Hard anodise where the aircraft sprays or flies over sea.
• Mask the contact area if the mount is part of the motor ground path. Anodise is an insulator, and this catches people out every year. See anodising UAV parts.

Heat, briefly

A motor mount is a heat path whether you designed it to be one or not. Stator heat goes into the flange, through the mount and into the arm. Two practical consequences:

• Do not put a plastic thermal break between motor and mount unless you have checked the motor can shed its heat another way.
• Do not pocket the material directly under the motor flange to nothing. That is the path.

If the aircraft runs hot at hover, the mount is one of the cheapest places to add conduction area. Heat sinks and cold plates covers the rest.

A drawing that quotes cleanly

• Motor model and bolt pattern named, not just dimensioned.
• True position on the bolt circle, referenced to the centre bore as datum.
• Flatness on the mating face.
• Clearance stated as clearance, not as a fit.
• Screw engagement depth, or the maximum screw length the design allows.
• Finish, with masked areas listed.
• Material and temper.

Seven lines and a model. Send it through the RFQ page and the price, the lead time and any question we have come back within 24 business hours. The worked examples page walks through a 90 mm mount in detail.

Frequently asked questions

Keep reading

• UAV component machining: what to put on the drawing
• Taking weight out of a machined drone part without losing the part