# 2208 gimbal motor: outside envelope and mounting holes (Ø27.5 × 23, base M3 on 19 × 16, rotor M2 on 12 × 12)

> What are the dimensions and mounting holes of a 2208 brushless gimbal motor, and how much does it make as a generator?

- Category: Hardware › Stirling & thermoacoustic
- Kind: Hardware
- Basis: T-Motor GB2208 KV128: Ø27.5 × 23, KV 128, internal resistance 14.4 Ω, 12N14P (maker's page); Tiger GB2208 listing: 4.5 hollow shaft, stator holes 4-M3 on 19 × 16, rotor holes 4-M2 on 12 × 12; tap depths assumed
- Page: https://xemstudio.com/objects/gimbal-motor-2208-envelope/ · JSON: https://xemstudio.com/api/geometry?object=gimbal-motor-2208-envelope · Plain text: https://xemstudio.com/objects/gimbal-motor-2208-envelope/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## Notes

- An envelope, not a model of the motor: the can, the hollow-shaft bore and the base and rotor-top holes at the published sizes. The base is the stator side that bolts down; the rotor top turns and carries what it drives.
- Electrical: 128 rpm per volt and 14.4 Ω (published). As a generator on the sealed engine it turns 2.8 times the crank speed through a 42 : 15 gear pair: 19.7 V open circuit at 900 crank rpm (2520 motor rpm).
- Why this motor: copper loss in a generator is (T / Km)², with Km = Kt / √R the motor constant; the gearing divides the torque. Of the low-KV motors whose size, KV and resistance are published and that fit the 2 in cross with the gearing that fits there, this one gives the most watts; the table shows the others (4 and 8 W at the crank, 2 % gear loss, drag not included).
- The GM4108 would do far better (Km five times higher) but at Ø47 it has no running clearance in a 2 in bore.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| can | Ø27.5 × 23 (envelope) | motor-mount face and the crank below (1.75 clear) | designed | all |
| base holes | 4 × M3 (Ø2.5 tap drill, 4 deep): pairs 16 and 19 apart | csk-m3x6 through the mount face | designed | all |
| rotor-top holes | 4 × M2 (Ø1.6 tap drill, 3 deep): pairs 12 apart | shcs-m2x4 through the pinion adaptor flange | designed | all |
| hollow shaft | Ø4.5 bore | nothing (clear) | designed | all |

## Sizes

| motor | can_mm | KV | R_ohm | fits | ratio | E_open_V_at_900 | W_out_at_4W | W_out_at_8W | V_at_8W |
|---|---|---|---|---|---|---|---|---|---|
| A2212/13T 1000 KV (2212 drone outrunner) | Ø27.5 × 30 | 1000 | 0.133 | yes | 2.80 | 2.5 | 3.6 | 6.55 | 2.1 |
| A2212/13T 1000 KV (2212 drone outrunner) | Ø27.5 × 30 | 1000 | 0.133 | yes | 1.00 | 0.9 | past matched load | past matched load |  |
| T-Motor GB2208 KV128 (gimbal) | Ø27.5 × 23 | 128 | 14.4 | yes | 2.80 | 19.7 | 3.35 | 5.56 | 14.0 |
| T-Motor GB2208 KV128 (gimbal) | Ø27.5 × 23 | 128 | 14.4 | yes | 1.00 | 7.0 | past matched load | past matched load |  |
| iPower GM2804 (gimbal) | Ø35 × 15 | 168 (from no-load rpm) | 9.0 | yes | 1.83 | 9.8 | 2.47 | past matched load |  |
| iPower GM2804 (gimbal) | Ø35 × 15 | 168 (from no-load rpm) | 9.0 | yes | 1.00 | 5.4 | past matched load | past matched load |  |
| iPower GM3506 (gimbal) | Ø40 × 17.8 | 141 (from no-load rpm) | 5.6 | yes | 1.50 | 9.6 | 2.98 | 4.09 | 5.0 |
| iPower GM3506 (gimbal) | Ø40 × 17.8 | 141 (from no-load rpm) | 5.6 | yes | 1.00 | 6.4 | past matched load | past matched load |  |
| iPower GM4108H-120T (gimbal) | Ø47 × 22.7 | 27 (from no-load rpm) | 11.1 | no | 1.00 | 33.3 | 3.84 | 7.36 | 30.7 |

## Where the files are

- T-Motor (maker's store page): [GB2208 gimbal motor KV128: Ø27.5 × 23, internal resistance 14.4 Ω, 12N14P, 39.5 g with cable](https://store.tmotor.com/goods-447-GB2208.html) — web — maker page; values are facts (verify at link)
- GetFPV (product listing): [Tiger GB2208-80 gimbal motor: 27.5 × 23, 4.5 hollow shaft, stator 4-M3 on 19 × 16, rotor 4-M2 on 12 × 12](https://www.getfpv.com/tiger-gb2208-80-brushless-gimbal-motor-hollow-shaft.html) — web — vendor page; dimensions are facts (verify at link)
- iFlight (maker's shop): [iPower GM2804: Ø35 × 15, 9 Ω, no-load speed at 10 V (KV derived)](https://shop.iflight.com/ipower-gm2804-gimbal-motor-pro1153) — web — maker page; values are facts (verify at link)
- iFlight (maker's shop): [iPower GM3506: Ø40 × 17.8, 5.6 Ω, 2149–2375 rpm no-load at 16 V (KV derived), 0.17 A no-load](https://shop.iflight.com/ipower-motor-gm3506-brushless-gimbal-motor-pro967) — web — maker page; values are facts (verify at link)
- iFlight (maker's shop): [iPower GM4108H-120T: Ø47 × 22.7, 11.1 Ω, 513–567 rpm no-load at 20 V (KV derived)](https://shop.iflight.com/ipower-motor-gm4108h-120t-brushless-gimbal-motor-pro217) — web — maker page; values are facts (verify at link)
- iFuture Technology (product listing): [A2212/13T 1000 KV outrunner: Ø27.5 × 30, shaft Ø3.17 × 12, mounting holes M3 on 16 mm × 19 mm, 1000 rpm/V](https://ifuturetech.org/product/a2212-1000-kv-bldc-brushless-dc-motor-for-drone/) — web — vendor page; dimensions are facts (verify at link)
- Flying Tech (product listing): [SunnySky X2212-13 980 KV II: internal resistance 133 mΩ, no-load 0.3 A at 10 V, rotor Ø27.5, body 30, shaft Ø3.175, 12N14P](https://www.flyingtech.co.uk/product/sunnysky-x2212-13-980kv-ii-2-4s-multirotor-brushless-motor/) — web — vendor page; values are facts (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/gimbal-motor-2208-envelope/index.json

- gimbal-motor-2208-envelope: [STEP](https://xemstudio.com/files/hardware/gimbal-motor-2208-envelope/gimbal-motor-2208-envelope.step) · [STL](https://xemstudio.com/files/hardware/gimbal-motor-2208-envelope/gimbal-motor-2208-envelope.stl) — ENVELOPE, not a motor: 2208 gimbal-class outrunner at its published outside size, can Ø27.5 × 23 with the base (stator mounting face) at z 0 and the rotor top at z 23, Ø4.5 hollow-shaft bore; 4 × Ø2.5 (M3 tap drill, 4 deep, assumed) in the base, pairs 16 and 19 apart; 4 × Ø1.6 (M2 tap drill, 3 deep, assumed) in the rotor top, pairs 12 apart

## Questions

**What are the mounting holes of a 2208 gimbal motor?**

Base: four M3, pairs 19 and 16 mm apart. Rotor top: four M2, pairs 12 mm apart (Tiger GB2208 listing).

**Is this a CAD model of the motor?**

No: it is the space the motor takes and where it bolts, at the published outside sizes.

**Why not drive it straight from the crank?**

At 900 rpm it makes only 7.0 V and its 14.4 Ω winding could deliver at most 0.86 W into a matched load; geared 2.8 : 1 it makes 19.7 V and can take the engine's few watts.

## Fits with

- [Motor-mount plate for a 2 in tri-clamp cross: clamps between two faces, holds a 2208 gimbal motor inside](https://xemstudio.com/objects/tri-clamp-motor-mount-plate/)
- [Step-up gears for a generator inside a 2 in tri-clamp cross: 42 : 15, module 1](https://xemstudio.com/objects/tri-clamp-generator-step-up-gears/)
- [Build list: Tri-clamp sealed diaphragm generator, 2 in](https://xemstudio.com/objects/list-tri-clamp-sealed-diaphragm-generator/)