ElectroClassic EV
Classic Cars Reborn into the Electric Future

Scaling Mount Motor

Time to tackle the motor mount. My understanding is that collar-type mounts cannot be used with the AC50 because they could warp the motor case, so instead an L-bracket mount is bolted to the motor end face. I had originally fabricated one based on RR’s 914 bracket for his AC50 motor, assuming my adapter alignment would be the same. No such luck – His ElectoAuto adapter allows positioning of the motor to any degree, but my CanEV adapter offsets the motor mounting holes by exactly 13.7 degrees. According to Randy at CanEV, this accommodates ADC and WarP DC motors, which both have a threaded lifting lug on their cases at that same offset. Unfortunately, this obsoleted my prefabricated motor mounting bracket, because drilling new offset holes would only allow room for two mounting points, which didn’t seem strong enough. After all, this bracket carries the combined forward weight of the motor and transaxle. Another revelation from HPEV was that the AC50 motor itself has an approximate 45 degree offset between the mounting holes on the drive side and encoder side of the case, where the motor mount will be attached. The approximate aspect adds some complication, which I will soon explain.


Back at the drawing board, I fiddled with Google SketchUp to see if I could design the motor mount and produce a usable CAD file. If I could nail the exact measurements, then M&K Metal could plasma cut my piece from quarter inch steel plate, make the precise 90 degree bend, and the motor mount would bolt right in without complaint. That would be supercool. HPEV confirmed the offset between the threaded holes on the drive end and encoder end of the motor is within a few clicks of 45 degrees, so adding the transaxle adapter offset of 13.7 gives a total of approximately 58.7 degrees. The design on the upper left has slotted holes to allow for a plus or minus 5 degree variation from exact hole placement. The design on the right wishfully assumes any variation will be forgiven by a 1/16 inch larger bolt hole. Crossing fingers and wishing hard. This SketchUp Pro trial version outputs CAD files in both .dwg and .dxf format, which I emailed to Ron at M&K Metal. He imported them into his plasma cutter from a thumb-drive, and ran the job. It’s like a huge computer vector plotter that writes with superheated gas on metal. Just watching it in operation gave me a million ideas. Click HERE to see a clip of the plasma cutter in action.


Seeing the perfection of the shapes and buttery smoothness of the cut edges was really impressive. Then we discovered the mounting holes were slightly larger than expected. Further inspection revealed all other dimensions were scaled up by about 10 percent. Back at his computer, Ron opened my CAD files in a free program called DraftSight, and confirmed all the measurements were off. Somehow, the SketchUp output to CAD introduced a scaling error, and the cutter had simply followed those bad directions impeccably. It was on me to figure it out.


First I went into the SketchUp Model Info pane and switched the Units from Architectural to Fractional, increasing the precision to 1/64. Then I exported an AutoCAD 2010 DWG file and opened it in DraftSight for Mac OSX, shown above. Ron’s business relies on DraftSight, and it speaks a language his system understands, so I was satisfied when it reported the dimensions were good. After resending the file to Ron, he confirmed the dimensions and ran the job free of charge. That small token of generosity made my week, and I left with perfectly rendered motor mounts in hand.

.Just to show the precision of the plasma cutter, the first set of brackets on the right were accidentally scaled 10.1 percent larger due to an output error in Google SketchUp, and yet they are perfectly proportioned. The second set on the left were scaled and cut to the correct size.


A test fit was very encouraging and underscored the precision of the plasma cutter. The motor mount with unslotted holes fit flawlessly on the end of the motor, but did not provide the offset needed for the bracket to sit level on the crossmember. This proved that crossing fingers is statistically effective only some of the time.  On the other hand, the mount with slotted holes allowed enough play for the bracket to level itself before tightening down all the bolts. To prep for permanent installation, the working mount was buffed with a wire wheel, and given a few coats of etching primer and appliance epoxy black for protection.


Here’s a shot of the final install. Notice the 1/2 inch collars used as spacers between the bracket and motor, which provide a gap for the encoder and temperature sensor wires to pass under the mounting bracket. That gap is needed because the white temperature sensor wire emerges from the motor end face itself (visible in the picture at the very top). I was careful to add the spacer thickness to the inner dimension for the bend, and am glad I actually measured them: They are sold as half-inch, but are actually 7/16 inches thick. Alas, the short distance to perfection is infinite.

All told, the fabricated mount cost me less than buying an off-the-shelf bracket, which would have needed modification for the offset anyhow. I’ve also gained a rudimentary knowledge of both Google SketchUp and DraftSight, a working CAD file of the motor mount, and a really fun experience with a plasma cutter. Something tells me my plasma cutting days are not over.

** Note of celebration – This was my 100th blog post!



14 Responses to “Scaling Mount Motor”

  1. Amazing. Looks like one of the more complicated and most important issues you’ve had to deal with. How exciting to succeed!

  2. That is real pretty, Mark. Maybe you can get a 10% larger motor for the next one. 🙂

  3. Stumbled onto your site and I’m interested because I’m converting a 911. In college, I did a motorized couch and a recliner. Nice job documenting the build.

  4. Hi there – I am not sure if you are still checking responses, but I was curious what you used as the motor mounts underneath the AC50 bracket that you fabricated, and how you connected these to the crossmember if not part of the original 911. Thanks!

    • Hey Dave,

      Those were new stock Porsche 914 rubber standoffs for the original gas engine. I designed the electric motor mount to utilize the stock mounting setup.

      Still checking!


  5. […] based my mount on the dimensions of “Chuckles” McGroover’s Porsche motor mount, here. Somehow he had more success with Sketchup, probably because he was using the eval of the Pro […]

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