This adapter plate has cost $50CAD in materials, and $64 in tools!

It took a lot of measuring and re-measuring, it was hard because I wasn't sure if I was seeing metric or imperial, plus not having the right tools for anything.

Since this time, I have had a nice visit in Florida with Steve Clunn, of Green Shed Conversions, and learned of a much simpler and extremely accurate way of creating an adapter plate, but an engine hoist and good welding skills are required, I have neither, but I'm good at teaching! So let me know if you need help!

Here is a photo with the existing plate that I found between the transmission and the old ICE engine. I believe it is called a block saver/spacer plate/gasket. I think it is about 1/16" thick. After making my coupler, I measured that my required adapter plate will have to be 1" thick (well, 1-1/16", but i'll re-use the spacer plate)
Many EV converters use aluminum, however, after looking into the price of a 16"x14"x1" block, vs the price of steel, the choice was easy. I'll use 1/2" steel plating, but I got a 16x16 instead so I would be able to cut off 2 bars at about 1" each to use a spacers for the electric motor. (only $30 no tax!)

I had a theory based on the shape of the transmission and spacer plate. That at least 4 of those holes are on a line that is circular directly surrounding the drive shaft center. Using some old reliable center of an arc finding techniques, I eventually determined that the radius of this circle is exactly 6-1/4"
What I wasn't able to find so accurately was the size of the holes. Only 2 of the holes were important in that they were calibrated in the factory on the aluminum plate to snugly fit the size of the doweling pins that the weight of the transmission rested on. I had initially measured these to 15mm (before I concluded that it was all imperial) After seeing that you can buy 5/8" doweling pins, and measuring that the existing pins were really really close to 5/8" (remember, i don't own calipers) I assumed the pins were 5/8". WRONG!
I managed to borrow a 5/8" drill bit, and its a good thing I tested it first because it was too big, as it turns out, the actual size is 1/64th smaller than 5/8 (39/64)... man, imperial is stupid.
Here is a picture of my fancy new 24$ 39/64" high speed steel bit, if anyone wants to buy/borrow it from me when I'm done, let me know, I'll probably never need such an obscure size again.
(the smoke is from the oil I was using)

Getting back on topic, I tested my circular theory on a piece of wood here with positive results.


I used the drill bit as a temporary dowel pin.
You can see the perfect alignment through the picture on the right, along with the nice cutting job on the drive shaft guide pin.
I made a tool that would help me draw a perfect circle while remaining ridged. Simply, I put a thumbtack through some scrap aluminum sheet metal on one end, and exactly 6-1/4" away, I put a hole for my .7mm mechanical pencil. I started by placing the tack into a center punched dent that I guessed to be where I want to be the center of the plate.

Here is a step-by-step photo description on what happened next. What isn't shown is that I drilled my first hole without tracing, (see drill bit picture above). This hole was placed along the circle and once drilled, i used the drill bit to secure the spacer plate in place to ensure that this part of it is aligned with the circle's circumference.
I aligned the other holes along the circle and traced them, (this part introduces the most error, so I made sure to be accurate as possible) the rest of the photos are pretty self explanatory.

After testing the alignment, i was pleased to see it worked out well! This last photo of a dowel hole is taken with the drill bit as a dowel on the other hole.

I have all the other holes center punched and ready to drill, but alas, my job called me away, and I can't get to this till later.
Next up is getting the holes drilled for mounting the electric motor. I've figured out an easy way to do this using trigonometry. Allow me to rant some more about how much imperial sucks.
Here is the drawing diagram of the electric motor's face and bolt pattern.
(took me a while to find this file, but here you go)
Notice the diameter of the mounting bolts... 8.4 inches. Seems pretty normal at first, (especially if you grew up with metric), but I challenge you to go rummage around your old school supplies or even basic hardware store for a ruler that has increments of a tenth of an inch. 8.4! that means 8-2/5" ... two fifths?! wtf? that's as stupid ask asking for 5/7"
(2/5 would not be unreasonable in metric, such as 2/5 of 1 cm = 4mm, easy right? however 213.36mm is a little bit beyond my 1.0mm scale)
I figured out an accurate work around, and I'll share it with you now (I don't know why the motor company didn't do it this way)
I'll leave out how I derived it and show you the results just for the top hole, and the rest follow suit.

1. From the center point, go up at 90 degrees for exactly 4-1/16" - mark this point
2. From that point, go right (at 90 degrees again) exactly 1-1/16", mark this point for center tap
3. do this for 3 other sides.

Here is the math proving it.
√(4.0625² 1.0625²) = 4.199
according to the drawing, the radius needs to be 4.2"
This works to an accuracy of .00084" (error is 0.020375%). close enough eh?
I'm sure my own hand with the center punch, and potential drift from the drill press will have much more error than this.

*Update* So the above portion looked perfect on paper, but alas, there was still a little bit of error in drawing the lines. I wouldn't do it this way again. I started with the holes tapped as above, but they didn't line up perfectly, I had to smooth them about about 1/64" or so with my high speed dremel. At this point I only had 2 holes drilled and I was much more careful with the other 2. I ended up making a circle drawing tool at approximately the right distance. ( I very hard to pinpoint 4.2" ( I still resent that number).
Long story short, the adapter plate is complete enough to be useful.

I didn't cut out the shape of the transmission due to time constraints. I drew a line outlineing where to cut should I have time in the future to do so. Another tool that brought the price up to 64$ was the hole-cutter you see in the above pictures. You may think that those cutters are meant for wood, but this one is ice tempered and is rated for hard iron! (low rpms)