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Coupler overview

Since I spent a majority of my EV conversion budget on a Lithium battery pack I needed to cut back the spending and save money wherever possible. One of the areas I figured I could save a few bucks was on the home fabrication of the coupler and adapter plate.

I started my quest for a DIY coupler by searching the catalogs of commonly available industrial taper lock bushings. After looking over the many different products that use the taper lock setup I selected a synchronous gear belt pulley and split taper bushing combination made by Browning. I chose the synchronous gear belt pulley to make the coupler since it was made out of steel instead of cast iron and would be easier to machine. The pulley was Browning part No. 24HP100 (Grainger No. 2L700) which had an outer diameter of approximately 3.8” and thickness of 1.25”. The bushing was Browning part No. P1 X 1-1/8 (Grainger No. 3X483) for a 1.125” keyed shaft. I chose the split taper style bushing over the Q-D style bushing because the split taper type has an external key which mates with the pulley for positive drive and greater torque carrying capability. Whereas the alternate Q-D style bushing does not have an external key and relies solely on the friction of the taper and the bolts to transmit the motor torque to the load.


Bushing and pulley as purchased


Coupler CAD dwg

One nice aspect of using a later model donor vehicle such as the A4 Jetta (4th generation) is the pilot-less input shaft in the transaxle. No pilot bushing is required for the transmission input shaft. This eliminates the need to fabricate and install a bushing in the end of the motor shaft or coupler body.

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Reverse mounting of bushing

One problem with the split taper style bushing was that it did not have the reverse mounting capability (bushing flange towards motor) that other style bushings have. This problem was easily solved by installing threaded inserts in the bushing through-holes. I used 5/16-24 fine thread thin walled key-inserts with a 7/16-14 outside thread for this purpose. Reversing the bushing mounting scheme also required drilling and counterboring the existing threaded holes in the pulley to sink the fastener heads below the surface of the pulley which is used for mounting the flywheel. I used a 31/64” endmill to a depth of approx 0.750” for the counterboring operation. After this I trial-fitted the bushing and pulley together to verify proper fitment and that the 5/16-24 socket head bolts would be below the surface of the pulley after the facing of the pulley was complete.


Threaded inserts installed in bushing

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Turning the pulley – Flywheel mounting surface

Next I removed the belt guide rings around the edges of the pulley and mounted the pulley and bushing on a dummy shaft so I could turn it on the lathe. I used a 4-jaw chuck instead of the 3-jaw chuck to minimize runnout. Before removing any material from the pulley I verified the pulley and shaft were true with a dial indicator. I took 0.375” off the face of the pulley leaving a protrusion or hub in the center of the pulley to properly locate the center of the flywheel. My goal was to mimic the shape of the flywheel mounting surface on the crankshaft while maintaining the proper flywheel depth when using a 1” thick adapter plate. The flywheel locating hub on the crankshaft had an outer diameter of approximately 1.693”. This measurement was smaller than the inner diameter of the bushing opening in the pulley (1.856”). This meant that the nose of the bushing and not the pulley would be responsible for locating the center of the flywheel. I realized this was not an ideal method of locating the flywheel since the outer diameter of the bushing nose could vary somewhat depending on the diameter of the motor shaft it was mounted on. With this in mind I made sure the dummy shaft I used had the same diameter as the motor shaft to within 0.0005”.


Machining complete on pulley face

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Drilling and tapping the pulley

Once the material was removed from the face of the pulley and the flywheel hub was sized appropriately I aligned and marked the 6 flywheel bolt holes on the pulley for drilling and tapping. The flywheel bolt holes were not all evenly spaced at 60°. One of the holes was set approximately at 49° / 71° from the others. Initially I was worried that this meant the engine was externally balanced by the flywheel. But close inspection of the flywheel assembly revealed no external weights or non-symmetric features on which meant the flywheel was not used to balance the engine (this was later verified by trial assembly and spin test on the motor). I drilled the flywheel mounting holes in the pulley to 8.75mm and tapped with a M10-1.25 tap. The original flywheel bolts had a 1.0mm thread pitch but I could not find a M10-1.0 tap with a rear centering dimple locally. I used M10-1.25X30mm grade 12.9 socket cap screws to mount the flywheel to the pulley.


Flywheel bolt holes marked and center-drilled


Tapping flywheel bolts holes in pulley


Finished coupler – Front view


Finished coupler – Side view


Finished coupler – Rear view

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Adapter plate overview

Since I did not have access to a CNC milling machine I figured I would use a two piece adapter plate (one plate attached to the transaxle bellhousing and the other plate to the motor) rather than one thick plate. I lucked out and found some scrap ½” aluminum plate for $1.50 per pound at the local metal recycling yard for this purpose. Prior to removing the flywheel from the gas engine I measured the ‘critical distance’ behind the flywheel to the transmission mounting surface. This measurement was 0.175”. My goal was to accurately duplicate this distance while using a 1” thick adapter plate and have the taper lock coupler fully inserted onto the motor shaft.

Due to the limited space in a transverse engine compartment (19.875” between tranny bellhousing and frame rail) anything much greater than 1” for the adapter plate thickness would prevent the use of the Kostov 11” motor which has a length of 18.05” from the mounting surface to the end of the fan housing. Motor options for transverse engine applications are limited due diameter and length restrictions.

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Transmission plate

I temporarily removed the two hollow locating dowel pins and RH side CV drive flange from the transaxle so I could trace the bell housing outline and locate some of the mounting bolt holes onto the aluminum plate. Once the plate was marked I cut it to size with a jigsaw and smoothed out the edges with a belt sander. I used a 5/8” counterbore drill on two of the mounting holes to mate with the locating dowel pins in the transaxle. Once the plate was mounted to the transaxle I marked the location of the remaining bellhousing bolts and input shaft center onto the plate. I cut a 9.125” diameter hole in the center of the tranny plate to clear the motor mounting bolts in the motor plate.


Tracing outline of transmission on aluminum plate


Index holes drilled and outline marked


Cutting center of tranny plate


Finished tranny plate mounted on transmission

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Motor plate

I selected the Kostov 11” DC motor for this conversion project. Since the Jetta bellhousing was not much larger that the body of the motor I decided to make the motor mounting plate outer diameter the same as the motor drive end plate diameter (11.45”). Centering of the motor plate to the motor was accomplished by a 4.0” diameter hole to accommodate the nose of the motor. The mounting bolt holes were drilled oversize and subsequently not used for centering the motor. Special attention was given to the allow clearance of the CV drive flange so it would not interfere with motor plate or motor. In fact both the motor plate and faceplate of the motor had to be notched to clear the CV drive flange and CV joint mounting bolts.


Notching motor plate for CV flange

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Centering the motor plate on the tranny plate

Since this transmission had a pilotless input shaft there was minimal radial play associated with said shaft and therefore it could be used to center the motor plate with respect to the tranny plate. I mounted a dial indicator to the transmission input shaft and used it to center the 4” circular cutout in the motor plate.


Centering motor plate on tranny plate with dial indicator

Once the motor plate was properly positioned I clamped it to the tranny plate, drilled and installed four 5/16” roll pins to securely and precisely locate it in this position. Note: prior to this step I made sure the motor was properly oriented with respect to the transmission (lift eye on top). After the roll pins were installed I removed the plates from the bellhousing and drilled holes through both plates for eight 3/8-16 socket head cap screws. I installed threaded inserts into the tranny plate to accommodate the cap screws. I also had to counter bore the holes in the motor plate to prevent contact between the cap screw heads and the motor face. These fasteners are used to solidly clamp the motor and tranny plates together.


Finished adapter plate – motor side


Finished adapter plate – tranny side

At this point I trial fit the finished adapter plate assembly onto the tranny and temporarily bolted the motor faceplate to the adapter. This allowed me to gauge how much material needed to be removed from the motor faceplate to clear the CV drive flange.


Trimming motor faceplate for proper clearance of CV flange and bolts

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Final assembly of adapter plate, coupler and flywheel

With the motor plate and tranny plate bolted together I attached the adapter plate assembly to the motor using eight 3/8-16 socket head cap screws. Once the adapter plates were installed onto the motor I installed the coupler on the motor shaft and trial fit the flywheel. Special attention was given to the method used to torque the three taperlock clamping fasteners. This was performed in a repeated sequential fashion to the desired value of 192 inch-pounds. The first attempt at positioning the coupler depth was not correct so the coupler had to be loosened to allow axial adjustment of the coupler on the motor shaft. This took a few times to reposition the coupler and get the correct distance of 0.175” between the back of the flywheel and mounting plate.


Adapter plate and coupler installed on motor


Verifying correct flywheel position with feeler gauge


Once the coupler was properly positioned and torqued I installed the flywheel and verified the axial runnout and wobble was acceptable with a dial indicator (< 0.001”). The A4 Jetta 1.8L Turbo engine and possibly others uses a dual mass flywheel assembly which adds a size constraint to the head of the flywheel mounting fasteners. Because of this I had to reduce the height of the socket head to approximately 0.325” with a bench grinder to clear the movement of the friction surface portion of the flywheel. After the flywheel was mounted onto the coupler the next step was to install the clutch disc and pressure plate. This was followed by the tell-all spin test of the motor – no vibration!!


Final assembly and spin test of flywheel and clutch

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Final assembly of motor to transaxle

After I was satisfied that the flywheel and clutch were vibration and wobble free I installed the motor onto the transaxle. Once the motor and tranny were mated I then could install the right side CV flange which was removed earlier. The final test of the assembled motor and tranny was to energize the motor with a moderate voltage and cycle the clutch a few times with the motor spinning. This verified proper alignment of the motor with respect to the input shaft of the transmission. All was good…


Motor and tranny – Top view


Motor and tranny – Front view


Motor frame and faceplate modified for CV flange clearance