Last update: May 15 2017

Motor Adapter

At the time I did this conversion, there was no commercially available adapter for the Subaru EJ engine, so I designed my own and had it made at a local machine shop. It has some drawbacks, primarily the cost.

The aspects of Subaru EJ engines and the Impreza that impact the adapter design are:

Custom Flywheel

A custom machined flywheel accommodates the many constraints of fitting the NetGain 11 and keeping the A/C. It allows locating the motor as close to the transmission as possible. I spent months researching and designing. The custom flywheel needs neither the 8 mounting bolts nor allowance for the boss, so an ordinary QD bushing can be used. This design uses a SDS series bushing (which cost $16). An additional advantage of this design is that you can easily adjust the flywheel runout since the flywheel does not need to be removed in order to make adjustments. The machining cost $1400, took three months and several corrections.

flywheel 3d rendering Custom flywheel

The flywheel uses a Fidanza 229101 replaceable friction surface insert (which cost $66). Here is some Subaru Impreza clutch size data I collected when searching for the friction surface insert. Specifications are outside diameter x inside diameter in mm.

2002-03225 x 150230 x 150230 x 150
2004-05225 x 150230 x 150240 x 160
2006-07225 x 150230 x 155240 x 160
Flywheel InsertSize
Fidanza 229001228.60 x 146.05
Fidanza 229101231.14 x 146.05
Fidanza 229501241.30 x 152.40

The SDS series QD bushing has a torque capacity of 417 ft·lb. For comparison, the torque ratings for the 2006 Impreza base engine, the WRX engine and the STi engine are 166ft·lb, 235ft·lb, and 300ft·lb. The table below contains QD bushing specifications, taken from TB Wood'sPDF. The three 1/4-20 bolts are torqued to 108 in·lb (12.2 N·m).

Bushing Torque Ratings and Dimensions

To remove the flywheel from the QD bushing, the three cap head socket screws are removed from the unthreaded holes, then driven into the adjacent threaded holes. The screws push against the QD bushing, spreading the two components apart.


The housing is a single piece design. I had a local machine shop make it at a cost of $2000 and two months time. Pricing with eMachineShop shows a cost of $1150 and less than a month.

adapter assembly adapter housing 3d rendering adapter housing drawing

The adapter described here is for a NetGain WarP 11. The NetGain WarP 9 mounting holes are tilted by 15° compared to the WarP 11.

Center Bearing

In the original ICE configuration, the transmission input shaft rides in a ball bearing in the center of the flywheel. In the electric configuration, the input shaft rides in a bronze sleeve bearing inserted into the counterbore of the motor shaft. A sleeve bearing is adequate because the flywheel and motor rarely turn at different speeds - only when shifting gears. The input shaft diameter is metric (12mm) while the motor counterbore is fractional inch (11/16 = 0.687). I purchased a metric sleeve bearing 12mm ID x 18 OD x 16 LG (McMaster 6658K18) and had a machinist reduce the OD to 0.686in.

Measuring the Transmission

I measured the locations of the EJ transmission mounting holes using an 18in caliper, with the transmission in the car. The one important dimension is the distance between the dowels, holes 3 and 8. The input shaft is at the midpoint of these two dowels. The bolt holes could be enlarged to work around a fitment problem. But the dowels must be positioned as accurately as possible. I converted the distances from the dowels to the other holes into X-Y coordinates using the eMachineShop CAD program by drawing circles and finding the intersections.

18in caliper measuring transmission spare engine convert caliper to X,Y

The table below shows the data I collected, as well as data published on the Ultimate Subaru Message Board by WoodsWagon. Note older versions of the EJ engine have 4 bolts rather than 8.

HoleTransmission in Car
by caliper
Junkyard Transmission
by CNC
Spare Engine
by caliper
by CNC

I also measured an EA transmission for a EA-EJ adapter project.

measuring transmission

HoleTransmission in Car
by caliper
by CNC

To double check the hole positions, the plans were printed actual size, cut and and placed right on the transmission.

checking hole placement

Checking the flywheel alignment with a dial indicator.

dial indicator