Measuring the "Magic Number"

Measuring the “Magic Number”

Although there have not been any posts recently, in fact a great deal is happening.  The adapter plate remains the primary focus.  Two different suppliers are working on two significantly different designs.  One will use the key way and the other is designed around a tapered hub.  Each process of manufacturing the hub and transmission adapters requires the determination of the precise distance between the face of the flywheel engine flange and the face of the engine block.  This number in the EV parlance has been called “the magic number” for the adapter plate.  This distance must be duplicated in the final design of the adapter.

Picture 4704

Flywheel engine flange after removal of the flywheel and the 19 mm hex bolts.

To obtain the measurement, place a machinist ruler ( in this example a Starrett, 19.5”/500mm, from McMaster Carr #2117A53) against the face of the flywheel engine flange, and then using a caliper (Mitutoyo 6”/150 mm, available from McMaster Carr # 8647A42) positioned perpendicular to the long dimension of the ruler, open the jaws to extend the depth measuring pin.  See picture 4714.

Picture 4714

Showing the orientation of the caliper and the steel machinists ruler.

Then with gentle pressure push the caliper against the engine block.  The pin will withdraw until the base of the caliper is flush and perpendicular to the ruler.  The reading on the caliper at that point, is the distance between the engine housing and the outermost edge of the ruler.  Upon subtraction of the width of the ruler, the difference reflects the distance between the engine block and the face of the flywheel engine flange.

Picture 4716

Close up of the caliper positioned perpendicular to the ruler, with the center depth measuring pin in contact with the previously cleaned engine surface.

In the case the 1992 BMW 325i Getrag transmission, there is also a metal plate (1.0 mm thick) that is situated between the block and the transmission.  The plate appears to close off the open areas of the bell housing when it is attached to the engine block.  This thickness must be subtracted from the above calculation to obtain “the magic number”.

The challenge is to hold the ruler perpendicular to the flange plate, and to hold the caliper depth surface both parallel to the ruler and perpendicular to the engine block.

By the method described here, "the magic number" was determined to be 20.17 mm.