The info on this topic is from Tom Osborne. He put this together to help his customers at Central Georgia Generator to properly measure SAE Flywheel Housings and Flywheels. We have permission to post this on our forum.
Determining Your Bell Housing & Flywheel Size
In order to determine your correct SAE bell housing and flywheel size, we will need for you to take three measurements:
XD which is the inside diameter of the bellhousing
XE Which is the diameter of the drive plate or the machined-out part on the flywheel where the drive plate bolts. NOT THE DIAMETER OF THE FLYWHEEL
XG Which is the distance the drive plate extends into the bellhousing. The XG is measured by laying a straight edge such as a level across the rear of the bell housing and measure from the inside of the straight edge to the flywheel machined out part on the flywheel where the drive plate bolts.
Compare your measurements to the measurements in the chart to find your SAE size.
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Tom Osborne's Video clips of measuring your SAE Bell Housing & Flywheel Size
https://youtu.be/MO6S0DLQ_pw
https://youtu.be/bc0dISeIZN8
Well, the housing measures exactly 18" on the XD dimension, so it looks like some sort of a bastard size. To the scrap yard it goes.