This SAE Recommended Practice establishes a single bolt pattern for the No. 1 clutch housing (see Figure 1) and the No. 2 clutch housing (see Figure 2).
These four bolt patterns are designated to give commonality of mounting brackets in existing frame rails.
The 420 mm (16.5 in) span, pad face to pad face, allows the ease of installation in existing frame rail widths. This is also the minimum spacing which will accommodate commonly used clutches.