This SAE Recommended Practice establishes a single bolt pattern for both No. 1 and No. 2 clutch housings (see Fig. 1).
This four-bolt pattern is designated to give commonality of mounting brackets in existing frame rails.
The 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 a 15.5 in two-plate clutch. The bolt pattern due to its symmetry allows reversing or inverting of brackets to attain change in vertical or horizontal positioning with fewer brackets.
The phi (ϕ) symbol is for the convenience of the user in locating areas where technical revisions have been made to the previous issue of the report. If the symbol is next to the report title, it indicates a complete revision of the report.