The B.O.P. "581" is an all aluminum bellhousing.
 These clutch and flywheel assembly covers were manufactured for multiple applications,
as evidenced by two starter housings and dual transmission mounting bolt patterns.
    In 1965 General Motors used Dearborn built (Ford) 3 speeds as a heavy duty option to the Muncie 3 and 4 speed  trans on some models.
 Instead of casting different bells they made these more universal.





       Front view
    Here you can see the two trans mounting bolt patterns.
 Four holes tapped 7/16"-14  for Ford 3 speed manual transmissions
and four tapped 1/2"-12 for the optional GM production trannys.
Although the housing looks skewed the General Motors transmission bolt pattern
is parallel with the upper engine mount holes and the gravel guard flange.






     Part #
   The part number is cast into drivers side starter "ear".
All original "581" clutch housings I have seen have the 7
Raised like a speedometer rolling over.





      Top view
   This picture shows the "V" in the engine mating flange and is indicative of a Buick, Oldsmobile and Pontiac clutch cover.
There are two 1/4" x 1" slots above the transmission mounting surface and another smaller one on the engine mounting flange.






    Fork side
The fork hole is 2 1/4" x 3" and accepts the "355" clutch fork boot.
There are two 3/8"-16 taped holes for the B body z-bar bracket.







  Bottom
    These bells utilize a one piece sheet metal inspection cover that wraps around from engine side to bottom,
and is attached by three 1/4"-20 bolts.
  






  Passenger Side View
Passenger side is unencumbered except for the Buick
starter bump.






 Engine Side View
    Here you can see the two starter "ears" and the engine mounting flange,
with six clearance holes for bolts and two dowel holes.
The starter "ear" on the right is for Oldsmobile and Pontiacs
while the "ear on the left accepts Buick starters. 






          Inside View
    This view reveals the fork mounting stud location and beefy reinforcements around all mounting holes.
The BOP bell housings are much stouter and heavier than Chevrolet bells.