57-70 (3728), 1/4“x20x1/2” hex bolt.
71-85 (2420), 1/4“x20x3/4” posidrive bolt.
Any 1/4'x20 bolt will do as a replacement.
If the threads are jacked up when this bolt is removed, you can drill / tap to next size up. ¹⁾

1982 Primary Oil Site Plug 2)

1957-1976 models vent the primary thru the motor internally via the oil transfer valve. Click here to read more on the transfer valve in the Sportsterpedia.
1977-1985 models vent out the back of the motor from the top of the transmission near the rear motor mount just behind the solenoid (1/4” hose fitting).

Routing the vent hose:

• Note: The transmission vent hose routing on Late 1981 XLH/XLS models was changed per Service Bulletin M-813 to reduce excessive oil leakage from the transmission.
  • Early 1981 models can be changed to the new routing.
  • The vent hose should be routed under the starter, over all the oil lines and the rear brake line, then down between the pivot bolt and swing arm. ³⁾
    

Generally speaking, the rubber hose should run up under the seat and curl pointing down. If it has rotted or come loose from the fitting it can make a mess of that area. ⁴⁾
There should be no sags in the hose between the attachment to the engine and the open end of the hose.
This will allow air to escape and oil to separate from the air and drop back down into the tranny compartment.

1977-1980 models use a fitting (34744-77) with a SAE 3/8“ x 16 bolt pattern that has a 1/4” hose nipple on the other end.
1981-1985 models use a pressed in metal tube (34744-77) instead with a 1/4“ hose nipple on the end.