Oil filler cap O-ring 

Ref. H-D : 11178

As shown in the pictures

Brand : Harley-Davidson

Type : New

Fits on :

The part is compatible with the following motorcycles:

Softail  
Heritage : FLST (1987-1990)  
Heritage Classic : FLSTC (1987-1990)  
Fat Boy : FLSTF (1990)  
Standard : FXST (1987-1990)  
Custom : FXSTC (1987-1990)  
Springer : FXSTS (1988-1990)

Dyna  
FXR Low Rider : FXLR (1987-1990)  
FXR Super Glide : FXR (1987-1990)  
FXR Custom : FXRC (1987)  
Police Low Rider : FXRP (1984-1990)  
FXR Convertible : FXRS CONV (1989-1990)  
FXR Low Glide : FXRS (1987-1990)  
FXR Sport : FXRS-SP (1987-1992)  
FXR Touring : FXRT (1987-1990)

Touring  
Electra Glide Sport : FLHS (1987-1990)  
Electra Glide Standard : FLHT (1988)  
Electra Glide Classic : FLHTC (1987-1990)  
Electra Glide Classic Ultra : FLHTCU (1989-1990)  
Tour Glide Classic : FLTC (1987-1990)  
Tour Glide Classic Ultra : FLTCU (1989-1990)