O ring size for block fitting for mercruiser fuel rail
 

I have a gen 6 454 mag mpi gen 3. There is a brass block with 2 o rings that go into the port side in the middle of the fuel rail. The schematic only shows 1 in the drawing and says 1 required. Any idea if they are the sane size? I'm assuming yes but my eyes start playing tricks on me. This is the part number listed 25-805970

Guessing you have a VST system?? By the looks of the diagram, you would need one O ring for each hose that goes into the end of fuel rail

VAPOR SEPARATOR TANK AND FUEL RAIL (mercruiserparts.com)

No vst. Cool fuel. Schrader valve at front end on fuel rail. Fitting in the back is plugged