I have a 1991 F250 7.3 IDI and used a size 014 o-ring (1/16" cross section 1/2" x 5/8") for the inner seal and a size 018 o-ring (1/16" cross section 3/4" x 5/8") for the outer seal. I found them at a local Parker hydraulic hose store. They sealed up the leak perfectly and they had the proper amount of resistance during assembly. I originally tried a size 114 o-ring and found it was too thick and required exessive force for assembly. I now have over 1000 miles on this fuel heater o-ring job and its still working great.