I swapped the cap off the junkyard pump today. While I had the original pump out and laying on a board before taking the broken cap off, I noticed that gas was dripping out from between the junction of the cap and plastic body it connects to.
When I got both of them apart, I put my calipers on the junkyard O-ring then on the Viton -138 O-ring I put in ours. The -138 measured .105" thick, while the junkyard O-ring measured a full .125" thick. I still had our original O-ring and while it was somewhat distorted from where it had popped out, I got an average of about .118" thick.
That's a huge difference!
I coated the OD of the junkyard O-ring plus the ID of the junkyard cap with a little clean motor oil to prevent the O-ring from rolling over and assembled them onto our original pump assembly since it was a whole lot cleaner than the junkyard one. Assembling the cap took a little more force than it did with the -138 O-ring, which is good as it shows that there is a bit more interference now. I put it all back together and we'll see how it works, but I feel confident that this will take are of the problems with restarting after it had been shut off.
Based on my findings, I would have to say that the -138 O-ring someone else specified is NOT the correct one for this application. From charts, the -138 has a cross-section of .103", ID of 2.112", and OD of 2.318".
The plastic section of the pump assembly that the O-ring slides over is ~54mm (2.126") and the ID of the metal cap where the O-ring rests is ~59mm (2.323").
I think it is actually supposed to be a #-928 metric O-ring with a cross-section of 3mm (.118"), ID of 53.09mm (2.090") and OD of (2.326"). This would seem to provide the necessary "squish" to seal it up.
Amazon currently has a 12-pack of 75 durometer Viton -928 O-rings listed for $12 or a 5-pack of 70 durometer nitrile (Buna) listed for a buck. They also list EPDM O-rings in that size, don't use them as EPDM is NOT compatible with gas!
Hope this thread helps someone who comes along later...
"Felix qui potuit rerum cognoscere causas"
[Happiness is understanding how things work]