Here are a couple more pictures of the tip of the pump cylinder for Thermos Camp Stove Model 8491. Note the first pic with the valve pip seated. The second pic shows the valve pip raised so access can be gained to the O-ring on which the valve rests when in the seated position. The O-ring on mine was 9 mm in diameter, but I substituted a 3/8 inch (outside diameter) by 1/16 inch (thickness) Maybe this might help someone else who has the same problem that I had.
I just purchased a Thermos 8423 recently and I'm having the same issue with a leaking o-ring on my pump. The pump is identical to yours. My question is how on Earth did you get that 0-ring out of that tight little space and what tool did you use and what HARD drink did you have on your table beside you. Not having an NRV that will unscrew like the Enders pumps for easy maintenance is mind boggling to me. Stupid actually.
It's quite simple to put a Euro style NRV in there if you wish. It' my usual repair on these now. And/or change that seal. Stick a pencil of similar into tube till it opens valve, then secure with tape. Small screwdriver /pick will remove that little oring, they are often rock hard. I have seen several orings, the first few times you pump up stove ROLL OUT OF GROOVE. Which of course is a disaster, fuel leaking everywhere. Make sure Oring is well seated in groove. Pump these up and let them sit several times before using. Also, there is another "hidden" oring between pump tube and the aluminum plug w valve in it, you can see the seam in the picture above. This oring often leaks, you will get fuel into pump tube. You can knock out plug and replace Oring OR you can smear a bit of epoxy around that seam. These pumps tend to leak, don't run em inside, be careful. I have one Thermos lantern and one stove with the original style pump, neither of em have I fixed the Oring, both work perfectly. That's not often the case. Again, be careful.
I did this exact mod to my pump on my 8423, however every time I pumped it the whole end piece would pop out into the fuel tank. That was a treat trying to fish it out. To fix this issue I put a thin coat of epoxy on the channel in front of the new o-ring then pressed it on, it only required a bit of pressure with my thumb..that's how loose it was. Waited 24hrs before testing it out. Seems to be working great, I tested it every day for an hour on full blast for a couple weeks (running it on regular gasoline) for reliability purposes before I use it this summer on camping trips. Runs perfect . I wonder why Thermos didn't thread their NRV assemblies into the pump shaft like Enders did.
