Thorp Air Command - T18.net

I just found a picture on-line of a Dukes fuel boost pump 4140-00-17, which looks like yours.
The threaded holes for the fittings have a 45 degree chamfered O ring seat. I'm thinking that the washer (not a gasket) is supposed to flatten the O ring until it seals from that seat to the un-threaded gap in your (modified) bulkhead fitting.
If that is going to work, I think you may need an O ring made from thicker cord. (just my perception from looking at your pics)
https://picclick.com/4140-00-17-Dukes-F ... 51830.html
These take AN to O ring adapters, check this link:
https://www.summitracing.com/search/dep ... ing&page=3
You have a home made version of the 90 degree variety. you can see them in the link. (also some unrelated power steering fittings etc)
These all have a smooth spot under the nut (no threads) for the O ring to seal when it is compressed into the chamfered space.
A pull from the product description:
ICT Billet AN to straight thread O-ring (ORB) adapter fittings connect AN threaded flexible hoses or rigid metal tubing to straight thread O-Ring Boss (ORB) fittings. ORB fittings are recommended for optimal leakage control in medium and high-pressure fluid systems. The seal takes place by compressing the O-ring; the threads hold the connection mechanically.

If you install the correct engineered part, it might last more than 100 hours.

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Scott Emery
EAA Chapter 326
T-18 N18TE

x2

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson