I would try a fiber or rubber washer before I hauled out the wallet and cleaned out the Benjamins.
And please add a little to your profile, like a general location. Marine shops have a wealth of goodies for sealing that dry landers never need. My first choices would be a Caterpillar Dealer or a John Deere Dealer. Both have tremendous inventories of O-rings and washers. And their parts people are pro's. They can adapt. Bring your plug with you and explain what you need to achieve. Pictures are worth a thousand words, and a good parts man ain't all that chatty.
This isn't the first time somebody got off a tad on a re-tap or a helicoil. There's a bike running around here that has been using a leather washer on the oil plug for about 6 years cuz somebody was in a hurry and kinda messed up..
ďmany a trip continues long after movement in time and space have ceasedĒ- Steinbeck, [I]"Travels with Charlie"
[FONT="Century Gothic"][I]Sometimes your only available transportation is a leap of faith[/I] [/FONT]