Oil Filler cap

Hi: I have a friend that is building a plane designed by a farmer 60 years ago.
The engine is from a model T, max speed 75 mph .
The original plans has a sliding cable with a pin that locks the oil filler cap, that is screwed on. He has changed that, not wanting the tube to stick out of the cowling. The change is to use a non threaded plug design with just 2 "o"rings for a friction fit without the cabled pin as a lock. My view is that the filler plug could come out from vibration or just the air going past it, Bernoulli's principle. I have an engineering back round and have used redundant safety wires and cotter pins on many fasteners in the printing industry.

He wants to fly to every state a write a book on his adventure. I just want his experimental plane to make every trip being as safe as humanly possible.

Any advice would be greatly accepted.
Thank You for listening.
dwdw47

Just in general terms, loosing an oil cap and having all the oil sucked out is bad news. I'd do everything to assure that never happens.

I can't exactly picture the present setup- do you have a pic?
Bob H