Hi Mark-

I’ve built several BOVs with the rotating spool layout and I’ve learned a lot in the process. Here is what is most likely going on:

The EXTERNAL seal- to keep water out of the loop- is the O ring on the spool near whatever serves the function of a face plate/ spool retainer. The gas leaks that cause failure of the neg test occur within the loop- thru the mouthpiece as stated above. A BOV indeed CAN be built to allow a spool type design a decent neg test, but it ain’t easy. I found that the tolerance spool to body has to be in the 0.0005” range- yes 5 ten thousandths of an inch. The final seal is then achieved with lube in the spool to body interface. No O rings involved.

With this tight a clearance my BOV does good +/_ tests, but the spool is not an easy “flick” to twist ala Jetsam. The slight extra effort required to twist a spool vs how other more complex designs function is worth it to me because the internal bore is a LOT larger = lower WOB.

A spool design could be designed gas tight another way as well but would require some pretty exotic milling or molding gear that would allow O ring grooves to be produced on the face of a cylindrical shape. WAAAAAY beyond the capability of my stone age shop…………

Clear as mud, right?

Best,
Ken