boost pressure gauge for a boost bottle?

Threaded barbed fittings are almost impossible to find with the appropriate sized inside diameter that are not to large on the outside. The problem is most threaded barbed fittings have pipe threads which makes them relatively big given their small inside diameter.

With a small 13.5mm carb venturi, the smallest restriction in the tube (including the fittings) should be no smaller than about 7mm or about 5/16". My preference would be 10mm with such a carb size.

For the Do-It-Yourselfer, thin walled metal tubing (brass is easy to find) epoxied into place might be the best bet since the appropriate barbed fitting with an inside diameter that large would probably be too big to thread into the intake manifold depending on the application.

If you get your brass fittings from a auto parts store their threads are smaller than if you get your brass fittings from the plumbing department at a hardware store (home DePot,lowe's,Ace's). I'll give that a try frogger the next time I do a boost bottle.

You need what are usually called thin-walled fittings. The problem with using a large enough fitting is where the tube needs connect to the intake manifold. Often there just isnt a lot of room to drill a big enough hole for a large diameter fitting.

I've seen a lot of boost bottles (expensive ones at that) using 1/4" fuel tubing. The barbed fiiitngs for that tubing has an inside diameter of about 3mm or slightly less. No way that will work correctly.