I have two combination manifolds with the boss for a second vacuum nipple over the #1 intake. These are installed on my 68 sedan and 67 wagon. I also have an old style aluminum intake with a threaded vacuum nipple hole at the forward end of the balance pipe. The aluminum plug twisted out and made a huge mess. This has made it impossible for me to install a vacuum hose connector without re-tapping the hole.

Since I have an 1800E brake booster and a B20E on the 67 Wagon, and I will be shortly going to a 140 booster for the 68 sedan, I would like to drill and tap all three of these manifolds to install a second vacuum nipple (or the first one in the case of the aluminum manifold). Every late model 122 with the updated brake setup I've seen has had a dedicated vacuum connector for the brake booster (separate from the PCV vacuum). I figure this is important and the 67 wagon's brakes do not boost properly when it's jacked into the PCV vacuum...


The nipple appears to be 1/2, but it's neither 1/2-16 nor 1/2-20. A check using a comb tool from my tap and die kit suggests that it is 18 threads per inch. I've checked all the local hardware sources for a tap of this size, but nobody seems to have one.

So is my measure of this correct? is it really 1/2-18? If so, where can if find the right tap? If not, what is the real thread count and size??

I'm not dying to take care of it now, but I'll need to be able to do it soon as I've got a new MC and booster on the way for my sedan, and I'd like to be able to use the proper PCV valve on the B20E...