I need your help with my vacuum tap (and my manifold studs, too).

[cruiser]

Mopar Guys/Gals: I could use your advice on my 1974 Duster's intake manifold power brake booster vacuum tap. The first photo is of the existing tap in the intake manifold of my slant six. Like many things on my motor, it had been changed out with an incorrect part years ago. My engine should have the single tap with no nipples coming off it since it is a non-a/c car. The second photo is the correct tap that I'm installing on my engine. The diameter is 11/16" with either NPT or fine threads on it, and it takes an 11/16" open end wrench to remove it. As you can see, the old tap was only threaded about halfway into the manifold. When I removed it, the bottom half of the female threads inside the intake manifold appeared to be dirty and a bit rusted, although not damaged or stripped. My first question is this: How far should this fitting be threaded into the intake manifold? Logic tells me that it should be threaded all the way down to the shoulder on the fitting itself. But as you can see, the old tap won't go in any further because it stops threading about halfway down. I suspect the threads in the manifold are contaminated and prevent the fitting from threading fully into the intake. What do you think? Should this thing go all the way down? The length of the threads on the fitting is the same as the depth of the female threads inside the intake manifold. Should I put some nylon tape or pipe dope on it prior to installation to ensure a vacuum tight seal? Should this fitting be painted engine color or left natural metal? I've heard of something called a thread chaser before, and I believe it is used to clean out contaminated female threads. I've never used one, so forgive me for asking such a basic question, but should I chase the threads in the intake manifold so that my fitting will go all the way in? Thanks in advance for your advice. Finally, the studs that attach my intake/exhaust manifold to the cylinder head appear to be several different lengths. Shouldn't they all be the same? Slant six experts, what is the correct stud length and fastening bolt for my engine? Does anyone sell this in a kit, or do I just buy them at NAPA? Thanks for the help!

 

[Projectile Dart]

I'm going out on a limb here but it would make sense if the intake side of the threads were tapered to help ensure a good seal and since not all nipples are made the same I would think that it would be normal for some threads to be showing. If it were me I would put a little Teflon tape on it either way just to make sure everything seals up nice because vacuum leaks are a good way to create some gremlins that can take you forever to track down

 

[Professor Fate]

They are 3/8 pipe threads. Pipe threads have a taper to them. You can chase the manifold threads with a 3/8 NPT tap to clean them out, but do it CAREFULLY- you don't want to cut any new threads, just clean the carbon/rust out of the existing threads. Running the tap in too deep will enlarge the hole, and cause the fitting to run in too deep- you do NOT want it to bottom out on the shoulder, as that could actually cause it to not seal. The taper is what allows it to seal; threads on the fitting still showing when tight is normal. A few wraps of teflon tape won't hurt anything on a vacuum fitting or coolant nipple. (You don't want to use tape on oil pressure or coolant temperature senders since the bodies require a good electrical ground path to the block, through the threads).
Sorry, I've got no help to offer on the manifold studs.

 

[Badvert65]

The factory manifold studs are double ended studs, 5/16-18 x 1/2, 5/16-24 x 3/4, and 1 3/4 inch long. The 5/16-18 end threads into the head. Broken studs can be a pain to replace, so 'if it ain't broke'......
jus sayin....

 

[RustyRatRod]

The factory port in the intake is 3/8npt (national pipe thread) and is tapered. The fitting you have is a tube thread and is straight threads like a bolt. The fitting you need will have the same size hose barb as the new fitting, but will have the correct 3/8 NPT threads for the manifold. Since the threads are tapered, you do not tighten it all the way down, or you will break the manifold. They only tighten in a few threads. Make sure to seal the threads with something too.

 

[cruiser]

The factory port in the intake is 3/8npt (national pipe thread) and is tapered. The fitting you have is a tube thread and is straight threads like a bolt. The fitting you need will have the same size hose barb as the new fitting, but will have the correct 3/8 NPT threads for the manifold. Since the threads are tapered, you do not tighten it all the way down, or you will break the manifold. They only tighten in a few threads. Make sure to seal the threads with something too.

Thanks, Rusty. Just cleaned up and installed the new tap with Teflon tape. Works and looks great. Another question for you, if you don't mind. When I install my engine block drain bolt into the side of my slant six, do I put some pipe joint compound on it to seal it against coolant leaks? Thanks - cruiser