this is an autometer gauge I have and it has this fitting in the back, how do I convert it or how do I use it to read my manifold pressure? My vacuum lines are think rubber tubing.
that is a npt or pipe thread fitting. napa or most other auto parts stores should have a fitting that will convert that to a hose barb. you may need two fittings to get to the correct size. a coupler and a adapter.
You could put a fitting on the gauge with a copper line. Cut off the copper line leaving about two inches. Then slip the rubber hose on it.
NOTE: the rubber hose you select has to be oil line rated to take the pressure and temeratures of the oil. Do NOT use fuel or vacuum hose - it will burst! The brand name is the same used for transmission line and other low pressure applications - "Push-Lock" and is made by aeroquip and others rated at 250 psi and 350F. You can use AN, SAE or double flare (inverted flare) with the correct adapters to the pipe thread. I prefer rigid line and use inverted flare steel lines (brake and fuel lines) in my gauge installations. It makes them a bit harder to "service" but they don't leak and there is less dampening effect than with hose or capilary tubes.
I think you are correct but I have to agree with Paul on not using the vacuum line just for accuracy. I would find a hose house and see if they can do a stainless over Teflon hose for you. It can take the heat of the engine compartment, it's way over-kill for the pressure and won't give you false readings (not that the other would be that far off).
