I am running a TR4A intake manifold on my TR3. There was an outlet port on the top that I had plugged. Can I use this port to attach a vacuum gauge? Does anyone know the size it is offhand and what size vacuum gauge threads typically are? I just read a recent article on the forum by Bob Haskel on how he used the gauge on his Austin Healy to dial in the ignition timing and I would like to try it on the 3. Does the vacuum advance to the distributor need to be plugged when doing this? Karl