This is how the intake trumpets fit into the plenum. The two are held together by eight 6 mm nuts and the seal is maintained through the use of a big (expensive @ $18) O-ring. If you ever have your plenum apart be careful not to pinch this O-ring during assembly. It is easy for the O-ring to slip out of place while you are tightening the 8 nuts. If this happens you will have a small vacuum leak that will be hard to find down the road.

It is not uncommon for the forward vacuum fitting on the intake plenum to come loose over time. Heat and vibration start to take their toll and the fitting gradually works its way out of the plenum (it is pressed in). If the fitting pops out when you are WOT at the track, the sudden increase in un-metered air can cause a lean condition which may damage your engine. The best bet if your fitting is loose is to remove and clean it. Then reinstall it with Loctite Sleeve Retainer.

Whether your fitting is loose or not, it is good insurance to drill and tap the plenum so that a small set screw can be used to keep the vacuum fitting from working its way out in the future. I used a small 3 mm Allen screw with red Loctite for this purpose. Since drilling and tapping the plenum will allow aluminum shards to fall into the plenum it is not a bad idea to take it apart so that you can thoroughly clean the inside. You will probably find a lot of oil in there too 8^)