The Stovebolt Site Forums The Shop Area Engine Shop 49 intake manifold fitting thread

GM used several different sizes. There all pipe threads, so your best bet is to remove what was installed and size it up or alternately, you can measure the hole. Measure as good as you can the outer dimension of the threads as the difference in pipe sizes is big enough to measure the difference with a ruler.

Here is a chart that you can compare sizes. Remember you are measuring the OD of the threads.