Someone had asked about the plenum and IH I have for sale. They have a 90 and so the issue of oil port restriction came up. Searched the forum and came up w this thread. For grins, and since different people were using different sized Dorman plugs, I decided to measure the 2 sets of IHs I have.
One is a NOS w a manuf date of Sept.89, the other was on a motor and they were made 1 year later. Here's a picture comparing the two ports.
The dimensions are 1 5/32" 29.32mm
1 1/32" 26.25mm
The larger ports are the earlier NOS IH.
