Sometime in '66/'67 Isuzu changed the diameter of the outlet on the manifold.
I found this out back in '75 when I took my first PR20 to the exhaust shop.
All I wanted to get done was a new inlet on the pipe. The fella measures up the main
pipe. Cuts a tube to the size. Welds it to the old pipe. Too small. Checks the outlet on the
manifold. Next size up tube. Start again. This time he makes a new seat for the gasket to sit in.
I've taken some measurements on the cars that I have here and the distance between the
manifold studs as there is also a difference in the retaining plate to hold the pipe into place.
Pipe. 45mm (1 3/4"). Plate. 73mm. (2 7/8")..........Pipe. 41mm. (1 5/8"). Plate. 66mm. (2 5/8").
These sizes are pretty close as I just went downstairs about a half hour ago with torch in mouth
and verniers in hand.
Dave.