[Safari Larry]

I diddled with the 56 heads yesterday and got one of the tubes out without bending it up too badly. Can't get the the other out, it is stuck.

I tried a 57 tube in the 56 head, it would go part way in but a rib near the center of head stopped it from going in further. It was same inserting it from either end. It might be rust buildup in the center two ribs that prevented it from going all the way in, not sure about that. Those inner ribs are difficult go get to.

Some photos attached of tube in 56 head and 56/57 tubes together for comparison (57 is cleaner).

Bill, I think your GMC parts book is incorrect.