As far as I can recall, every GMC I have ever looked at had a metal sleeve in it. I ran across one a week for so ago that the sleeve actually necked down from 1 5/8 to 1". I opened a discussion about it on http://www.oldgmctrucks.com/ and a guy in Washington State has one and wanted another for multiple carb setup. I sent it to him and he claims it is different from the one he has. I am still not certain why some of them are necked down. But I was not aware that Chev did not have a metal sleeve in it.