Jerry, I understand that...what you said is logical, sensible and I thought the same thing looking at the gears. In fact the first time this happened to me (around 1976) this would have been the last thing I would have thought possible. But the man who owned Cylinder Head Sales Co said he had seen this happen several times and the way to fix it was to take away the excessive clearance. Which he did in a few minutes when I brought the cam to him.
I know I've seen this happen twice, it happens at low rpm and the people who designed the engines and wrote the shop manuals must have put those clearance numbers in there for a reason. In fact in the 48 to 51 shop manual it is mentioned 4 times, and in the 1957 manual those numbers are mentioned 3 times. I also know somebody started adding the spacer to the thrust plate to make it easier to install the gear and hit the proper clearance. And in my case once the thrust plate was brought back to specs the idle problem was resolved. Over the years I've seen this mentioned on the VCCA, ChevyTalk and other forums, so I have to think maybe a few others have had the same issue and came to the same conclusion. I will say the two times I witnessed this the cam gear was the fiber style. Whether that made a difference I don't know.
