Smooth Fit And Heat

I just thought I would post this as although I knew what the problem was I hadnt thought about it till after the event.
I was turning down a stainless shaft to a smooth no play fit in a bore.
Mic it up, spot on. Then it cooled to the same temp as the bore.
Shrunk enough that I had to make another one.
This time I let it cool before finishing to the correct size.