Hi again, mate. Please Register or Log in to view the hidden image! Sorry if I am not making myself clearer. If the surface 'wrinkles', the surface may stay the same 'area' but its ridges effectively make the 'fitting space' smaller diameter without the actual inner surface area changing much. Then I also said that the inner diameter may not change because the inner surface layers may be 'compressed' and made a denser layer than the material behind it. That's all I meant mate. Cheers! Disclaimer Again: I do not know about the 'heating fit' side of things. I've only ever used (once) the 'cooling' method for the stud/bolt to go into such a hole. Please Register or Log in to view the hidden image! .