In the Corvair world where I spent many happy years, you were always admonished to use an anti-sieze compound when installing steel into aluminum. I'm sure this holds true for the D's also. Any auto parts store should have it, get a small tube and lightly coat anything you screw into the block or heads, especially sensors, thermisters, studs, and the like...you may want to get it loose sometime without a torch and hours of frantic misery. If you're in a hurry and can't wait, at least give the threads a shot of WD-40. Les