do you mean on the underside to measure the core or on the fins? if it's to measure the core then tape it to the plastic or ceramic bit with the tip touchin the core (but not on top unless you want to burn the chip out) if you want to measure teh temp of the heatsink by putting it on one of the fins then you can also just tape it too it
Go carefully here. The height of the core is more often than not, less than the thickness of the probe. If the HS base clamps down and hits the thermal probe first, it will, in all likeliness, not seat upon the core with sufficient force (if at all) to remove the heat effectively. Simple test using that method is to mount the probe, install the HS and tug on the probe wire. If it is stuck tight between the CPU substrate and the HSF base, chances are the base is not on the core correctly. About the only safe and accurate approach is to drill a hole through the base, in the area above where it contacts the core. That's what I wound up doing. Drilling a hole in from the side of the HSF, parallel to the baceplate face and mounting the probe inside is not quite as accurate as having the tip actually resting upon the core surface.