Core 0 = 77c
Core 1 = 87c
Core 2 = 85c
Core 3 = 82c

Now I'm thinking I'll need to reapply the cooler, which I will do but I wonder if the IHS isn't helping? Seems a massive difference 10c, I was running LinX

Same thing happens with my 3570K. 2550K before it too. My guess, and I think this is correct, is that core 0 is in the middle of the dye, with the other cores tightly packed underneath and the igpu above (or dead space in the 2550K's case). The igpu doesn't create any heat so core 0 has a large surface area to dissipate the heat whereas the other cores are tightly packed allowing lower heat transfer.

Or I am completely wrong and it's a programming error in software or something else.

Regardless I have remounted multiple different coolers and always had the same result so just lived with it. Still happens on my delidded 3570K.

Same thing happens here with my delidded 3770k. Core 0 is a good 12c less than the others. Seems fairly common.

Don't worry about it. My 4670k has been delidded, had CLU applied to the core, IHS lapped so it's level and stuck back on and it's still 8-10 degrees cooler on one core. I thought it may have been poor thermal paste application on Intels part which is one of the reasons I delidded it but it's the same results after delidding as it was before (apart from the 12-15 degree drop across all cores). it's annoying but we just have to live with it.

Heh, I was looking at mine a couple of days ago and seeing exactly the same thing. Core 0 is invariably cooler, even under water.

But with the age of my 2500K I really can't be bothered trying to do anything about it. If it throttles, I'll act, otherwise it's so far beyond it's expected lifetime I don't actually mind too much if it blows up

*edit* Could it be down to CPU layout? Does something else (cache?) cluster around the other cores, but 0 gets something less-used?

the temps are normal. all cores are in a row and core 0 is next to the gpu so its cooler, 1 and 2 are in the middle and 3 at the end

Glad it's not just me, imagine how much better it would be if we didn't get any integrated graphics!