lapping questions

Hey,

so im trying to lap an i7-920 and have a couple of questions.

Ive started of with 400grit and did like 50 ups and downs, then turned 90degrees and repeated, and again and again until i had done a full turn, then repeated all of this 3 times. I then moved on to 600grit and then 800grit. Now after all of this there is still some aluminium showing near the sides. Most of it has gone but my question is "at what point should ALL of the aluminium have gone and there is only copper showing?". I just dont think that the finer grits are gonna remove much so should i carry on with the coarser grits until I can only see copper?

My 2nd question is- if I lap the cpu do I HAVE to lap the heatsink as well? It's a thermalright ultra 120 extreme. I read somewhere that some heatsinks are created with a slight dome to fit the cpu, so if you were to only lap the cpu then it would no longer fit the dome of the heatsink and actually result in worse temps? is this true?

thanks for any advice.

Question 1: I'd continue until there is nothing but copper on the top. Al on the edges means the CPU was convex (bulging in the middle) and you're not done getting it flat.

Question 2: No, it doesn't mean you have to lap the heatsink. With a TRUE, it might be a good idea though. They're designed with a bulging base on purpose. If you lap both, you'll insure the best contact possible.

What are you using to lap the CPU? Are you using a lapping stone, or sandpaper on glass? At a certain point you stop 'removing material' and end up polishing. If you want a mirror surface on it, you can work your way up to 4000 grit (my cutting tools are polished up to 12000+). I suspect I don't have to mention that you need to use a perfectly flat surface to lap the CPU otherwise you'll end up with a imperfect result.

Some blocks have a slight bowing that is designed to apply more pressure in specific areas to improve heat transfer. Unless your block has a concave base I'd run with it unlapped.

You should lap the CPU some more; I doubt that there is a CPU with such a large bulge that will harm the CPU itself. I would lap the heatsink anyway because if you are lapping you CPU why not go all the way and get more out of your work?

im using sandpaper and a piece of glass to lap. Wish I had known that the heatsink was domed, well ill have to buy some more sandpaper for that then cause the sandpaper i have been using on the cpu is knackered now :/