Why skim a cylinder head?
When a cylinder head is removed from the engine block due to gasket failure, it is essential that you check the head and block for flatness. Generally you'll find that the block is fine due to the size of it and usually being made of steel, but the head may show signs of warping or rot. The way most manufacturers recommend checking the flatness is with a long steel rule or straight edge. Check across the length, across the width of the head in various places, and also diagonally from corner to corner.
Even this can be a little hit and miss, which is why a light skim is probably the best way to ensure true flatness!
If a cylinder head is only a few thou (if you don't know what a thou is, check the faq's page as you may be surprised how small it is!) out of flat, it can cause premature gasket failure, either on first start up or a few thousand miles down the road. Do it once, do it properly or you'll be back to square!
It is also a good idea to check the waterways on the face of the head for any signs of rot. It'll look like grey rusting, and if it gets too bad it can fill your cylinders with coolant. Not ideal! We also offer a welding service, so don't panic too much if you're head is showing signs of rot as in most cases it's a fairly simple repair!