What is cylinder head skimming?

Cylinder head skimming is a way of refacing the mating surface of the head. The method we use is sometimes called ‘fly cutting’ which involves mounting the cylinder head on a milling machine which has a fixed head and a moving table. The milling head has a bespoke attachment with a cutter which spins at around 600 rpm, and the table of the mill moves the cylinder head across the cutter. This is probably the most accurate way to resurface a head. Check out the video below to see a head being skimmed. The depth of cut being taken in the video is 2 thou.

Can you skim all cylinder heads?

Steel insertWe can skim most aluminium and cast iron cylinder heads with a maximum width of 200mm, and length of 600mm. The main exception to this is some diesel cylinder heads have a hardened steel insert (see pic) pressed into the aluminium head. These type of head need grinding rather than flycutting as the cutter will just bounce over the insert leaving high spots on the surface. Sometimes it is possible to remove the inserts prior to skimming.

What is a ‘thou’?

A Thou is a unit of measurement which is 1000th of an inch (0.001″). As a rough guide to those of us that moved along with the times and now work in metric 40 thou (0.040″) is roughly equivalent to 1mm. Therefore 20 thou (0.020″)= 0.5mm, 10 thou (0.010″)= 0.25mm, 5 thou (0.005″)= 0.125mm.