Frequently asked questions
Hopefully, these will answer most of your questions, but if not, feel free to contact us!
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 edge. This is probably the most accurate way to resurface a cylinder head.
Can you skim all cylinder heads?
We 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 pressed into the aluminium head. These types of heads need grinding rather than fly cutting as the cutter will just bounce over the insert leaving high spots on the surface. Sometimes it is possible to remove the inserts before skimming.
What is a ‘thou’?
A Thou is a unit of measurement 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, and 5 thou (0.005″)= 0.125mm.
What is crack testing/pressure testing, and do I need it?
Generally, as soon as you remove the head and inspect the gasket and head surface, you can see where and why the gasket has failed. In some circumstances, if it is not apparent after inspection and checking the head for flatness where the problem lies, it is probably worth having pressure or crack testing done. This basically involves sealing off all the waterways on the cylinder head and fitting an airline with a pressure gauge to one of the water inlets. The head is then pressurised to a certain pressure. If the head holds this pressure, it’s okay, if it doesn’t, then it is likely to be cracked somewhere, which usually means throwing it in the bin! Although we do not offer this service, we can connect you with companies that do.
What causes head gasket failure?
Failure can be for numerous reasons. The most likely causes are using cheap antifreeze, engine overheating, or just the gasket reaching the end of its life. Obviously, there are loads of other reasons, but these are the most common causes.
How long does it take to skim a cylinder head?
Most of the time involved is setting the head on the milling table. Start to finish on most flat/square heads is around 45 minutes. Wedge-shaped heads take much longer to set up. We offer a ‘while you wait’ service if you’d prefer to bring your head in rather than post it. Please get in touch with us to arrange a time. Most heads received by post will be turned around the same day.
What do I need to do to the head before sending/bringing it in?
Any brackets, coolant hose flanges and camshafts should be removed. Anything that protrudes the mating face of the head (valves/spark plugs) will need removing. If the valves sit lower than the face of the head, these can be left in. Clean as much oil and grease off the head as possible, or we’ll charge you for doing this!