Frequently asked questions
Hopefully, these will answer most of your questions, but if not, feel free to contact us!
Is it worth skimming a cylinder head when replacing a head gasket?
Yes. At Roberts Fabrications, we always recommend having a cylinder head skimmed once it has been removed from the engine, provided it is suitable for machining.
Even if a cylinder head appears perfectly flat to the naked eye, minor distortion or surface imperfections may still be present.
Replacing a cylinder head gasket is a labour-intensive and often costly job, particularly when new head bolts, gaskets and other components are required.
For the relatively modest cost of cylinder head skimming, it makes sense to ensure the mating surface is flat and correctly prepared before reassembly.
Having the cylinder head professionally inspected and skimmed where appropriate can significantly reduce the risk of having to repeat an expensive head gasket replacement due to an uneven sealing surface.
Skimming must always remain within the manufacturer’s permitted machining tolerances.
Does a cylinder head always need skimming after head gasket failure?
Although not every cylinder head will be visibly warped or damaged following head gasket failure, our recommendation is to have the head skimmed whenever it has been removed, provided the manufacturer’s specifications permit it.
Even small amounts of distortion can affect the seal between the cylinder head, head gasket and engine block.
Considering the time and expense involved in removing and refitting a cylinder head, replacing the gasket and fitting new stretch bolts where required, skimming is a relatively inexpensive precaution.
It provides reassurance that the cylinder head’s mating surface has been properly prepared before the engine is rebuilt.
However, skimming cannot rectify cracks, internal defects or problems with the engine block itself.
What size cylinder heads can you skim?
At Roberts Fabrications, our milling equipment can accommodate cylinder heads measuring up to approximately:
- Maximum length: 500mm
- Maximum width: 250mm
We can machine most aluminium and cast iron cylinder heads within these dimensions, subject to their design, condition and suitability for fly cutting.
Certain cylinder heads, particularly those containing hardened steel inserts within the mating surface, may require specialist grinding rather than conventional fly cutting.
If you’re unsure whether your cylinder head is suitable, please contact us with the engine details and approximate dimensions.
How long does cylinder head skimming take?
The time required depends primarily on the cylinder head’s material, design and the amount of preparation needed.
Aluminium cylinder heads – approximately 30–40 minutes
Most standard aluminium cylinder heads can be set up and skimmed in around 30–40 minutes, making them ideal for our convenient while-you-wait service.
Cast iron cylinder heads – approximately 90 minutes
Cast iron cylinder heads take longer to machine because the cutting process must be carried out at a considerably slower rate than aluminium.
As a guide, customers should allow approximately 90 minutes for a standard cast iron cylinder head.
Wedge-shaped cylinder heads and those requiring additional preparation may take longer due to their more complicated setting-up requirements.
All while-you-wait appointments must be arranged in advance.
What needs to be removed before bringing in my cylinder head?
To ensure your cylinder head can be safely and accurately mounted on our milling machine, it must be suitably prepared before arrival.
All camshafts must be removed before cylinder head skimming.
Please also ensure the following components have been removed where applicable:
- Camshafts and associated components that obstruct machining
- External brackets and attachments that interfere with mounting
- Coolant hose flanges and protruding fittings
- Spark plugs or valves that extend beyond the cylinder head’s mating surface
- Any other components that could obstruct the milling equipment
Valves can generally remain fitted, provided they are fully closed and do not protrude beyond the surface being machined.
Cylinder heads should also be reasonably clean and free from excessive oil, grease and loose debris.
Additional cleaning or dismantling may incur an extra charge.
If you’re unsure about preparation, please contact us before bringing your cylinder head to the workshop.
Can you skim a cylinder head with the valves and camshafts fitted?
Camshafts must always be removed before we can skim a cylinder head.
In many cases, the valves themselves can remain fitted, provided they are fully closed and sit below the cylinder head’s mating surface.
Any valves or other components protruding into the machining area must be removed or made safe before work can begin.
Please contact us if you’re unsure whether your cylinder head is ready for machining.