A VM cylinder head checking service is offered free of charge. This includes a visual examination for overheating damage, distortion, corrosion, internal cavitation, cracking, loose combustion chambers and valve seats.
If it appears that the cylinder heads are good enough quality to be reused then we go on to pressure test them and remove the valves to examine them and check the valve seats. If they pass the pressure test, the valves are refaced and the valve seats are recut and lapped in and the heads are then skimmed (if required), cleaned and reassembled.
We do not normally skim VM cylinder heads unless it is absolutely essential to get a flat sealing surface, as most models of the VM cylinder heads are nickel-plated. Skimming of the cylinder heads removes this hard nickel plate, which is there to resist corrosion and erosion.
There are a few things to look out for when reusing VM cylinder heads regards to cylinder head cracking. Certain models of VM cylinder heads tend to crack under the valve seats where they are not visible. The thickness of the cylinder head casting in this area is approximately 7mm thick which happens to be one of the thinnest parts within the cylinder head casting. If the cylinder heads pass the pressure test this does not mean that cracks are not present under the valve seats. They may have already travelled an unknown distance, such as the crack may have travelled 1mm or even 6mm. So this means that when you reuse a VM cylinder head which has passed the pressure test does not guarantee that the cylinder head will last a specified length of time. Pressure testing will only tell you that at that moment in time the cylinder head is not leaking which causes cooling system pressurisation. The pressure testing of VM cylinder heads is not a crack test.
Our expertise on the VM engine enables us to assess whether the engine has been over heated or not as we know exactly where to look for cracking, corrosion and internal cavitation, all of which contribute towards thinning of the cylinder head casting which can lead to coolant loss and cooling system pressurisation. The information above enables us to advise our customers about the risks involved in reusing cylinder heads which have been over heated or whether they are suitable to be reused.