Machining 101: Cylinder head
With the proper procedures for servicing the engine block and rotating assembly reviewed in the previous installments of this series, I now turn attention to the cylinder head.
The cylinder head is the most commonly serviced core component of an engine. This is due to its complexity of the operation of the valves, rocker arms and shaft (if used), valve springs, locks, retainers and guides along with the fact that this assembly of parts is exposed to both the liquid coolant and the majority of the combustion heat.
It is not uncommon for a cylinder head to be removed due to a head gasket failure. This is usually the result of a severe over heating event or the lack of maintenance from not changing the anti-freeze or adding the necessary supplemental ingredients to keep corrosion at bay with a diesel engine. Regardless of the reason if the head is removed, proper and thorough machine shop procedure need to be followed.
Due to the environment that the cylinder head operates in it is paramount that it is checked for cracks especially in the area between the valves before any work is performed. A good machinist will assume that if the engine is being rebuilt or the head alone is removed, then at least one time in its life it has been brought to a dangerously elevated temperature. The operator may not have been aware of this since the liquid coolant temperature may not have ever been spiked. The complete engine or one cylinder could have been exposed to excessive combustion heat from lugging, hard work or a lean air-fuel mixture. If any cracks are determined a repair employing techniques such as pinning, stitch welding, epoxy or traditional welding may be applied. But be prepared for bad news since some cracks cannot be repaired due to their location or nature. In these cases a replacement head is required.
Once the cylinder head is determined to be usable the following steps need to be applied during the reconditioning process.
Every cylinder head must have the following surfaces visually inspected and checked for warpage with a straight edge and feeler gauge. They are the head gasket surface, intake manifold mounting surface and the face that attaches to the exhaust manifold. If any of these surfaces are warped beyond allowable specifications for the gasket to seal then machining is required.
Some diesel engine makers do not recommend machining the cylinder head deck surface (the part that attaches to the engine block) and require complete replacement. In actual practice you will find that if the warpage is not too excessive a good machinist will be able to correct it with the proper procedure.
As the author, I do not know of any gasoline engine on a farm whose makers rejects decking of the head surface.
Cylinder head resurfacing can be accomplished a number of different ways that are all acceptable. Each machinist finds favor with a particular method for its benefit. That is fine as long as the end result is correct. The standard surfacing procedures include milling machine, wet grinder and a broach.