Machining 101: The cylinder headJune 26, 2018
With the proper procedures for servicing the engine block and rotating assembly reviewed in the previous installments of this series, I will 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 — the operation of the valves, rocker arms and shaft (if used), valve springs, locks, retainers and guides, along with the fact that it 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 the cylinder head is removed, proper and thorough machine shop procedure needs 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 equipment 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. Be prepared since some cracks cannot be repaired due to their location or nature. In these cases another cylinder 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, along with the face that attaches to the exhaust manifold. If any of these surfaces are warped beyond the allowable specification for the gasket to seal, then machining will be required.
Some diesel engine manufacturers 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 that would be found on a farm that its manufacturer rejects decking of the head surface.
Cylinder head resurfacing can be accomplished a number of different ways that are all acceptable. You will find that 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 are, milling machine, wet grinder, and a broach.
Valve guide service
The valve guide is an area that is often neglected in an inexpensive cylinder head service. The valve guides are the hole in the cylinder head that the valve stem travels through.
There are two different types of valve guides: integral and replaceable. An integral guide is usually found on a cast iron cylinder head. It is a raised boss that has a passage gun drilled through it and is part of the cylinder head casting. In contrast, a replaceable guide may also feature a cast-in boss but has an insert that is usually made from bronze that controls the movement of the valve stem.
The valve guides are important to not only guide the valve in its proper path but for controlling oil from getting into the cylinder bore. Each engine has a specification for the valve-stem-to-guide clearance. If the guide and or the valve stem have excessive clearance, it is mandatory that this be brought back to the specification before the cylinder head is placed on the engine. In contrast, if the guide is too tight the valve may stick when the engine is extremely hot (especially an exhaust valve under load) and cause a collision with the piston and the possibility of major damage.
Due to the low rpm nature of agricultural engines many shops employ a procedure called knurling to recover some of the additional clearance caused by excessive wear. A special arbor is used to produce a spiral groove with raised plateaus that decreases the inner diameter of the guide. Though an acceptable procedure it is not ideal for an engine that you hope to get many more years of trouble-free service from your investment. The cost is low for knurling and it is effective at bringing the clearance to specification for a while.
The preferred method on an engine with integral guides is to bore out the worn guide so that it can accept a bronze liner insert. This is a far superior method for a heavy-duty engine and in relation to the entire cylinder head service, the cost is minimal.
Another approach would be to hone the guide to the next available valve stem dimension and if the cylinder head is going to be fit with new valves, just buy the ones with a larger diameter. If the old valves are being reused then it is not a cost effective approach.
If the cylinder head was made with replaceable guides then they just need to be pressed out and new ones installed to bring the clearance back to specification.
Regardless of the procedure employed, new valve stem seals if used must be installed.
Valves and seats
The intake and exhaust valves in a farm engine endure a great amount of abuse. Ponder these basic facts: Within 15 minutes of working in the field the valves may have opened and closed more than 10,000 times all while being exposed to the stress of spring tension and combustion heat that can reach 3,000 degrees F locally, for short excursions.
Due to the environment valves and seats (the area the valve head closes against) distort, wear, and recess from the constant pounding. In addition, it is common from the valve to pit from white-hot carbon deposits and attacked chemically from the byproducts of combustion. Thus, the valve and seats are very important service areas.
The wear areas of a valve that need to be confirmed are the stem diameter, tip, and an area called the margin. The margin is the region below the last angle and before the face. The margin is used to keep the valve both physically and thermally stable. If the margin is worn then the valve should not be put back into service. It will be very susceptible to warping and burning shortly there after.
If the valve margin and the other regions are in specification then it can be resurfaced to create the angles. The angles placed on both the valve and the seat is used not only for cylinder sealing but to direct and promote airflow into and out of the cylinder. Poor valve angles though they may seal will cost the engine power and fuel efficiency.
The valve seats can be integral (part of the iron casting) or an insert (pressed-in). If an integral seat is extremely worn, burned or cracked, then it is counter bored larger and an insert is installed.
The standard method to install the angles and number of them on the seat is via a grinding stone. This is a very well-respected procedure but if the shop you use has the equipment to cut in lieu of grind the seat angles, it more desirable. Cutting the seat and installing multiple-angles to gingerly direct the incoming charge and out going exhaust will greatly improve the performance of the engine. The cylinders will be filled more with combustible charge and it will exit easier. In engine speak it is described as increased volumetric efficiency. Racers and tractor pullers know what I am referencing.
When machining the cylinder head it is essential that the installed height of the valve spring is set properly along with the pressure. Cutting the seats and valves alters the geometry and this needs to be accounted for and corrected.
It must be recognized that parts of the cylinder head such as the rocker arms, valve, springs, locks and retainers are all considered consumables along with the rocker mounting studs, if used. A proper cylinder head service replaces these with high-quality and not inferior imported parts. Reusing these components would be akin to putting worn laces on a new pair of work boots. Sadly, many shops ignore this and use most if not all of the old parts. In addition, head bolts should not be reused since they will be in a stressed state from all the years of thermal cycling and being clamped.
A cylinder head that is treated to proper procedure will provide years of trouble-free service but if any corners are cut, it will become an enemy that strikes at the worst time.