Part 1. Preparation and safety
Objective
- Machine cylinder heads to operational condition.
Personal safety
Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:
- Work clothing - such as coveralls and steel-capped footwear
- Eye protection - such as safety glasses and face masks
- Ear protection - such as earmuffs and earplugs
- Hand protection - such as rubber gloves and barrier cream
- Respiratory equipment - such as face masks and valved respirators
If you are not certain what is appropriate or required, ask your supervisor.
Safety check
- Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.
Points to note
- In the machining process its important to remove the least amount of material possible.
Part 2: Step-by-step instruction
- Bore and reline valve guides
First, the old valve guides are machined so that their bore is fractionally larger, and new bronze liners are pressed into the head.
The excess of the new liners are sheared off the top and bottom and then a steel ball which is the exact diameter of a finished guide is pressed all the way through and out the other side. This leaves a perfect size hole for the valves to slide into.
- Level the head
The head is placed in a leveling fixture and set up on the decking machine.
It is important to remove the least amount of metal possible to make the surface flat again. The more metal shaved off the heads, the more the compression ratio raises in the combustion chamber and the closer the valves will be to the tops of the piston when they open.
- Cut new valve seats
On the valve seat cutting machine the head is leveled so that the valve guides are positioned straight up and down. Next, an example valve that has a perfect angle cut in it, will be set up with a pointer. The position of the pointer is then used to set the position of the cutting tool.
After the cutting tool is set up and double checked, the valve seat will be cut.
This particular tool cuts all three angles of the valve seat at the same time. Two of the angles are to smooth the flow of air in or out of the chamber and the middle angle is the one where the valve face will contact.
The setup is the same for the intake and exhaust seats except the exhaust seat areas are bored out so that hardened steel inserts can be used to handle today's lower fuel octane ratings.
This cut with all the smoke is the inner diameter of the hardened seat being sized out. Then the actual exhaust seat is cut, all three angles at once. The size of the middle angle where an exhaust valve sits on the seat is much wider than the intake however, because the exhaust valves get much hotter and need more contact.
- Grind the valves
Now that the heads are perfect and the valve guides are back to specification, the valve stems are checked with a micrometer to make sure they are reusable.
If they are, the valve tip is ground off a little to flatten it and then the angle of the valve face is freshened up. Valves come out of the organizer tray one by one and the angle and the position of the grinding wheel is worked until all of the darker dirty spots on the old surface of the valve face are returned to a nice clean and smooth ground finish.
If a valve is put in the grinder and the grinding wheel is unable to contact the entire circumference of the valve face, the valve stem is bent and that valve is thrown away.
- Measure installed spring height
Once the valves and seats are done, a valve is inserted in the head and the correct retainer is installed in place with a set of keepers. An installed spring height gauge or a ruler is used to find the distance between the bottom of the retainer and where the spring sits in the head. This measurement is the installed spring height.
- Measure valve spring pressure
Once the installed height is known for the intake and exhaust, the springs are put in a spring checker and compressed to the size of the installed height. The integrated scale tells the machinist the pressure that the spring is exerting at the installed height. If the pressure is not enough to keep the valve seated properly, sometimes a shim can be installed under the spring to give it more tension. If the shim still isn't enough, the springs will have to be replaced by a new, stronger set that will close the valve properly with the correctly rated tension.
