Question: What is a "valve job" and when is
it necessary?
Answer:A valve job is removing the cylinder head(s) from
the engine so the valves, guides and seats can be refurbished
to restore compression and oil control. A valve job may be necessary
by the time an engine has 80,000 or more miles on it, or to fix
a "burned valve," compression or oil burning problem.
Before we describe all the steps that a typical valve job
involves, we should warn you that some shops don't necessary do
all the steps. In other words, you get what you pay for. A "cheapie"
valve job might skip a lot of things that saves you a few dollars
in the short run, but may end up costing you a lot more in the
long run. So look for a shop or service facility that does quality
work.
A valve job typically begins by disassembling, cleaning and
inspecting the cylinder head. Cast iron heads are "Magnafluxed"
to check for hairline cracks. This involves applying a strong
magnetic field to the head and sprinkling iron powder on it.
Cracks disrupt the magnetic field and attract the iron powder,
making invisible cracks easy to see.
Cracks are bad news because they can leak coolant into the
combustion chamber damaging the cylinders and/or causing the engine
to lose coolant and overheat. If cracks are found in any critical
areas of the head, the head must either be repaired or replaced.
Cracks in cast iron heads are most often repaired by "pinning"
(installing a series of overlapping threaded pins). Cracks in
aluminum heads are very common and can often be repaired by welding.
If a head has been repaired (pinned or welded), most shops
will usually pressure test the head afterward to make sure there
are no leaks. Some may also apply a sealer compound to the inside
of the water jackets as added insurance against future leaks.
Once the head passes this point, it is also checked for flatness.
The surface of the head must be flat to seal the head gasket
against the block. Excessive warpage, roughness or any damage
can cause the head gasket to fail. If the head exceeds the maximum
allowable out-of-flatness specs, it must be resurfaced or replaced.
Usually there's enough metal in the head to allow for a certain
amount of resurfacing. But on many import aluminum cylinder heads,
the amount of resurfacing that's possible is minimal.
Overhead cam aluminum cylinder heads are often found to be
warped (usually the result of overheating). If the condition
cannot be corrected by resurfacing, the head can often be straightened
by heating it in a special oven and then bending it until it is
straight.
Next come the valves, guides and seats. The guides are checked
for wear. They're almost always worn, so they either need to
be replaced, relined or knurled (a process whereby grooves are
cut into the inside diameter of the guides to decrease the bore
size). Few shops knurl guides anymore. Most install new guides,
guide liners or bore out the old guides to accept new valves with
oversized stems. Aluminum heads have cast iron or bronze guides
that can be replaced but most cast iron heads do not.
If the valves are to be reused, they will be inspected, checked
for straightness then refaced. Many shops automatically replace
all the exhaust valves to reduce the risk of failure (exhaust
valves run much hotter than intakes and are much more likely to
fail).
The seats in the head are either cut or ground to restore
the sealing surface. If a seat is cracked or too badly worn to
be refaced, the seat must be replaced. If that isn't possible
(as is the case on many late model cast iron heads because the
casting is too thin), then the entire head must be replaced. All
aluminum heads have hardened steel seats that can be replaced.
The valve springs are all inspected and tested to make sure
they are still capable of maintaining proper pressure. The spring
retainers, keepers and other hardware is likewise inspected.
Any worn or damaged components are replaced. New valve guide
seals are always used.
The valves are then installed in the head and shimmed to restore
proper valve height. This is necessary because machining the
valves and seat alters their dimensions. Valve height is important
because it affects valvetrain geometry and guide wear. If it
is an overhead cam engine, the cam is also installed and the valve
lash adjusted prior to returning the head to the customer.
