of carbon on the contact surface of the seata condition
that keeps the valve from closing properly. To further
reduce carbon build up, there is an interference angle
(usually 1 degree) between the valve and seat. In some
cases, a small portion of the valve seat has an additional
15-degree angle ground into it to narrow the contact
area of the valve face and seat. When you reduce the
contact area, the pressure between the mating parts is
increased, thereby forming a better seal.
The valve seats may be an integral part of the
cylinder head or an insert pressed into the cylinder head.
Valve seat inserts are commonly used in aluminum
cylinder heads. Steel inserts are needed to withstand the
extreme heat. When a valve seat insert is badly worn
from grinding or pitting, it must be replaced.
The valve guides are the parts that support the
valves in the cylinder head. They are machined to fit a
few thousandths of an inch clearance with a valve stem,
This close clearance is important for the following
It keeps lubricating oil from getting sucked into
the combustion chamber past the intake valve
stem during the intake stroke.
It keeps exhaust gases from getting into the
crankcase area past the exhaust valve stem
during the exhaust stroke.
It keeps the valve face in perfect alignment with
the valve seat.
Valve guides may be cast integrally with the head, or
they may be removable (fig. 3-52). Removable guides
are press-fit into the cylinder head.
Figure 3-52.Valve guides.
Valve Springs, Retainers, Seals, and Valve
The valve assembly is completed by the spring, the
retainer, the seal, and the valve rotator (fig. 3-53). The
spring, which keeps the valve in a normally closed
position, is basically the same for all engines; however,
the number and types of coils can vary. Most valves
have only one spring, but, in some cases, there may be
twoan inner spring and an outer spring. The second
spring increases the pressure holding the valve closed.
Low-spring tension can cause valve float (spring too
weak to close the valve at high rpm).
A valve retainer and keepers lock the valve spring
on the valve. The retainer is a specially shaped washer
that fits over the top of the valve spring. The keepers, or
locks, fit into the valve stem grooves, holding the
retainer and spring in place.
The seal keeps the valve operating mechanism oil
from running down the valve stem and into the
combustion chamber. Valve seals come in two basic
typesumbrella and O ring. Both are common on
modern engines. The umbrella valve seal is shaped like
a cup and can be made of neoprene plastic or rubber. An
umbrella valve seal slides down over the valve stem
before the spring and retainer. It covers the small
Figure 3-53.Valve spring, retainer, and seal.