Mazda Training manual - part 244

4 – VALVE TRAIN

Piston Engine Fundamentals

TC010-05-01S

Gear-Driven Camshaft with Friction Gear

Mazda also uses another type of cam drive that features a gear-driven
camshaft with a friction gear. In this design, shown in Figure 44, the timing
belt drives one camshaft on each head. The other camshaft is driven by
helical gears. The helical gears turn the driven-side camshaft
counter-clockwise. This design creates a more compact valve train, which
allows a lower hood line on the vehicle.

The arrangement of the cam lobes on the two cams causes a clicking
noise as the cams turn. To eliminate this noise, the driven gear is
equipped with a friction gear.

The friction gear has one more tooth than the helical gear. The friction gear
causes a slight bind between the drive and the driven cam gears,
eliminating any clicking noise resulting from backlash/freeplay between the
gears.

FIGURE 44 The
drive camshaft is
driven by the
timing belt. The
driven camshaft is
driven by helical
and friction gears.

Driven
camshaft

Drive
camshaft

Helical gears

Timing belt

Friction gear

4 – VALVE TRAIN

Piston Engine Fundamentals

TC010-05-01S

FIGURE 45.
Camshafts
typical- ly have a
thrust flange to
control
front-to-back
movement, and a
groove to drive
the distributor.

Intake
camshaft

Distributor
drive groove

Exhaust
camshaft

Journals

Cast
Hexagons

Thrust
flanges

CAMSHAFT DESIGN

Figure 45 shows a typical pair of camshafts used in a DOHC engine,
with one shaft for the intake side and one for the exhaust side.

Camshafts have thrust flanges either on the front or rear. In the illustration,
the flanges are on the front. These flanges match up with thrust surfaces in
the cylinder head to control front-to-back movement of the camshaft. These
flanges work much like thrust bearings do on the crankshaft. Some
camshafts have a separate thrust plate that performs the same function as
the flange.

Mazda OHC engines do not have insert bearings. Instead, the camshaft
rides directly on the polished journal of the head, which serves as the
“bearing” surface.

4 – VALVE TRAIN

Piston Engine Fundamentals

TC010-05-01S

These camshafts also have hexagons cast right into the shafts. The hexagons are used to
hold the camshaft and keep it from turning when the camshaft pulley is removed or
replaced. The exhaust camshaft includes a groove that drives the distributor. The distributor
shaft rotates, allowing the distributor to direct electrical current to the spark plugs as they
fire in turn.

Auxiliary Drives

Some camshafts also have extra lobes, gears, or grooves that drive the fuel pump or oil
pump.

Lubrication

Oil passages in the cylinder head distribute oil through holes in the bearing surface to
lubricate the journals. The cam lobes are lubricated by oil dripping back into the oil pan or
by tubes delivering oil directly to the cams.

Camshaft Measurements

Critical measurements include:

•

Clearance between the camshaft journals and bearing surface

• Journal

roundness

•

Camshaft end play

If any of these measurements are outside factory specifications, the engine may run poorly.

4 – VALVE TRAIN

Piston Engine Fundamentals

TC010-05-01S

REVIEW EXERCISE 5

Fill in the words that correctly complete these sentences. Check your answers with the
answer key on page 66.

1. The surface of the valve head that contacts the valve seat is called the

____________________

2. The _____________ holds the valve stem so the valve fits squarely on its seat.

3. Exhaust valves are more likely to be damaged by _______________ than intake

valves.

4. The length of an installed valve spring between the retainer and the spring pad is

called the __________________ .

Match the numbered illustrations to the valve train descriptions below.

5. _______

A. Overhead valve train (OHV)

6. _______

B. Single overhead camshaft (SOHC)

7. _______

C. Dual overhead camshafts (DOHC)

8. In an indirect camshaft drive, the camshaft pulley is driven by a _______________ or

______________

connected to a pulley on the crankshaft.

9. Which of these symptoms would you expect to see in an engine that has worn valves?