Mazda Training manual - part 242

 
4 – VALVE TRAIN 

 

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OVERHEAD CAM VALVE TRAIN

 

 
The overhead cam (OHC) design provides more direct control over valves 
than the OHV design. This advantage may explain why overhead cams are 
the most commonly used design. The overhead cam design includes 
engines with a single overhead cam (OHC or SOHC), as well as engines 
with dual overhead cams (DOHC). 
 
 
 
Overhead Cam (OHC or SOHC)

 

 
In an overhead cam engine, the camshaft is installed in the cylinder head 
above the valves. The camshaft, shown in Figure 36, is a solid or hollow 
cast iron shaft with several cams on it. Each cam has an off-center bulge 
on one side called a cam lobe. On most Mazda OHC engines, the 
camshaft runs directly on the cylinder head journal surface without insert 
bearings. 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

FIGURE 36. The 
camshaft rides 
directly on the 
cylinder head 
journal surface.

 

 

Cam lobes 

Camshaft 
journal 

Cams

 

 

 

 

 
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FIGURE 37. As 
the camshaft 
turns, the lobe 
contacts the 
rocker arms, 
opening and 
closing the 
valves.

 

 

Rocker arms

 

Camshaft

 

Valves

 

Cam lobes

 

 

 

 

 

 

 

 

 

 

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

To make sure that the valves open and close at the right time in the 
four-stroke cycle, the camshaft is driven by the crankshaft. The camshaft 
turns at one half the speed of the crankshaft because it takes two 
revolutions of the crankshaft to complete one cycle. So the camshaft 
completes a full cycle in one revolution. 

 

As the camshaft turns, a lobe on one of the cams contacts a rocker arm 
which presses on the valve stem end and opens the valve, as shown in 
Figure 37. As the lobe turns past the rocker arm, it releases the valve, 
allowing the spring to close it. The single overhead camshaft includes 
two or more cam lobes for each cylinder — one lobe for each valve — so 
that the intake and exhaust valves can open and close at different times. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
On V-6 and V-8 engines with an SOHC, two separate camshafts are used, 
one for each bank of cylinders. Even though two camshafts are used, this 
is still an SOHC design because one camshaft operates all the valves for 
the cylinder bank. 

 

 
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Dual Overhead Cam (DOHC)

 

 

The dual overhead cam (DOHC) design is also very common in 
light-weight, multi-valve engines that operate at high speeds. Dual 
camshafts reduce the weight of the valve train parts, which means the 
engine can generate more power without an increase in displacement and 
weight. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

V-6 and V-8 engines with dual camshafts actually have four separate 
camshafts, two for each bank of cylinders. 
 
In addition, the OHC design also allows four valves per cylinder — two 
intake valves and two exhaust valves, as shown in Figure 38. This 
arrangement improves the intake and exhaust efficiency of the engine. 
 
 
 
CAMSHAFT DRIVES

 

 
This section covers four types of camshaft drives: 
 
• OHV 

Drive 

 
• OHC 

Drive 

 
• 

Belt and Chain Drive 

 
• 

Gear-driven Camshaft with Friction Gear 

 

 

 

 

 

 

 

 

 

 
 

FIGURE 38. Dual 
camshafts 
reduce the 
weight of the 
valve train and 
improve intake 
and exhaust 
efficiency.

 

 

Camshafts

 

Exhaust 
valves

 

Intake valves

 

 

 
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FIGURE 39. OHV 
drives typically 
use a timing 
chain to drive the

 

camshaft.

 

 

Camshaft

 

Camshaft 

gear 

Timing chain

 

Crankshaft 

gear

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

OHV Drive

 

 

In an OHV engine, where the camshaft is mounted below the valves in the 
block, a crankshaft gear is used to drive a timing chain, which drives the 
camshaft gear. Figure 39 shows a typical drive for an OHV engine.