This is the second part of a 4-part series on engine cycle theory and classification. Part 2 covers the valvetrain architectures that transfer motion from the camshaft to the valves, including I-head (OHV), overhead cam (OHC), and dual overhead cam (DOHC) designs. Understanding how these mechanical linkages convert rotation into linear motion explains why some engines can rev higher while others are built for durability at lower speeds.
Reciprocating Motion Transfer in I-Head OHV Designs
The I-Head, or Overhead Valve (OHV) design, centralizes combustion by placing valves directly above the piston crown. This requires a mechanical linkage to transfer camshaft rotation in the engine block to linear valve actuation in the cylinder head. Rotation of the camshaft lobe displaces a lifter, which is also called a tappet, and that lifter moves a pushrod vertically. The pushrod then acts upon one end of a rocker arm. The rocker arm pivots on a shaft or a ball stud, which reverses the direction of force to depress the valve stem against the spring tension. Because this design involves multiple reciprocating parts, including the lifter, the pushrod, and the rocker arm, the mass of these components limits the maximum engine speed, measured in RPM, before valve float occurs.
Rocker Arm Mounting and Valve Arrangement Constraints
For rocker arm mounting, there are two common configurations. Rocker arms mounted on a common shaft provide high lateral stability and consistent geometry across multiple cylinders. Rocker arms individually mounted on ball-shaped pivots allow for self-alignment but require precise torque on the adjustment nut to maintain the proper lash. In the I-head design, the valves may be arranged in a straight line or in a staggered pattern. Staggering the valves allows for larger valve diameters within the fixed diameter of the cylinder bore, which increases volumetric efficiency. The pushrod and rocker arm relationship typically involves a mechanical advantage called the rocker ratio. A small displacement at the pushrod results in a larger lift at the valve, which optimizes gas flow without requiring excessively large camshaft lobes.
Comparative Geometry of L, T, F, and I-Head Valvetrains
For the L-head configuration, the valve location is in the block on one side of the cylinder, the camshaft location is in the block, and the actuation method is direct tappet to valve. For the T-head configuration, the valve location is in the block on both sides of the cylinder, the camshaft location is in the block with dual cams, and the actuation method is direct tappet to valve. For the F-head configuration, the valve location is one valve in the head, usually the intake, and one valve in the block, which is the exhaust, the camshaft location is in the block, and the actuation method is a hybrid of pushrod and direct. For the I-head configuration, the valve location is in the cylinder head, the camshaft location is in the block, and the actuation method is pushrod and rocker arm.
Pushrod Seating and Pivot Lubrication Requirements
During assembly, the pushrods must be seated perfectly in the lifter cup and in the rocker arm socket. A failure to align these components before torquing the rocker stands can result in bent pushrods or damaged lifters upon the initial crankshaft rotation. The rocker arm pivot, whether it is a shaft or a ball, is a high-friction interface. The assembly logic dictates the use of high-pressure lubricants or ensuring proper oil gallery alignment to prevent galling during the critical dry start phase.
Inertial Reduction Through Camshaft Relocation
The Overhead Camshaft (OHC) design improves volumetric efficiency by relocating the camshaft from the engine block to the cylinder head. This eliminates the need for pushrods and lifters, significantly reducing reciprocating mass and mechanical deflection. By removing the pushrods, the valvetrain possesses lower inertia, which allows for higher stable RPM and more precise valve timing at high frequencies. In many OHC designs, the camshaft lobe acts directly upon a rocker arm or a follower, which then depresses the valve stem. Zero valve stem to rocker clearance is maintained via hydraulic lash adjusters. These components utilize engine oil pressure to take up mechanical slack, which ensures quiet operation and compensates for component wear and thermal expansion.
Camshaft to Crankshaft Timing Ratio and Cam Lobe Profiles
In OHC and I-head engines, the camshaft is driven by the crankshaft via a timing chain or a timing belt. The relationship is fixed at a two to one ratio, meaning there are two crankshaft revolutions for every one camshaft revolution. The profile of the cam lobe, specifically the base circle versus the lobe lift, determines the valve duration and the valve lift. In high-efficiency designs, rollers are used at the lifter to camshaft interface to convert sliding friction into rolling friction, which reduces parasitic power loss and wear on the cam lobes.
SOHC, DOHC, and Transverse Valve Alignment
In a Single Overhead Cam or SOHC design, one camshaft per cylinder head operates both the intake and the exhaust valves. In a Double Overhead Cam or DOHC design, there are two camshafts per cylinder head. One camshaft is dedicated to the intake valves and the other camshaft is dedicated to the exhaust valves. This allows for optimized port angles and the placement of the spark plug in the direct center of the combustion chamber. The alignment of valves in an I-head engine, whether staggered or inline, is dictated by the intake and exhaust manifold positioning. A transverse view reveals how the rocker arm bridges the gap between the cam-actuated pushrod and the valve stem.
Timing Mark Alignment, Hydraulic Priming, and Torque Sequences
Precise alignment of the timing marks on the crankshaft gears and the camshaft gears is a functional requirement. A misalignment by even a single gear tooth can result in improper valve to piston clearance, which can potentially cause catastrophic mechanical interference. During assembly, hydraulic adjusters must be properly lubricated and primed. Any air trapped in the adjuster will cause excessive valve clatter and a reduced valve lift until the system reaches operating oil pressure. The cylinder head bolts and the rocker shaft pedestals require a specific torque sequence, which is usually from the center outward, to prevent warping the head or binding the camshaft journals.
Valve Lash as a Thermal Expansion Requirement
Internal combustion engines require a specific mechanical gap, known as valve lash or valve clearance, to account for the thermal expansion of metal components during operation. As the engine reaches operating temperature, the valve stems and the pushrods lengthen. Without a predetermined cold clearance, the valves would fail to seat fully, which leads to compression loss and burned valves. Many modern overhead valve (OHV) and overhead cam (OHC) engines utilize hydraulic lifters or lash adjusters. These components use pressurized oil to automatically maintain zero lash, which eliminates the need for manual adjustment and reduces mechanical noise.
Mechanical, Hydraulic Plunger, and Shim-and-Bucket Adjustment Methods
For mechanical adjustment using a screw and lock nut, which is found on rocker arms, an adjusting screw is rotated to set the precise gap between the valve stem and the rocker arm. A lock nut is then torqued to maintain this setting. On engines with hydraulic lifters but adjustable rockers, the initial setting is often established by placing the lifter plunger at its halfway point of travel. This ensures the hydraulic circuit can compensate for both wear and thermal expansion. In some DOHC designs using direct actuation, known as shim and bucket, the camshaft lobes act directly on followers at the ends of the valves with no rocker arms. The clearance is adjusted by replacing hardened steel shims of varying thicknesses.
Dual Overhead Camshaft (DOHC) Architecture
The DOHC configuration utilizes two separate camshafts per cylinder head: one camshaft for the intake valves and one camshaft for the exhaust valves. The followers transfer motion from the cam lobe to the valve stem in high RPM applications where rocker arms may be too heavy or prone to deflection. The drive system typically uses a timing belt or a timing chain driven by the crankshaft pulley. The DOHC layout often integrates the fuel injectors, the spark plugs which are centrally located for flame front efficiency, and the distributors or coil-on-plug systems directly into the head architecture.
Oil Pressure Effects, Interference Engine Risks, and Mass Reduction Benefits
The hydraulic lash adjusters are functionally linked to the engine oiling system. Low oil pressure or aeration, which is foaming of the oil, will result in tappet noise and a reduced valve lift. The crankshaft, the connecting rods, and the pistons must maintain exact synchronization with the DOHC valvetrain. In interference engine designs, a failure of the timing drive will result in the piston striking the open valves. By using dual camshafts and direct followers, the valvetrain eliminates the pushrods and the rocker arms, which lowers the total mass of the moving parts. This allows the engine to achieve a higher power density without mechanical failure.
Technical Identifiers for DOHC 24-Valve V6 or I6 Engines
The block houses the crankshaft and the pistons. The head houses the dual camshafts, the valves, and the followers. For induction and exhaust, the intake manifold and the fuel injectors are positioned to optimize the airflow into the intake ports. For lubrication, the oil pan, the oil pump, and the pickup screen ensure a continuous supply of filtered oil to the camshaft journals and to the hydraulic adjusters.
The key takeaway is that valvetrain designs range from the simple but heavy pushrod I-head to the lighter, higher-RPM overhead cam configurations, with each design having specific assembly and adjustment procedures. Proceed to Part 3.
Local Shop Note:
You know, this takes me back to a conversation I had with a mechanic over on E Orvis St in Massena, N.Y. We were at a SUNY Canton Automotive Diagnostics Center seminar, and he was telling me about a sedan that came in with a complaint that it had a persistent valvetrain noise and a slight miss at idle. The customer had already replaced the lifters and the pushrods. Still had the noise and miss.
He checked the oil pressure — it was good. Checked the lifter preload — within spec. Then he started looking at the rocker arm geometry. He noticed the pushrods were making contact with the cylinder head at the top of the travel. The previous rebuild had used pushrods that were too long, which was changing the rocker arm pivot angle and putting side load on the valve stems. That side load was causing the valve guides to wear, and the noise was the rocker arm hitting the head at full lift.
He replaced the pushrods with the correct length and re-adjusted the rocker arm geometry. The noise disappeared, and the engine ran smooth.
The lesson for you guys is: the valvetrain is a mechanical linkage. Every part has to be in its place — length, angle, and clearance all matter. If you change one part without checking the geometry, you’ll create problems that sound like something else. Always verify pushrod length and rocker arm geometry when you’ve got a noise complaint. Sometimes the parts are the right type, but the wrong size.