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THE JOB OF THE VALVETRAIN IS TO TRANSFER THE CAM LOBE’S MOVEMENT TO THE VALVE ITSELF .
Keep the valve spring and retainer attached to the valve stem.
Must work with the keepers being used (7-degree or 10-degree) and the valve springs (single spring, dual spring, or triple spring).
Have little to do with the basic valvetrain function but have a lot to do with the engine’s oil usage on the street .
The valve spring is the key to the proper function of the valvetrain.
Fits into the pivot in the top of each tappet. The pivot in the mechanical tappet is part of the way down the inside of the tappet body, while the hydraulic tappet’s pivot is very close to the top.
Converts the upward, vertical motion from the cam lobe, tappet and pushrod into downward
The valve size and shape affect the port’s airflow. vertical motion of the valve.
One size does NOT fit all, nor does one design solve all valvetrain problems. Likewise, each engine tends to have its own issues and challenges. The valvetrain starts with the pushrods, leading to the rocker arms, the rocker shaft (A & B & HEMI® engines) or pivot (Magnum) and then to the valves, which are generally discussed as part of the cylinder head, then on to the keepers or locks, retainer, valve seal and the valve springs.
While the camshaft itself is actually part of the short block, the cam motion is the action that the valvetrain is supposed to transfer. The typical B (383/400) or RB (440) cams such as the B/RB mechanical cam P4120661AC has .557″ lift. For B/RB hydraulic cams, refer to the 284 degree duration version (P4120235AC ground on 108 centers for racing) and the similar profile (284-degree – .484″ lift) ground on 114 centers (P5007697) for street use, plus the bigger 292-degree duration P4120237AC ground on 108 centers for racing and the similar one (292-degree – .509″ lift) ground on 114-degree centers P5007698 (street). All four of the hydraulic cams use the standard B/RB hydraulic tappet, while the mechanical cam uses the mechanical tappet.
Most of the A and B wedge engines use a rocker ratio of 1.5. The Magnum V8 and the Jeep® Brand 4.0 6 use 1.6 ratio. The stamped, hydraulic A-engine rockers (set of 16 P4529742) tend to have a left (P4529474) and right (P4529475) aspect caused by the pushrod socket being offset slightly toward one side or the other of the rocker. The Magnum rockers do not have any offset. The Magnum engine has a complete set of aluminum roller rockers with a 1.7 ratio (P5007404). Why swap ratios – a .480″ valve lift cam on the Magnum (1.6 ratio) becomes a .510″ valve lift cam with the 1.7 ratio rockers. There are 1.5 and 1.6 aluminum roller rocker kits available from Crane, Comp Cams and Harlen Sharp (www.HarlenSharp.com).
» Rocker Shafts & Pivots – Typically the change of movement direction depends upon a center pivot (which is called the rocker shaft for most of the engines). The A-engine shaft is (P4510636) and the B-engine shaft is (P4529101). The hydraulic rockers are very forgiving about the shaft but mechanical rockers are not. The aftermarket aluminum roller rocker systems usually come with their own shafts. The Magnum engine uses a small pivot (one per rocker) and bridge (one per cylinder) which acts as a mini-shaft.
» Shaft Holddowns – As the valve spring loads go up because of bigger, higher performance cams, the loads holding the rocker shaft in place go up proportionally. With the greatly increased loads, the shaft tends to become oval, not round. Mechanical rockers of any style do not like oval shafts, so to help keep the shaft round, use hold-downs like (P5249714) (B-engine).
» Rocker Stands – In some cases like the W2, the early Max Wedge, and the 426 Gen II HEMI engine, rocker shaft stands are used to hold the rocker shafts rather than the cast-in pedestals. The 426 Gen II HEMI engine stands are aluminum (P5249505), while the A-engine W2 stands are (P5249049).
The basic difference tends to make mechanical pushrods somewhat longer than hydraulic pushrods. The mechanical cam requires an adjustable rocker arm while the hydraulic cam does not. Because of this difference, hydraulic rockers are generally stamped steel rockers with no adjustment. The actual adjuster in the mechanical rocker arm is a threaded screw with a lock-nut that tightens against the top surface of the rocker arm itself. The exception to this neat package is the 426 Street HEMI engine (1966 thru 1971) and the 1970 340 Trans-Am engine which both used adjustable rockers with hydraulic cams. In 1992 the Magnum engines were introduced. They used hollow pushrods with a hole in each end and oiled the valvetrain through the pushrods rather than up through the block & head like the A-engine, B/RB-engine and the Gen II HEMI engine.
All the HEMI engine cylinder heads use different length intake and exhaust pushrods. An example is the new 5.7L/6.1L HEMI engines (P5153629) (set).
VALVES , VALVE SEALS AND VALVE SPRINGS
The valves are actually part of the cylinder head because the valve’s seat must be matched to the valve seat, ground into the head. The keys for the valvetrain are overall length (on installed height), stem diameter (on keepers) and number of grooves in the stem (on keepers).
Valve seals are very important. The typical production valve seal fits to the valve guide so the stem diameter becomes very important as well. 3/8” – (P4120492) works on both the A and B engines. Some seals fit over the valve guide like the special HP seal designed for use with hi-lift cams and dual valve springs – 3/8” valve stem, and .530” guide – (P3690933).
If the valve spring does not do its job correctly, the valves will float (not follow the cam). Very bad things happen if the valves float. The key spec for the spring is the valve lift of the cam – The .484” lift B-engine cam mentioned above would use (P3690933) spring. The mechanical cam had .557” lift and would require a dual spring. The B-engine springs install at 1.86” while the A-engine springs install at 1.68“. Both can be converted to 2.00” installed height by using longer valves. The A-engine springs do not fit the Magnum heads because the recess machined into the head casting requires springs with an OD of about 1.42” where the A-engine springs are in 1.48/1.50” area. Use special Magnum spring (P5249464) and its matching retainer (P4432032) for most valve lifts in the .400” to .525” area and (P4876062) for lifts up to .600“.
RETAINERS AND KEEPERS
The most common retainer is the 3/8“, 7-degree, chrome-moly retainer which works for most A and B engines. Retainers are made of steel, chrome-moly, aluminum and titanium.
The stem diameter (5/16” or 3/8” are common) and the number of grooves in the valve stem (one, two or three are common although single grooves are used in almost all performance applications) are the key specifications for keepers (like P4120618 single groove, 3/8 stem). The other aspect of the keeper-retainer relationship is the angle of the keeper (7-degree is the most common and 10-degree keepers are used in hi-load, racing applications). In very high load situations, the 7-degree keepers can pull-through the retainers and the 10-degree design keeps failure from occurring.