Firing Order, Small Block & SVO
•• EXCEPT IN MASS AIR VEHICLES ••
This is one of the most common questions asked by our Ford customers. The firing order for the early 221-302 engines and the early 5.0 engines is 1-5-4-2-6-3-7-8. This is the firing order for all prefix “31” cams and is the standard replacement cam for all early engines. The later 5.0 engine and all 351 engines are designed to use the 1-3-7-2-6-5-4-8 firing order. This is the firing order for all prefix “35” cams, and cams ordered for these engines should use this prefix. Other than the firing order, the cams are identical. By changing the spark plug wiring at the distributor these cams can be interchanged.
By far, the most common problem encountered when installing a new high performance camshaft is the incompatibility of the existing valve springs to the new cam. All of the factory valve springs are designed to work with a certain lift cam, and since most aftermarket cams have higher lift, the spring must be addressed. It is highly recommended and a requirement of the warranty that the suggested springs be installed along with any COMP Cams® camshaft.
Most Ford cylinder heads utilized a step cast into the head that acted as the valve spring locator. When installing a dual spring, it is highly recommended that this step be removed by machining to minimize the possibility of coil binding the inner spring.
Whenever installing a high-tech racing cam in any engine, the cylinder heads must be equipped with the correct valve springs, screw-in studs, guide plates and hardened pushrods. The increased loads and ultra high speeds of the racing engines make this a necessity for valve train stability.
Whenever you are using a high performance camshaft and have problems with the valves not staying properly adjusted, one of the first things to check is the rocker arm studs. Most early model small block heads utilized pressed-in studs. When high spring loads and high engine speeds are used with these stock type studs, they tend to pull out of the heads. You can check for this by laying a straight edge across the top of the studs to see if any of the studs are too high and out of alignment. If so, the heads should be removed and machined for screw-in studs.
Positive Stop Stud
This type stud was used on 1969-76 302 and 351W engines, as well as 1968-72 429 engines with hydraulic cams. They do not allow for lifter adjustment and work only with smaller cams when the dimensions of the engine (block, head deck height, etc.) remain close to stock. They also don’t work on solid lifter cams.
COMP Cams® offers an adjustment kit (Part #4610-16 on page 276) for use with the stock positive stop studs. For high performance applications, this type of stud should be replaced with the more conventional screw-in type, along with the pushrod guide plates.
The conventional stud is usually found on early model 221-302 engines and all engines originally equipped with a solid lifter camshaft. This type of stud uses a locking nut or polylock to keep the valve adjustment fromchanging.
Self Aligning “Rail” Rocker Arms
Originally the small block engine used a machined slot in the head to guide the rocker arm on the valve. It has been common to enlarge this hole and install a guide plate when switching to a high performance valve train. Later model engines utilize a small alignment slot or “ears” on the valve tip end of the rocker where it contacts the valve. These rockers must be used with long stem valves. These applications can be easily identified by a large (1/2”) hole where the pushrod passes through the head and the fact that there is no pushrod guide plate. If the head in question has either a guide plate or a slot to guide the pushrod, the rail type rocker arms cannot be used.
Conventional Rocker Arms
This type rocker arm was used on 289 hp and 1963-1966 289 engines. The cylinder head had a slot cast in the head where the pushrod passed through. This slot guided the pushrod and aligned the rocker arm with the tip of the valve. Some heads have been modified to use a pushrod guide plate instead of this slot. Since there are no rails on the end of the rocker arm, a shorter tip is used on the valve. This type of rocker arm can be used only in conjunction with either a slot in the head or a guide plate but not both.
Fulcrum Style Rocker Arms
Fulcrum type rocker arms are used on most 351C and 351-400M engines originally equipped with hydraulic cams as well as 429-460 engines made without guide plates. These rocker arms used a fulcrum or “sled” in conjunction with a bolt to secure the rocker arm to the head. Pre- 1977 models used a slotted pedestal cast into the head to keep the rocker arm aligned with the tip of the valve, while later 5.0 351W engines used a stamped steel guideplate under the rocker arm fulcrum to align the rocker arm. To replace rocker arms of this type with the adjustable Magnum or any roller rocker arm, screw-in studs and guide plates will be necessary. These engines can be easily converted by using Part #4504-16 studs which feature a 5/16” thread on the lower portion of the stud. This will screw directly into the holes in the head, and since these engines use a long tip valve, the rail type Magnum (Part #1431-16) adjustable rocker arm can be installed.
”FE” Shaft Rocker Arms
The 332-428 “FE” engines use a shaft rocker arm design. The standard nonadjustable rocker arms will work well with the smaller hydraulic cams, but when installing any solid lifter cam or any hydraulic cam larger than a 292H, the rocker arms must be replaced with adjustable rockers. They can be found on page 274.
Timing Chain Set
When installing a cam in any small block engine, be sure to check items such as the upper and lower cam gears, cam gear spacer, fuel pump eccentric, cam retention plate and front cover clearance. Ford has changed the arrangement of these items through the years and interference and misfits can occur. A Part #3220 timing chain set should be used on pre-1972 221-351W engines. On 1972 and later engines, a Part #3230 chain set should be used. Hydraulic roller equipped 5.0 engines (1985 and later) must use a Part #2138 timing set to ensure proper upper gear fit to the nose of the camshaft.
Camshaft Dowel Pin/Fuel Pump Eccentric
Two different length dowel pins were used in the front of the cams in 221-351W engines. In 1972 and earlier engines, a longer (1.375”) dowel pin was used so that it would extend through the one piece fuel pump eccentric used on these engines. The 1973 and later engines utilized a two piece fuel pump eccentric which required a shorter (1.125”) dowel pin. If no eccentric is used, a thicker than standard retaining washer must be used to make up for the thickness of the eccentric. The cam gear MUST be pulled tightly against the snout of the cam. If the gear is not tight against the step at the front of the cam the bolt will come loose and engine failure is sure to occur.
Dowel pin failure is fairly common in Small Block Ford engines. This is almost never the result of a defective or soft dowel pin. It is most often caused by the bolt in the center of the cam coming loose and allowing the dowel pin to be loaded and shear. The center bolt should always be torqued to the manufacturer’s specification and a suitable thread lock used to prevent the bolt from coming loose.
Valve Stem Oil Seals
When changing to a higher than stock lift camshaft, it is common to have a clearance problem between the bottom of the spring retainer and the top of the valve stem oil seal. Before final assembly of the heads, install one seal, one valve and one retainer without the spring. Measure the distance between the top of the seal and the bottom of the retainer to be sure that it is greater than the lift of the valve by at least .050”-.060”. Be sure to take into account any extra lift due to higher ratio rocker arms.
Flat Tappet Break-In
All higher lift hydraulic and solid flat tappet cams will require special attention during the break-in process. Special springs and certainly tender loving care will be required to ensure long life of the cam. Please refer to the instructions in your cam box for complete procedures. If ever in doubt, please call the COMP Cams® CAM HELP® line at 1-800-999-0853.
High Ratio Rocker Arms
A higher than standard ratio rocker arm moves the pushrod closer to the rocker arm stud. It then becomes necessary to check the clearance between the pushrod and the head where the pushrod passes through the head. This is a very common problem and should be checked whenever a rocker arm ratio change or pushrod diameter change is made.
Rocker Arm Geometry
Proper rocker arm geometry is necessary to ensure the maximum benefit from any cam design. Camshaft base circle, block deck height, cylinder head design and lifter design all contribute to possible errors in geometry which must be compensated for with pushrod length. Usually a longer than stock pushrod will be necessary in a high performance engine, but care must be taken to choose the correct length. A comprehensive explanation of the checking procedure can be found on pages 264-265.
Multi Groove Valves
No longer is it necessary to convert to “Chevrolet” style single groove valves to benefit from the superior strength of COMP Cams® Super Locks™ and the variety of spring retainers available with this lock. Super Locks™ are now available for the multi groove valves used in most 351C and 351M-400M applications. They are available in pairs or in sets and can be found on page 311.
Hydraulic Roller Cams
In those engines originally equipped with hydraulic roller camshafts, conventional flat tappet hydraulic and solid lifter cams can be used. It will be necessary when making this change to use the corresponding cam, lifters, pushrods, rocker arms, valve springs and timing chain set.
Retro-Fit Hydraulic Roller Cams
COMP Cams® has developed a special kit to allow the installation of hydraulic roller cams in standard Ford V8 engines (289-351W, 351C, 351- 400M) not originally equipped with hydraulic roller cams. This kit uses many of the same parts as the factory roller cam equipped 5.0 engines use, including the lifter guides and retention tray. This kit can be used only with specially designed COMP Cams® Retro-Fit Hydraulic Roller Camshafts with special base circle size.
To ensure that you have the correct base circle size: install the cam, lifters and all lifter retention hardware. Slowly rotate the camshaft, looking closely at the top of the lifter where it contacts the guide bar. As the lifters move up and down, the lifter guides should remain flat on the top of the lifter bores. The lifters must not push the guides up as the lifters rise, and the lifters must not drop below the guide bar as they go all the way down. If either of these conditions exist, the base circle of the camshaft is incorrect and must be changed prior to complete installation.
COMP Cams® has developed new Pro Magnum™ hydraulic roller lifters that will eliminate the need for the different base circles. This lifter, Part #8931-16, is a captured link bar style lifter that is a simple drop-in design for most Small Block Ford applications. We also offer a Big Block Ford version, Part #8934-16.
Camshaft Retention Bolt
Most V8 Ford engines used a 3/8” bolt to secure the upper cam gear to the cam. Almost all racing engines use a 7/16” bolt for this application. Be sure to check the compatibility of the bolt to the cam, as a 3/8” bolt in a 7/16” cam will almost certainly result in catastrophic engine failure. Most of the COMP Cams® racing roller cams will come with the 7/16” hole in the cam.
Camshaft Journal Diameter
Many of the newer all out racing engines utilize a larger than standard cam bearing journal diameter. The advantages of the larger diameter are less flex and a larger base circle to smooth out the lobe design, making this a very desirable addition to any extreme racing engine. The most common sizes other than stock are: 2.051” (babbit bearing, all 5 common size journals), 2.081” (roller bearing, all 5 common size journals) and 2.165”/1.968” (roller bearing, commonly referred to as the “Large Roller Bearing”).
Any of these sizes should be available, but none are interchangeable. Make sure to specify journal size when ordering your cam. If no special size is requested, the standard journal size will be chosen.