AdamR
Big Member
By Dan Dvork
There are two important aspects to proper valve train geometry, and both are adjustable by the user, if you know how and why.
This exercise is many times more crucial in engines that employ a "shaft" type rocker arm system where ALL the valves MUST have the same valve stem height (the part of the valve that sticks out of the head) as opposed to individual stud mounted rocker arm system which can vary greatly, and still be able to correct the geometry.
The first consideration is called the rocker arm SCRUB ANGLE. Simply defined as the relationship of the tip of the rocker arm to the tip of the valve at all positions during the valve lift cycle.
Improper scrub angle WILL result in short valve guide life, loss of lift and horsepower, and eventually poor valve seal. Severe cases will result in tip wear and even parts breakage.
When everything is new from the factory, these important geometric considerations are science out and are engineered perfectly. Once its engineered, it is easy to duplicate over and over again, a must for trouble free, warranty driven long-term operation.
The problems occur during the RE-ENGINEERING PROCESS. The rebuilding and repair of existing engine parts, the substitute of other than OE parts, and the never-ending quest to "make things better"
Some of the things that will CHANGE this scrub angle relationship from that which was originally engineered, are:
Simple valve job where seats are ground (sunk) and valves are ground (thinner) forces the valve INTO the head changing the place on the rocker arm that the valve contacts.
Longer than standard valves, or longer than that which is necessary to make up for "sunk".
Addition of lash caps, which make the tip of the valve think it is wider and protects the valve tip from the pounding of racing springs and camshafts.
Addition of hardened valve seats used in repair or strengthening. Most places just install them without much regard to how it affects the height of the valve.
Installation of O/S valves that must be but are not usually "cut in" to OE depths. It's much easier to fudge the installation and leave the valve sitting the combustion chamber roof instead of grinding and contouring for maximum flow. *(Coming Soon)
Variation in valves. Valves are sold and measured in total length instead if the more proper seat to tip measurement that is the significant specification. Two different valves could have the identical total length, let's say 4.900". One value has a MARGIN (thickness) of .120 and the other has a margin of .070. The tip of the valve will appear .050 different to the rocker arm when it comes time to measure an adjust the scrub angle. .050" is becoming significant. .100" is critical and .150" becomes disaster.
And disaster is simple to create, because of all the combinations and permutations of the above possibilities. For example, a valve seat can be installed to repair "pulled through" damage and left high, then an oversized 1.81 valve could be installed to seat, but not cut in and further it can be a "wide" margin valve. At the same time, the intake valve could be "sunk" even just slightly, and you could have a difference of A QUARTER OF AN INCH! I have seen more than that!
Even if there is no real obvious problems, just the natural movement of the valve train dimensions during the re-machining process, makes the observation of the scrub angle mandatory in any high performance engine.
OBSERVATION AND CORRECTION
NUMBER ONE PRIORITY is to ensure and correct as necessary, the relative valve tip heights (fig 1). The closer you can get them to each other, the better the effectiveness of your efforts.
This can be accomplished by doing the valve job, install the valves, locate the one that sticks out the farthest, and then go about the task of matching the "short" ones to the "long" one. This can be accomplished by regrinding the seats, grinding a wider 45 degree angle on the valve, effectively reducing the margin of those valves, and in some severe cases, adding a lash cap to the tip of the valves to make them think they are "longer". You can also dress the tips down of the longest valves if that helps equalize them. CAUTION, LIMIT WHAT YOU REMOVE OFF THE TIP OF ANY VALVE to about .020".
ONCE YOU HAVE EQUALIZED THE TIPS, you are now ready to deal with the geometry and scrub angle.
It is best to install the lightweight "checking" springs for this procedure. Actually, you need only start out with a pair of valves, one intake and one exhaust, a shaft, two shaft hold-downs and of course two rocker arms. For the purposes of this discussion, we will concern us with standard ADJUSTABLE IRON ROCKERS, such as Isky, Crane or other MAX-WEDGE type rockers. These are the most common street type rockers and offer tremendous gains over the OE stamped steel models.
It should be pointed out at this point that roller- tipped rockers have the same geometric requirements. The procedure to correct the scrub angle is similar, but small amounts of imperfect geometry can be tolerated much better than their cast iron counterparts. A roller tip does not actually "scrub" as we have been defining it, but rather the tip ROLLS across the tip of the valve instead of "pushing and/or pulling" as FIXED tip rocker arms do.
We do not like roller rocker arms for street use. Especially aluminum bodied ones or needle shaft mount rockers. If you feel strongly about using ROLER ROCKER ARMS ON THE STREET, I would recommend the Comp Cams Steel bodied roller tip. Use the same procedure to identify and correct geometry problems.
WHAT YOU ARE LOOKING FOR
For the purposes of identification we will break the tip of the valve into 3 parts. The center, the shaft side of center and the exhaust manifold side of center.
The old theory of having the center of the valve contacting the center of the rocker pad (roller) at one half of valve lift isn't close enough, at least with a stationary tip rocker arm. What you are looking for is the action of the rocker arm across the valve tip that changes direction as the valve opens. The action will appear as rocker tip moving across the valve tip and then back.
The center of the valve tip should start contact (at zero lash) slightly to the shaft side of center. As the valve opens, the center of the rocker arm should move toward the center of the valve (or just past it) and then at some point, reverse direction and come back to where it started when the valve is fully open. This action spreads the load against the guide and averages it out.
Improper geometry will look like the rocker tip is scrubbing toward the exhaust manifold side of the valve tip for the entire lift cycle OR PULLING back on the tip of the valve toward the shaft side during the entire cycle.
IT IS IMPERATIVE THAT THE TIP OF THE ROCKER CHANGES DIRECTIONS AS IT "SCRUBS" ACROSS THE VALVE TIP.
A roller tip rocker is okay if the roller center is on the center of the valve tip at half lift in most cases. Raise (with shims) or lower (by machining the pedestals or the shaft stands until you can accomplish that position. But if you observe the movement across the valve tip it should do the same as described for the stationary tip rocker. And that is start contact to the shaft side of the valve while the valve is closed, move across the tip just to center or slightly past, and then return to the original position during full lift. Then back to center at one half closed and back again to the original position when the valve is closed.
It sounds complicated but it is not. Put a checking spring on the valve, adjust the lash (or pre load) and rotate through a few cycles and you will be able to identify what is happening to the scrub angle. It will become clear to you and what to do about it will come easily.
Take care of this important part of the engine build. Your valve train will love you for it!
There are two important aspects to proper valve train geometry, and both are adjustable by the user, if you know how and why.
This exercise is many times more crucial in engines that employ a "shaft" type rocker arm system where ALL the valves MUST have the same valve stem height (the part of the valve that sticks out of the head) as opposed to individual stud mounted rocker arm system which can vary greatly, and still be able to correct the geometry.
The first consideration is called the rocker arm SCRUB ANGLE. Simply defined as the relationship of the tip of the rocker arm to the tip of the valve at all positions during the valve lift cycle.
Improper scrub angle WILL result in short valve guide life, loss of lift and horsepower, and eventually poor valve seal. Severe cases will result in tip wear and even parts breakage.
When everything is new from the factory, these important geometric considerations are science out and are engineered perfectly. Once its engineered, it is easy to duplicate over and over again, a must for trouble free, warranty driven long-term operation.
The problems occur during the RE-ENGINEERING PROCESS. The rebuilding and repair of existing engine parts, the substitute of other than OE parts, and the never-ending quest to "make things better"
Some of the things that will CHANGE this scrub angle relationship from that which was originally engineered, are:
Simple valve job where seats are ground (sunk) and valves are ground (thinner) forces the valve INTO the head changing the place on the rocker arm that the valve contacts.
Longer than standard valves, or longer than that which is necessary to make up for "sunk".
Addition of lash caps, which make the tip of the valve think it is wider and protects the valve tip from the pounding of racing springs and camshafts.
Addition of hardened valve seats used in repair or strengthening. Most places just install them without much regard to how it affects the height of the valve.
Installation of O/S valves that must be but are not usually "cut in" to OE depths. It's much easier to fudge the installation and leave the valve sitting the combustion chamber roof instead of grinding and contouring for maximum flow. *(Coming Soon)
Variation in valves. Valves are sold and measured in total length instead if the more proper seat to tip measurement that is the significant specification. Two different valves could have the identical total length, let's say 4.900". One value has a MARGIN (thickness) of .120 and the other has a margin of .070. The tip of the valve will appear .050 different to the rocker arm when it comes time to measure an adjust the scrub angle. .050" is becoming significant. .100" is critical and .150" becomes disaster.
And disaster is simple to create, because of all the combinations and permutations of the above possibilities. For example, a valve seat can be installed to repair "pulled through" damage and left high, then an oversized 1.81 valve could be installed to seat, but not cut in and further it can be a "wide" margin valve. At the same time, the intake valve could be "sunk" even just slightly, and you could have a difference of A QUARTER OF AN INCH! I have seen more than that!
Even if there is no real obvious problems, just the natural movement of the valve train dimensions during the re-machining process, makes the observation of the scrub angle mandatory in any high performance engine.
OBSERVATION AND CORRECTION
NUMBER ONE PRIORITY is to ensure and correct as necessary, the relative valve tip heights (fig 1). The closer you can get them to each other, the better the effectiveness of your efforts.
This can be accomplished by doing the valve job, install the valves, locate the one that sticks out the farthest, and then go about the task of matching the "short" ones to the "long" one. This can be accomplished by regrinding the seats, grinding a wider 45 degree angle on the valve, effectively reducing the margin of those valves, and in some severe cases, adding a lash cap to the tip of the valves to make them think they are "longer". You can also dress the tips down of the longest valves if that helps equalize them. CAUTION, LIMIT WHAT YOU REMOVE OFF THE TIP OF ANY VALVE to about .020".
ONCE YOU HAVE EQUALIZED THE TIPS, you are now ready to deal with the geometry and scrub angle.
It is best to install the lightweight "checking" springs for this procedure. Actually, you need only start out with a pair of valves, one intake and one exhaust, a shaft, two shaft hold-downs and of course two rocker arms. For the purposes of this discussion, we will concern us with standard ADJUSTABLE IRON ROCKERS, such as Isky, Crane or other MAX-WEDGE type rockers. These are the most common street type rockers and offer tremendous gains over the OE stamped steel models.
It should be pointed out at this point that roller- tipped rockers have the same geometric requirements. The procedure to correct the scrub angle is similar, but small amounts of imperfect geometry can be tolerated much better than their cast iron counterparts. A roller tip does not actually "scrub" as we have been defining it, but rather the tip ROLLS across the tip of the valve instead of "pushing and/or pulling" as FIXED tip rocker arms do.
We do not like roller rocker arms for street use. Especially aluminum bodied ones or needle shaft mount rockers. If you feel strongly about using ROLER ROCKER ARMS ON THE STREET, I would recommend the Comp Cams Steel bodied roller tip. Use the same procedure to identify and correct geometry problems.
WHAT YOU ARE LOOKING FOR
For the purposes of identification we will break the tip of the valve into 3 parts. The center, the shaft side of center and the exhaust manifold side of center.
The old theory of having the center of the valve contacting the center of the rocker pad (roller) at one half of valve lift isn't close enough, at least with a stationary tip rocker arm. What you are looking for is the action of the rocker arm across the valve tip that changes direction as the valve opens. The action will appear as rocker tip moving across the valve tip and then back.
The center of the valve tip should start contact (at zero lash) slightly to the shaft side of center. As the valve opens, the center of the rocker arm should move toward the center of the valve (or just past it) and then at some point, reverse direction and come back to where it started when the valve is fully open. This action spreads the load against the guide and averages it out.
Improper geometry will look like the rocker tip is scrubbing toward the exhaust manifold side of the valve tip for the entire lift cycle OR PULLING back on the tip of the valve toward the shaft side during the entire cycle.
IT IS IMPERATIVE THAT THE TIP OF THE ROCKER CHANGES DIRECTIONS AS IT "SCRUBS" ACROSS THE VALVE TIP.
A roller tip rocker is okay if the roller center is on the center of the valve tip at half lift in most cases. Raise (with shims) or lower (by machining the pedestals or the shaft stands until you can accomplish that position. But if you observe the movement across the valve tip it should do the same as described for the stationary tip rocker. And that is start contact to the shaft side of the valve while the valve is closed, move across the tip just to center or slightly past, and then return to the original position during full lift. Then back to center at one half closed and back again to the original position when the valve is closed.
It sounds complicated but it is not. Put a checking spring on the valve, adjust the lash (or pre load) and rotate through a few cycles and you will be able to identify what is happening to the scrub angle. It will become clear to you and what to do about it will come easily.
Take care of this important part of the engine build. Your valve train will love you for it!
