Valve Train Geometry
We've talked to countless hobbyists and read all the magazine articles and for some reason this subject always seems to come up. It also seems quite often that the magazines fail to describe the procedure completely or properly. When talking to hobbyists, it becomes clear they often don't truly understand what it means or its importance. When we dig a bit deeper we find they have either never checked it or have used the wrong procedure.
This article is to address that problem.
Read this and follow the steps below and you'll be sure to have a properly set up valve train.
This article is to address that problem.
Read this and follow the steps below and you'll be sure to have a properly set up valve train.
What is Valve Train Geometry?
Valve train geometry is simply the sweep of the rocker arm
on the valve stem as the valve opens and closes.
The method for setting this is to install the correct length
pushrods.
That's sounds pretty simple and it is but it's amazing how
many "Hot Rodders" get this wrong.
Time and again we've read on any of the countless online forums "What length pushrods for my 'xyz' headed motor with an 'abc' cam?" There are plenty of folks giving answers but the only correct answer is YOU HAVE TO MEASURE!!!!
There are no two engines alike unless they are coming off an assembly line, and even then, they simply average their tolerances and shoot for the middle…this is one reason why some factory motors are rockets, and some lemons. So here is a list of some of what goes into determining proper pushrod length.
Deck Height - The distance from the center of the main journal to the deck surface (where the head bolts to).
When an engine block is machined it is often decked to square/flatten out the surface the head bolts to with the crank shaft centerline, and sometimes to adjust the compression height (typically referred to as zero decking). This will bring the head surface closer to the camshaft and decrease the pushrod length. This is often a small change (.010 or less) but coupled with other factors it adds up.
Head Deck Milling Thickness - Loosely described this is the amount of material that can be safely removed from the head during milling operations.
Every head has a tolerance and they vary greatly. It is very common to mill a head to flatten the sealing surface and/or to increase the compression ratio by reducing the combustion chamber size. This brings the valve closer to the engine and further reduces PR length. Again these changes are small but they add up. On the flip side of this, replacing a factory head with an aftermarket one may actually lengthen the PR required as the deck thickness is often greater than stock castings.
Valve Length - This is, predictably…a simple term used to describe the length of the valve.
Not all cylinder heads are created equally. For example RHS Small Block Ford heads use a Small Block Chevy style valve. Not only are they Chevy style valves, they are +.100" in length (as compared to stock length Small Block Chevy valves). Then consider that depending on the application you may even need +.200" valves and this addition makes the pushrod even longer. +.200" Chevy valves are CONSIDERABLY longer than stock Small Block Ford. Something to consider; pushrods are sold by Comp Cams and many other valve train companies in increments of as little as .020"…there has to be a reason for it.
So by now you're likely saying. "OK...Well nothing is changing but the cam".
To this we reply:
Other than trusting the stock timing marks and not degreeing the cam on installation, changing the lift without changing the pushrod is the most common mistake on cam swaps.
We absolutely cannot stress this enough.
This takes us to the last major spec that affects pushrod length:
on the valve stem as the valve opens and closes.
The method for setting this is to install the correct length
pushrods.
That's sounds pretty simple and it is but it's amazing how
many "Hot Rodders" get this wrong.
Time and again we've read on any of the countless online forums "What length pushrods for my 'xyz' headed motor with an 'abc' cam?" There are plenty of folks giving answers but the only correct answer is YOU HAVE TO MEASURE!!!!
There are no two engines alike unless they are coming off an assembly line, and even then, they simply average their tolerances and shoot for the middle…this is one reason why some factory motors are rockets, and some lemons. So here is a list of some of what goes into determining proper pushrod length.
Deck Height - The distance from the center of the main journal to the deck surface (where the head bolts to).
When an engine block is machined it is often decked to square/flatten out the surface the head bolts to with the crank shaft centerline, and sometimes to adjust the compression height (typically referred to as zero decking). This will bring the head surface closer to the camshaft and decrease the pushrod length. This is often a small change (.010 or less) but coupled with other factors it adds up.
Head Deck Milling Thickness - Loosely described this is the amount of material that can be safely removed from the head during milling operations.
Every head has a tolerance and they vary greatly. It is very common to mill a head to flatten the sealing surface and/or to increase the compression ratio by reducing the combustion chamber size. This brings the valve closer to the engine and further reduces PR length. Again these changes are small but they add up. On the flip side of this, replacing a factory head with an aftermarket one may actually lengthen the PR required as the deck thickness is often greater than stock castings.
Valve Length - This is, predictably…a simple term used to describe the length of the valve.
Not all cylinder heads are created equally. For example RHS Small Block Ford heads use a Small Block Chevy style valve. Not only are they Chevy style valves, they are +.100" in length (as compared to stock length Small Block Chevy valves). Then consider that depending on the application you may even need +.200" valves and this addition makes the pushrod even longer. +.200" Chevy valves are CONSIDERABLY longer than stock Small Block Ford. Something to consider; pushrods are sold by Comp Cams and many other valve train companies in increments of as little as .020"…there has to be a reason for it.
So by now you're likely saying. "OK...Well nothing is changing but the cam".
To this we reply:
Other than trusting the stock timing marks and not degreeing the cam on installation, changing the lift without changing the pushrod is the most common mistake on cam swaps.
We absolutely cannot stress this enough.
This takes us to the last major spec that affects pushrod length:
Lobe Lift - What does lobe lift have to do with pushrod length? Well, it's all about the cam base circle.
What follows is a very simplified explanation, but it helps to visualize if you think about it like this. The cam is ground on a core of a fixed diameter. Max lobe lift is ground in just below the bearing journals and then the lobe profile follows down what ever the lift spec is say .280" (~.450" valve lift with a 1.6 rocker). To grind a larger lift cam, the base-circle is ground smaller. Remember max lift is still just below the bearing. So that .550" lift cam you want to install has a lobe that's .323" of lift, this makes the base-circle .043" smaller. Think you might need new pushrods for that?
There are other factors that affect the proper length of your pushrods, but the above are the major examples showing this is something that has to be done any time you assemble/reassemble a valve train in which ANY factor on the given motor has changed.
So let's take a look at doing it.
What you'll need
1.) Checking pushrod
2.) Solid lifter
3.) Checking springs
4.) Marker or machinists dye
5.) Machinist scale
Checking pushrods are available in a couple of types. Comp's Hi-Tech™ Checking Pushrods measure in gauge length and are precision made to give you .050 of length for every revolution. These are by far the easiest checkers to use especially with guide plates.
Diagram B shows how pushrods are measured and it varies between manufacturers so it's important to know what measurement you are taking and buy the pushrod measured the same way. COMP, TFS, and others measure in gauge length which is what you'll buy if you use the COMP checker.
The solid lifter is needed because hydraulics bleed down which adds length to your measurement. You can either purchase a solid lifter, or create your own from a modified hydraulic lifter.
The checking springs are very light valve springs that take the place of your springs. Even though you are using a solid lifter the valve springs will cause the pushrod checker to bend. Maybe not permanently (well, not all of the time anyhow…ask us how we know this) but it can definitely flex enough to affect your reading.
The marker and scale are to check and measure the contact patch.
Warning: The procedure is a piece of cake to write about but takes some patience to perform.
Drop in the lifter making sure the cam is on the base-circle, install the checking springs, checking pushrod, and rocker arm. Run the pushrod out until the rocker is contacting the valve stem as shown in figure A above. Pull the rocker and paint the valve stem with the marker/dye.
When using something like a sharpie it may be difficult to get the roller to rub off the marker and show the patch. We've used 1000 grit sandpaper facing the valve stem to make it easier to see. Also be aware that oil will wash the sharpie away.
We've also used dry erase markers but that requires a steady hand as you'll easily change the mark taking the rocker back off. Use which ever method you choose, or come up with one of your own, the important part is to make sure the mark that is made is by the sweep of the rocker.
Install your rocker at zero lash and roll the engine over. Remove the rocker and look at your contact patch. The trick is to get it as narrow as possible by adjusting the length of the pushrod and rechecking. Always go past and come back to what you think is right. Don't be too concerned about the mark being centered. You obviously don't want it running off the edge (or even close for that matter!), but unless this is a stock rebuild that needs to run 150,000+ miles without a tune up…having an absolutely centered contact patch isn't really all that important. Plus we figure since you've got roller rockers and a .550"+ lift cam you have performance in mind…and for that, the smallest patch is best.
Once you have your contact patch where you're happy, take the checker out and measure it. If running a solid lifter, you simply match this length to your vendor of choice, and order. If running a hydraulic lifter, be sure to add .020" to the measured length. This is for the preload you will put on the lifter when you assemble the engine. If you can't find an exact match you can go custom, or adjust to the next lowest increment. It is preferred of course to get the proper length but shorter is always better than longer. Going too long can cause problems with cranking compression and idle.
Repeat this procedure for the exhaust.
Technically you should check every valve but if you're using a hydraulic cam the lifters are going to adjust for small differences. Also hydraulic lifters use oil pressure to set the valve train to 0 lash. They accomplish this with a piston that the pushrod sits in. That piston tends to move around in a running engine, especially at high rpms, so your ideal pushrod length is going to move around a bit.
At the very least you must check a set on each other bank since they can be different. If there is a large difference that clues you into something else being off like one head that was milled and the other not.
COMP CAMS carries a full line of tools for performing this check and valve train parts when you are ready to buy. Whomever you buy your parts from if you follow these procedures you're sure to get it done properly, and by doing so you're likely to be one step ahead of your competition.
What follows is a very simplified explanation, but it helps to visualize if you think about it like this. The cam is ground on a core of a fixed diameter. Max lobe lift is ground in just below the bearing journals and then the lobe profile follows down what ever the lift spec is say .280" (~.450" valve lift with a 1.6 rocker). To grind a larger lift cam, the base-circle is ground smaller. Remember max lift is still just below the bearing. So that .550" lift cam you want to install has a lobe that's .323" of lift, this makes the base-circle .043" smaller. Think you might need new pushrods for that?
There are other factors that affect the proper length of your pushrods, but the above are the major examples showing this is something that has to be done any time you assemble/reassemble a valve train in which ANY factor on the given motor has changed.
So let's take a look at doing it.
What you'll need
1.) Checking pushrod
2.) Solid lifter
3.) Checking springs
4.) Marker or machinists dye
5.) Machinist scale
Checking pushrods are available in a couple of types. Comp's Hi-Tech™ Checking Pushrods measure in gauge length and are precision made to give you .050 of length for every revolution. These are by far the easiest checkers to use especially with guide plates.
Diagram B shows how pushrods are measured and it varies between manufacturers so it's important to know what measurement you are taking and buy the pushrod measured the same way. COMP, TFS, and others measure in gauge length which is what you'll buy if you use the COMP checker.
The solid lifter is needed because hydraulics bleed down which adds length to your measurement. You can either purchase a solid lifter, or create your own from a modified hydraulic lifter.
The checking springs are very light valve springs that take the place of your springs. Even though you are using a solid lifter the valve springs will cause the pushrod checker to bend. Maybe not permanently (well, not all of the time anyhow…ask us how we know this) but it can definitely flex enough to affect your reading.
The marker and scale are to check and measure the contact patch.
Warning: The procedure is a piece of cake to write about but takes some patience to perform.
Drop in the lifter making sure the cam is on the base-circle, install the checking springs, checking pushrod, and rocker arm. Run the pushrod out until the rocker is contacting the valve stem as shown in figure A above. Pull the rocker and paint the valve stem with the marker/dye.
When using something like a sharpie it may be difficult to get the roller to rub off the marker and show the patch. We've used 1000 grit sandpaper facing the valve stem to make it easier to see. Also be aware that oil will wash the sharpie away.
We've also used dry erase markers but that requires a steady hand as you'll easily change the mark taking the rocker back off. Use which ever method you choose, or come up with one of your own, the important part is to make sure the mark that is made is by the sweep of the rocker.
Install your rocker at zero lash and roll the engine over. Remove the rocker and look at your contact patch. The trick is to get it as narrow as possible by adjusting the length of the pushrod and rechecking. Always go past and come back to what you think is right. Don't be too concerned about the mark being centered. You obviously don't want it running off the edge (or even close for that matter!), but unless this is a stock rebuild that needs to run 150,000+ miles without a tune up…having an absolutely centered contact patch isn't really all that important. Plus we figure since you've got roller rockers and a .550"+ lift cam you have performance in mind…and for that, the smallest patch is best.
Once you have your contact patch where you're happy, take the checker out and measure it. If running a solid lifter, you simply match this length to your vendor of choice, and order. If running a hydraulic lifter, be sure to add .020" to the measured length. This is for the preload you will put on the lifter when you assemble the engine. If you can't find an exact match you can go custom, or adjust to the next lowest increment. It is preferred of course to get the proper length but shorter is always better than longer. Going too long can cause problems with cranking compression and idle.
Repeat this procedure for the exhaust.
Technically you should check every valve but if you're using a hydraulic cam the lifters are going to adjust for small differences. Also hydraulic lifters use oil pressure to set the valve train to 0 lash. They accomplish this with a piston that the pushrod sits in. That piston tends to move around in a running engine, especially at high rpms, so your ideal pushrod length is going to move around a bit.
At the very least you must check a set on each other bank since they can be different. If there is a large difference that clues you into something else being off like one head that was milled and the other not.
COMP CAMS carries a full line of tools for performing this check and valve train parts when you are ready to buy. Whomever you buy your parts from if you follow these procedures you're sure to get it done properly, and by doing so you're likely to be one step ahead of your competition.
