Piston to Valve Clearance
Cruising the multitude of internet forums out there we see the same question coming up over and over again. As with pushrod length the solution for many is to ask the question.
"I have this motor with these heads and cam will I have PTV issues?"
As with the pushrod question the answer is "You have to check". No two engines are the same. What worked for one person my not work for you and catastrophic engine damage may result. You may get it fired up and running fine then the first time down the track or trying to merge on the freeway you run the RPM's up and a valve kisses the piston. Not good
This is the result of a valve breaking off the stem
OUCH!!
OUCH!!
This is milder damage but the heads are still coming off
What goes into determining PTV clearance?
There are a multitude of factors that affect PTV Clearance but one thing that is not a factor is max lift. Sure if you put a larger lift cam in an engine you may have PTV issues but the lift alone is not the issue. The ramp rates and valve events that changed with that big lift cam is what is causing the problem. Here is why.
Lift around TDC is what determines PTV clearance.
Yes that's right. Max lift is irrelevant since the piston is way down the bore when that happens. It is the events around TDC, where the exhaust valve is closing and the intake valve is opening (overlap), that is important. You may have run a cam for quite a while with no problems. You are looking for a bit more top end so your buddy tells you to retard the cam (an explanation of this is contained in a separate article ). When you retard the cam you are delaying the valve events. This means at overlap the intake valve opens later, which helps keep it away from the piston, but the exhaust valve closes later as well. This means it will be further off the seat as the piston approaches the top. If you had just enough PTV clearance before you will not have enough now. Same thing goes with advancing the cam to give you more low-end. The issue now is the intake valve opens sooner and since this happens while the piston is still moving up you are in great danger of damage, especially if you have a very aggressive intake lobe.
"OK you've convinced me. Now how do I check it?"
As with anything there is more than one way to do this. How you choose will depend on what tools are available and what you are doing to the motor. If you're doing a cam swap on a motor that is installed in the vehicle using the clay method may not be the best choice as it requires pulling the heads. On the other hand only using the dial indictor or feeler gauge method is not the right choice if installing new heads with larger valves as the valve relief margin needs to be checked.
We start with the easiest method and move to the hardest. Remember that you may have to use more than one method depending on your circumstance.
We start with the easiest method and move to the hardest. Remember that you may have to use more than one method depending on your circumstance.
Feeler Gauge Method
This is a heads on the motor procedure. You'll need your checking springs you bought to check valve train geometry and it is best to use a solid lifter which means pulling the intake if you have hydraulics. This is not an issue when replacing the cam as the intake is off anyway, but does add an extra step and the cost of gaskets if you are just changing cam timing. Who ever said speed was cheap! The reason for using the solid lifter is to make sure that the lifter does not collapse while taking the measurement.
Degree the cam to where you want it and set it on the base circle for the cylinder you are going to measure. Remove your valve springs and install the light springs (this will allow you to open the valve by hand), drop in the solid lifter, the pushrod and install the rocker arm at 0 lash. SLOWLY rotate the engine around by hand.
Degree the cam to where you want it and set it on the base circle for the cylinder you are going to measure. Remove your valve springs and install the light springs (this will allow you to open the valve by hand), drop in the solid lifter, the pushrod and install the rocker arm at 0 lash. SLOWLY rotate the engine around by hand.
We emphasis slowly…if you have interference you can easily chip or bend a valve by going too far.
As you approach TDC on the exhaust stroke (starting at about 20 BTDC) push the valve down by hand and slip a .100 feeler gauge between the stem and the rocker. Push down on the rocker to ensure you have more than .100 of clearance. Remove the feeler and rotating the engine a small amount (a few degrees) and recheck.
Do not rotate the engine while "feeling" for the piston.
You will feel the clearance get smaller and smaller then go larger. As long as you have greater than .080 on the intake and greater than.100 on the exhaust you are good to go. The exhaust needs more clearance because the valve is closing as the piston approaches TDC, any valve float and the piston could strike the valve.
You should at least check both banks since differences in the heads can cause different readings. If you are close then you better check all cylinders as crank timing can vary by as much as 5 degrees cylinder to cylinder on a stock or even some aftermarket cranks.
You should at least check both banks since differences in the heads can cause different readings. If you are close then you better check all cylinders as crank timing can vary by as much as 5 degrees cylinder to cylinder on a stock or even some aftermarket cranks.
Dial Indicator Method
This procedure is that same as the feeler gauge method but you set up a dial indicator on the valve spring retainer directly adjacent to, and in line with the valve stem. To take your measurement you zero the indicator and push down on the rocker until the valve contacts the piston or you run out of travel on your springs, then read the dial. This method will give you an exact measurement of PTV Clearance which may be helpful when making future changes.
Clay Method
Or as we like to call it "New uses for Play Dough". This method takes an impression of the valve and the relief in the piston. It is very handy not only for PTV but to ensure you have adequate valve relief diameter to fit the valve.
If the valve winds up in the relief and there is only .003 between the edge of the valve and the relief, your PTV may be fine but you are still not ok. The relief will need to be opened up to provide a bit more room. Remember the piston changes size with heat and can rock in the bore and the valve can actually move around if you develop other valve train problems. You should keep a minimum of .010 between the edge of the valve and the relief.
This method requires removing the head…possibly several times. Aren't you glad you sprung for those high performance aluminum castings now?
There is no need for a head gasket when doing the checks as the gasket thickness will add to your clearance. Just make sure you know what the COMPRESSED thickness is of the head gasket you will use. This thickness will be added to your reading.
You'll need some modeling clay, a dial indicator and a razor knife.
Prior to performing this procedure you must check to make sure the valve will not hit the pistons. Follow the procedures above but there is no need for actual measurements. What you will do is just make sure you can push the valve down further than the rocker does to guarantee the valve will not hit the piston. If you just put the clay in and the valve contact the piston you'll wind up with a chipped or bent valve.
Pull the head and put a pad of clay on top of the piston. It's helpful to coat the piston and valves with WD40 or some other lubricant so the clay will release easily. Push it down to a minimum thickness of .100…..in reality up to .250 is just fine and may be the only way to get an impression. Keep in mind the quench area and keep the clay away from there as it will compress the clay and can push it up on top of the valve. This will ruin your results and then you have to clean off the valves and seats.
Set the head back down, assemble the valve train with your solid lifters same as above only this time you're doing both valves. Roll the engine over once by hand and remove the head. Take a look at the impression…Do you see clear distinct marks from the valves? Did the clay lift off the piston or stick to the valves in places? Once you determined you have a good impression carefully peel off your clay making sure not to deform the clay. You now have an exact imprint of the piston and valve.
In this picture the assembler used clay in several different places as he is using domed pistons. The extra clay is to check clearance between the dome and the combustion chamber. This method works really well as it allows you to measure the compression without removing the clay. Only use the lubricant on the valve if you are going to do it like this. Once the impression was made the clay was easily sectioned right on the piston
There is no need for a head gasket when doing the checks as the gasket thickness will add to your clearance. Just make sure you know what the COMPRESSED thickness is of the head gasket you will use. This thickness will be added to your reading.
You'll need some modeling clay, a dial indicator and a razor knife.
Prior to performing this procedure you must check to make sure the valve will not hit the pistons. Follow the procedures above but there is no need for actual measurements. What you will do is just make sure you can push the valve down further than the rocker does to guarantee the valve will not hit the piston. If you just put the clay in and the valve contact the piston you'll wind up with a chipped or bent valve.
Pull the head and put a pad of clay on top of the piston. It's helpful to coat the piston and valves with WD40 or some other lubricant so the clay will release easily. Push it down to a minimum thickness of .100…..in reality up to .250 is just fine and may be the only way to get an impression. Keep in mind the quench area and keep the clay away from there as it will compress the clay and can push it up on top of the valve. This will ruin your results and then you have to clean off the valves and seats.
Set the head back down, assemble the valve train with your solid lifters same as above only this time you're doing both valves. Roll the engine over once by hand and remove the head. Take a look at the impression…Do you see clear distinct marks from the valves? Did the clay lift off the piston or stick to the valves in places? Once you determined you have a good impression carefully peel off your clay making sure not to deform the clay. You now have an exact imprint of the piston and valve.
In this picture the assembler used clay in several different places as he is using domed pistons. The extra clay is to check clearance between the dome and the combustion chamber. This method works really well as it allows you to measure the compression without removing the clay. Only use the lubricant on the valve if you are going to do it like this. Once the impression was made the clay was easily sectioned right on the piston
Which ever method you use, section the clay and measure the thickness with calipers. Again, you want at least .100 exhaust, .080 intake, and a good (greater than.010) margin around the edge of the valve.
As I said at the beginning easiest to hardest…
Even the people who do check tend to use only one method and the feeler gauge method seems to win out. This is fine if you are sure you don't have clearance issues with the diameter of the valve relief's.
As I said at the beginning easiest to hardest…
Even the people who do check tend to use only one method and the feeler gauge method seems to win out. This is fine if you are sure you don't have clearance issues with the diameter of the valve relief's.
