How to measure piston-to-valve clearance?
This piston-to-valve clearance was adequate at 6,000 rpm but at 7,000 rpm, a loss of valve control caused this exhaust valve to contact the piston and bend the valve. There are several techniques that can be used to measure P-V clearance, but the two most popular are the clay method and the “valve drop” technique.
How much valve clearance do you need for your engine?
In the past, the standard safe clearance recommendation for most street engines was 0.100-inch for the intake valve and 0.140-inch for the exhaust. The exhaust valve clearance is generally spec’d wider because if at high rpm the exhaust valve bounces off its seat at closing, it’s possible it could smack the piston. So added clearance is necessary.
Is it hard to check piston to valve clearance?
Checking piston-to-valve (P to V) clearance isn’t difficult, but necessary especially when building a high performance engine with a big cam with lots of lift. Building a custom engine means you must make sure those valves will clear the pistons.
What is the minimum P to V clearance for a car?
So first we should establish what makes up minimum P to V clearance. In the past, the standard safe clearance recommendation for most street engines was 0.100-inch for the intake valve and 0.140-inch for the exhaust.
How much clearance do you need between the piston and head?
The generally accepted piston-to-head clearance for steel connecting rod engines is 0.040-inch although there are many instances where builders have tightened this clearance to as little as 0.036-inch. Much of this depends upon the engine builder's willingness to gamble on how much the piston rocks in the bore.
What happens if valve clearance is too small?
Too much clearance means the valves will likely clatter and, over the long term, cause damage to the valves, camshaft lobes or rocker arms. If there's too little valve clearance, the valves won't fully close, causing excessive heat, and the engine will lose power.
What affects piston to valve clearance?
The position of the cam in the engine will greatly affect piston to valve clearance. 1) With the camshaft installed, remove the cylinder head from the block. Clean the combustion chamber and the top of the piston and valve reliefs. The cleaner the piston, the better the clay will stick to it.
What is the piston clearance?
Piston clearance is the clearance or gap between the piston and metal cylinder, to avoid damage due to excessive expansion of piston on getting heated during combustion.
What is acceptable valve guide clearance?
A general rule of thumb is that intake guides need guide-to-stem clearance of . 001″ to . 003″ and exhaust guides need from . 002″ to . 004″ clearance.
What happens if the valve clearance is zero?
If the valve clearance was set to zero, the valve could theoretically still be set to close fully, but as the engine gets warm, the parts expand; with no gap, the valve wouldn't close completely. That's why valve clearances MUST be checked with the engine stone cold.
Why piston ring gap should be on a proper clearance?
The key to proper ring gap is ensuring there is room for expansion as the rings heat up, without creating excessive clearance and unwanted blow-by.
What will happen if piston clearance is too large or too small?
If there's too much clearance, the piston could rock back and forth inside the cylinder and damage the rings, cylinder walls, or skirts. If the clearance is too tight, then the piston can seize in the cylinder as it expands due to combustion heat.
How do you test piston to cylinder clearance?
2:588:31How to Measure Piston to Cylinder Clearance - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo another way to do it is with a telescopic gauge which is what this is they move in and out likeMoreSo another way to do it is with a telescopic gauge which is what this is they move in and out like this. And you typically put them in the cylinder. At an angle. And then you lock this down here.
What is piston tolerance?
Note: When measuring a piston it should be noted that most pistons have a skirt diameter tolerance of less than twenty microns, (0.020mm), which is checked in a temperature controlled environment. If micrometers are to be used, their accuracy is 0.01 mm, which is greater than half the piston's diameter tolerance.
How important is valve clearance?
This clearance is important because it compensates for the expansion of the various engine components as the engine heats up, thus allowing optimum engine performance and preventing engine damage.
Can tight valves cause misfire?
YES, valves with adjustment issues can cause a misfire. A misfire is when one or more cylinders are not producing enough power. If the valves are adjusted too tight, they will cause a misfire.
Can valve clearance affect compression?
Loose valve clearance will raise the compression because you are closing the valve earlier in the cycle. Compression doesn't start until the intake valve closes, so by closing it earlier you will be compressing the mixture for more degrees of crank rotation.
Does valve clearance affect top speed?
Valve clearance doesn't generally affect the engine's power. Valve duration, controlled by the camshaft, or the amount of time the valve stays open, affects performance.
Definition & Recommended Clearance
Piston to valve clearance is the minimum distance between the valve and piston. On the intake stroke, it's when the intake valve is opening with the piston near TDC. On the exhaust stroke, it's when the exhaust valve is closing and the piston is moving up towards TDC.
Why does it matter?
When assembling your engine, it is important to check the piston to valve clearance. This will ensure that the valves will not contact the piston while the engine is running. If any of the valves hit the piston during operation, significant damage will occur.
What is the minimum recommended clearance?
To avoid damage to your engine, the following minimum clearance should be maintained.
How to make a check valve lifter?
You can create a solid checking lifter out of an old hydraulic lifter (either flat or roller) by removing the internal hydraulic piston and shimming the pushrod cup in place with small washers. This will create an accurate lift curve for measuring P to V.
What is overlap in exhaust?
Let’s start with a quick run through of what’s happening inside your engine. In the closing stage of the exhaust stroke when the piston nears top dead center (TDC), the exhaust valve is closing while the intake is just opening. This is commonly referred to as overlap.
How to calculate intake P to V?
According to the Reher & Morrison engine building and blueprinting book, it recommends a more precise rule for calculating intake P to V is total deck clearance plus 0.010-inch. So with a steel connecting rod total piston-to-head clearance of 0.040-inch, then 0.040 + 0.010 = 0.050-inch for the intake valve. On the exhaust side R&M recommends 1.6 to 2 times the intake clearance. This makes the exhaust side 0.100-inch or 0.080 if you’re brave. These recommendations are more specific and take into account aluminum connecting rods versus steel by using deck clearance. R&M recommends a total deck clearance of 0.035 to 0.040 for steel rod engines and 0.055 to 0.070 for aluminum rod applications. These are general recommendations and certainly will vary with more specific builds, but these numbers are a great place to start for a strong street engine.
What does clay mean on pistons?
If you look carefully, the clay may also indicate the radial clearance between the valve pockets and the valve although often this can be difficult to measure accurately. Most street piston manufacturers will place more than adequate valve reliefs in their pistons so that machining is not required.
What is the minimum clearance for a P to V valve?
So first we should establish what makes up minimum P to V clearance. In the past, the standard safe clearance recommendation for most street engines was 0.100-inch for the intake valve and 0.140-inch for the exhaust. The exhaust valve clearance is generally spec’d wider because if at high rpm the exhaust valve bounces off its seat at closing, it’s possible it could smack the piston. So added clearance is necessary. If engine speeds are not high and the engine builder is confident that the valve train can be controlled, then tighter clearances are possible down to perhaps 0.070-inch on the intake and 0.100 on the exhaust. But as engine speeds increase, wider clearances can keep you out of trouble.
How to determine valve clearance?
Using the valve centerline marked in the piston, set the valve radius (half of valve diameter) with a set of dividers and scribe a radius around the valve relief. This will indicate if you have sufficient radial clearance. If not, the pocket may need to be machined.
How much lash to subtract from hot setting?
With an iron block engine with aluminum heads, the chart recommends subtracting 0.006-inch of lash from the recommended hot settings. This will increase valve lift and decrease P to V by that amount.
How to check valve pocket?
To check the valve pocket location, remove the cylinder head and set a pair of calipers to the radius of the valve head (for example, for a 2.500-inch diameter valve head, set the calipers at 1.250-inch). With one point centered on the punch mark, swing the other point around the valve eyebrow. If the caliper hits the edge of the valve pocket, so will the valve. I recommend a minimum of .050-inch radial clearance between the edge of the valve and the pocket.
How to draw a road map?
You can draw a “road map” by noting the position of the reference notch as you turn the valve. For example, if the valve stem rises near the top of the dome and falls at the bottom of the valve notch, then the angle of the valve relief is too steep.
What happens if you have too much clearance?
On the other hand, if you have too much clearance, the engine will run, but it won’t achieve anything close to its performance potential.
Can you use clay to measure piston clearance?
The amount of clay on the piston top, the density of the clay, the effect that the clay has on the relative positions of the valve and piston, and the difficulty of measuring the thickness of the compressed clay accurately are just a few of the sources of potential error with this method. You don’t use clay to measure piston-to-wall clearance ...
Do both engines have the same volume?
Both engines have the same volume above the piston at TDC, but the engine with the proper valve reliefs and shorter domes will have a substantial horsepower advantage. Measuring piston-to-valve clearance properly is one of the basic operations that every novice engine builder should master.
Can light tension checking springs duplicate load?
This includes the same lifters, the same pushrods, the same rocker arms, and the same valve springs. Light-tension checking springs simply can’t duplicate the load and deflection that the valvetrain experiences with stiff race springs.
Minimum piston to valve clearance?
I'm having a piston to valve clearance issue. What I'm coming up with is almost zero. I'm using a .039" thickness gasket. There are thicker gaskets available. The thickest I've seen so far is .051" which would give me an additional .020" of clearance. I could probably open that up at bit because I lash my valves on the tight side. .018".
Minimum piston to valve clearance?
I'm having a piston to valve clearance issue. What I'm coming up with is almost zero. I'm using a .039" thickness gasket. There are thicker gaskets available. The thickest I've seen so far is .051" which would give me an additional .020" of clearance. I could probably open that up at bit because I lash my valves on the tight side. .018".
How to adjust the BTDC on a cylinder?
First, position the dial indicator on the exhaust valve spring retainer and make sure the dial indicator can accommodate the entire valve travel distance. Next, rotate the engine until the degree wheel indicates 10 degrees Before Top Dead Center (BTDC). This 10 degree-BTDC position is usually very close to the tightest point between the piston and exhaust valve.
How to tell if a valvetrain is in clay?
With the valvetrain in place, rotate the crank through at least four full rotations to ensure the valves have made an impression in the clay. After removing the valvetrain and head, there should be an impression from the intake and exhaust valves in the clay. Carefully slice the clay with a razor blade to expose the thickness of the clay where the valves made contact. That thickness will reveal the actual P-V clearance.
What is valve drop?
The valve drop method is slightly more involved and requires a degree wheel, pointer, accurate TDC location, light-duty checking springs, a magnetic base, and a dial indicator. Set up the checking fixtures on Number One piston with the head gasket and cylinder head you will be using. Assemble both the intake and exhaust valves with a checking spring and include a cold lash setting if using a mechanical cam. The checking springs will make the test easier to execute.
How to make a test lifter?
To make the test lifter, remove the clip and internal piston from a hydraulic lifter, and replace the piston with a stack of small washers to position lifter cup in its stock location. Do this for both the intake and exhaust lifters and set the valvetrain at zero lash. If the engine uses a mechanical cam, include a cold lash clearance.
Why is it important to check piston to valve clearance?
Accurate testing of piston-to-valve clearance will become increasingly important as street engines become more aggressive, while pushing the envelope with rocker ratios, valve timing, and compression. Just slapping an engine together and hoping it works can lead to expensive lessons. You don’t want to be the guy standing over a broken engine and having to answer the question: “And I didn’t check piston-to-valve clearance, why?”
What to do if P-V clearance is too tight?
If testing reveals the P-V clearance is too tight, there are some adjustments you can attempt. For example, let’s say the exhaust valve clearance is tighter than you are comfortable with but the engine is already assembled and you don’t want to fly cut the pistons or change the cam. The first thing to try would be to advance the camshaft by a couple of degrees.
How to check P-V?
To perform the clay method of checking P-V begins, you have to start with the cylinder head removed. Place a quarter to half-inch layer of modeling clay in the valve reliefs of the piston. Lay a thin coating of oil on the valve faces to prevent the clay from sticking to the valves during the test.
