Text & Photos: Jeff Smith
This is the golden age of horsepower and among big changes that are leading this charge are better cylinder heads and more aggressive cam lobes. Big cams mean towering valve lift numbers. In the most recent Engine Masters contest, all five top finalists ran no less than 0.750-inch lift and some exceeded 0.850! Not too long ago, those might have been NHRA Pro Stock numbers.
Clearly, as cylinder heads improve flow capacity, valve lift will continue to escalate. As lift escalates and combustion chambers get smaller, piston-to-valve (P to V) becomes tighter. The way to find out that your valves don’t clear the pistons is not when the engine locks up from a bent valve.
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. Think of this as the piston chasing the exhaust valve closed while the intake valve opens into the piston.
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.
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 to Do First
Before we begin to measure P to V clearance, we first must be certain of where the camshaft is in relation to the pistons. Each cam manufacturer will recommend a specific position for the cam based on the intake centerline. This position tends to be the most accurate way to locate the cam. For example, the cam card will list the intake centerline as 108 degrees after top dead center (ATDC). We won’t go into the procedure for degreeing a camshaft as that subject has been well covered.
If you are considering changing the intake centerline by either advancing or retarding the cam, you will need to perform the P to V measurements at all of the considered intake centerlines. Advancing the cam moves all the valve events earlier, which would reduce the intake V to P clearance and increase the exhaust. Retarding the cam will do the opposite.
The Clay Method
If your engine uses hydraulic tappets, you will need to substitute a mechanical lifter to ensure accurate valve lift. 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.[/caption]
Let’s start with the easiest method for determining P to V, but only if the heads are already off the engine. Frankly, the only specialty tool needed is some modeling clay – which you can borrow from your kid’s toy box. Place roughly a 3/8-inch thick lump of clay in the piston intake and exhaust valve reliefs.
Lightly coat the valve faces with oil to prevent the clay from sticking to the valves. Now assemble the heads with a previously compressed head gasket. The most accurate test is to fully torque the entire head, but we generally only torque the head bolts around the cylinder in question.
If your engine is equipped with hydraulic lifters, these will compress under load, so the best procedure is to substitute a set of mechanical lifters. Assemble the intake and exhaust pushrods and rockers and then set the lash at zero if you’re using a hydraulic cam. If the cam is mechanical, set the lash at the adjusted hot setting as specified in the accompanying chart. Iron does not expand as much as aluminum so these changes to cold lash are based on the thermal expansion rates of aluminum versus iron, making the lash clearances a bit closer to the hot settings.
Once lash is set for both the intake and exhaust valves, you can now carefully roll the crankshaft over at least four revolutions to allow the valves to run through their entire lift cycles. With that complete, disassemble the valvetrain and remove the cylinder head and bring the piston up to TDC. You will notice that the valves will have pushed into the clay, revealing the amount of clearance. There are several ways to measure this. You can use a dial caliper to measure the depth or you can cut the clay in half with a razor blade and then use a dial caliper to measure this depth.
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.
There are several limitations to this procedure. First, it is not highly accurate, although this is only necessary when the clearances are very near the minimum. Most street engines end up with over a 0.250-inch of P to V clearance, which means accuracy isn’t critical. As mentioned, radial clearance – the clearance between the outside diameter of the valves and the piston reliefs is important. We talked to engine builder Kurt Urban, and he prefers 0.060 just to be safe because engines with really short skirt pistons can move around quite a bit. We will run through how you can measure for this clearance in a moment using a homemade tool and a dial indicator.
The Dial Indicator Method
If removing the heads from the engine is not practical, you can measure P to V with a couple of checking springs and a dial indicator. First, remove all the spark plugs and install light checking springs on Number One intake and exhaust. Next, determine exactly where TDC is with a degree wheel because you will need to place the piston 10 degrees before and after TDC. Once this is established, set up the dial indicator on the exhaust valve to measure valve lift from the spring retainer. Slowly rotate the engine clockwise by hand until the piston is 10 degrees BTDC. The exhaust valve will be on the closing side of its lift curve. Zero the dial indicator and then push down on the valve end of the rocker and record the amount of travel before the exhaust valve contacts the piston. This amount of movement is the P to V for the exhaust valve. Now rotate the engine to 10 degrees ATDC and move the dial indicator to the intake valve. Zero the dial indictor and then depress the intake valve with the rocker until it too hits the piston. Record this value as the intake clearance.
If P to V clearance is close to or at the minimum, you may want to consider performing this test again except with the intended valve spring package. High spring pressures will create deflection in the valve train that can change the P to V. This test will also require some kind of lever tool to compress the spring once it is in position.
Checking Radial Clearance
If the outside diameter of the valve appears to be close to the edge of the piston valve relief, this will require more accurate measurement. The important measurement is the valve centerline relative to the valve relief in the piston. We found this trick in the Reher & Morrison Engine Building book where David Reher recommends cutting the head off of a valve and having a machinist mill one end to a point. Then with the piston at TDC and the head in place, use the valve stem to mark the valve centerline in the piston. As an example, let’s use an LS engine with 2.165-inch intake valves. Dividing the valve diameter in half to establish the radius, this gives us a radius of 1.0825. Duplicate this distance on a pair of dividers and then scribe a radius from the piston centerline. If there is less than 0.050 to 0.060 inch between the scribed line and the vertical wall of the valve pocket, this will require machining to increase the clearance. If the clearance is more than 0.050 – the clearance is good and you’re ready for assembly.
Taking the time to measure P to V cannot be over-emphasized. It’s also the kind of thing that all good professional engine builders do as a matter of course. While these procedures may seem intimidating at first, they are simple to perform and worth the time it takes – if nothing else just for insurance against disaster.
Valve Lash Compensation Table
For accuracy with a cold engine and a solid lifter camshaft, this chart specifies compensation for heat expansion with cold lash settings. 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.
Block | Head | Lash Compensation (inches) |
Iron | Iron | +0.002 |
Iron | Aluminum | -0.006 |
Aluminum | Aluminum | -0.012 |
Parts List
Description | PN | Source | Price |
Checking Springs, pair | 4758-2 | Summit Racing | $3.97 |
Professional degree wheel | 4791 | Summit Racing | 190.97 |
Caliper, 6 inch | 4908 | Summit Racing | 49.97 |
Dial Indicator magnetic base | 4907 | Summit Racing | 67.97 |
Dial Indicator, 0-1” travel | 4909 | Summit Racing | 37.97 |
Reher & Morrison Racing book | Order Online | Reher & Morrison | 74.95 |
Sources
COMP Cams
(800)999-0853
compcams.com
Reher-Morrison Racing Engines
(817)467-7171
rehermorrison.com