 Checking Piston-to-Valve Clearance
What Is Piston-to-Valve Clearance And How Does It Affect Me?
This is best said in simple terms ... "If You Do Not Want Your Engine To Explode, and You Have Installed A Performance Cam, You'd Better Check The Piston-To-Valve Clearance!"
Piston-To-Valve clearance is just what it says ... the minimum distance between the engine's exhaust and intake valves to the top of the piston at the closest given position of the reciprocating cycle. Piston to valve contact is probably one of the most common causes of damage to a performance or racing engine. Piston to valve interference can be caused by many items, including: Cam Lift, Cam Timing, Piston Design, Timing Chain/Drive Failures, Deck Height, Cylinder Head Modifications, Valve Diameter, Broken Valvetrain Parts (valve locks, springs, retainers, lifters, pushrods and cam lobe failures), Valve Diameter, Connecting Rod Stretch, or a combination of these factors. In this article I will be addressing cam timing, cam lift, and basically anything that has to do specifically with the valvetrain.
Anytime you make a change to your engine in relation to the cam lift, cam timing, pistons, cylinder heads, rockers arm ratio, or anything else that can change, even so slightly, the position of the valve in relation to the piston, you MUST verify that you have the minimum clearances safely met. This procedure consists of setting up your engine and measuring the clearance at the valves closest point to the piston and validating a "safety margin" clearance. This is typically .080" on intake valves and .100" on exhaust valves when using steel connecting rods. With aluminum connecting rods this dimension changes to .100" on intake and .120" on exhaust. The exhaust always needs more clearance because of the added COE (Coefficient of Expansion) of the hotter running exhaust valve. These are standard numbers, in some cases you have to look at your entire combo and make your own decision as to the minimum clearance values.
Why Should You Check Piston-to-Valve Clearance?
This is an easy one to explain! Let me ask you; Do you like spending thousands of dollars on your engine and having it only run a few moments before it starts running like crap, or in a worst case, having a piston explode when a valve hits it? If a valve hits a piston, in the least it will bend that valve, possibly also doing damage to the valve guide, cylinder head, pushrod, lifter, cam lobe and more. In a more serious dilemma, the valve will bend enough to not close all the way and then when the piston heads back up to TDC (Top Dead Center) it ventilates the piston (punches a hole through it), possibly bending or breaking the connecting rod. When a connecting rod is damaged it is possible that it will then leave the block at the easiest point of exit. This can also cause oil shooting out of the engine, oiling the race track and your tires which will cause traction loss and the possibility of crashing your car into a retaining wall or even into your opponent's race car. This does not sound like much fun does it?
With the safety issues as well as the cost to repair the damage that occurred, it is mandatory that you verify the piston-to-valve clearance.
What Tools Do I Need To Be Able To Do This?
This is one of the rare instances that only a few simple tools are required. Some considerations are necessary that if you plan ahead of time your process will be easier. What is most important is the time and patience you use to perform this procedure correctly. Read the list of tools, the engine parts needed, and their descriptions below:
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Short block with at least one piston/rod assembly installed.
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It is required that on engines such as the big block Chevy that has different handed pistons that you do at least two cylinders so as to verify each design. You should have already completed machining of the engine block. Deck heights, align honing, bore/hone, etc
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The cylinder heads, with milling and valve job complete.
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All modifications need to be completed on the cylinder heads you plan on testing. You may use the head with light pressure testing springs, but chamber volume, surfacing, valve job, and lash settings must me complete or you will need to re-test.
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Cylinder head bolts or studs
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Head Gasket (preferably a used one for this test)
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Camshaft and specs for the cam you are using as well as what rocker arm ratio you intend to use.
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You must know what your cam lift values are going to be with this combo. If you plan on possibly using multiple cams through the season, it would be best to test each one. If you plan on running a high ratio rocker arm, you need to test with that rocker arm on the cylinder head.
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Razor Knife
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Modeling Clay
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Torque Wrench
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Crank Turning Socket or other turning device
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Dial Caliper or other "accurate" measuring device
Exactly What Do I Do To Test The Piston-To-Valve Clearance?
The process is really quite simple, but if you have not already figured out what you need to do, the description provided below will be a step-by-step procedure of the processes.
First, you need to have all your parts you plan on using for this engine ready to go. These include the block, crank, connecting rods (just need one or two for this test) with the pistons hung on them, piston rings, camshaft, cam bearings installed, rod and main bearings, lifters, pushrods, cylinder heads (with parts), timing chain set and a used head gasket.
You first assemble your short block with the crank, one or two piston rod combinations (remember that the big block Chevy as well as many other engines have "handed" cylinders. If this is the case you need to test each "hand"), the camshaft, and timing set.
 With the short block pieces assembled you need to degree in and set up your camshaft where you plan on running the engine. If you plan on making future cam or cam timing changes, you should repeat the entire testing procedure for each variation you plan on using. Each change you make can cause an interference problem even if the first time you checked your clearances you were fine.
With that done, you will place a small amount of the modeling clay on the piston(s). (see image at right) Lightly oiling the piston top, valve faces, and chamber on the cylinder heads will save you a ton of grief when removing the clay. You need to place the clay over and around the valve reliefs on the piston top. Be careful not to place too much clay or making it too thick around the edges of the pistons. The clay only needs to be about 1/4" - 3/8" thick.
You now have the clay on the piston top, so you are ready to install the cylinder head. Use your used head gasket and place it on the deck and install the cylinder head. Use normal torquing procedures for your cylinder head and engine requirements. Once the head is in place you can install the remaining valvetrain (lifters, pushrods, rocker arms) on the cylinder(s) that are to be checked. Install both the intake and exhaust lifters for each cylinder tested, and also the pushrods and rocker arms. Be aware that you must test using the rocker arm ratio you are going to use on the completed engine. If you plan on installing a high ratio rocker arm at a later date, test clearance for that now. This will save you problems in the future. When you set the lash, set it at "zero". It is preferred that you test hydraulic cams with a solid lifter installed (a used lifter is OK too), just to make sure there is nothing that can cause a variance of the valve lift in your measurement.
With all this complete, you will now be rotating the engine. Place your crank socket and wrench on the crank and rotate the engine two complete revolutions. Your engine operates over 720°, not 360° as many people believe. With your 4-cycle engine this mandates rotating the engine two complete revolutions. If when turning the engine you feel resistance beyond normal, be careful, you may have contacted the valve against the piston.
Now take the cylinder head off carefully so as to not disturb the clay. When you remove the head you will notice the valve indentations in the clay. Look closely at the clay and you will see thinner and thicker areas created by the valve contact. The main areas you need to look at are the depth and edges of the eyebrows (valve reliefs), and the edges of the piston against the block (especially on big block Chevrolets).
Carefully take your razor knife at various locations on the clay and slice it. You can cut a section out to give unobstructed access and then use your caliper to measure the thickness of the clay that is still sitting on the piston top. Do not not try to measure the clay you removed, because in most cases you will stretch it and change it's dimensions upon removal and handling. Check the clay in various locations to get a detailed view at what the valves need for clearance. If you find areas that have less than admirable clearances you will need to go to the next step, which means machining will be required.
If There is Contact or Too Little Clearance, What Must I Do?
With the dimensions you found above, you hopefully took notes and wrote down the findings. With this info in hand, you can go to your machinist and tell him what you found. An easy tip is to have one of the other pistons and you can write on top of it with a thin marker the various measurement locations. If you did get measurements that require machining, I hope you had not already balanced your engine's reciprocating parts, because you will be paying the machinist to do it again after your items are machined for valve clearance. Machining for valve clearance changes the piston weight and will require you to correct the balance of the reciprocating assembly.
In applications where you have contact, the most common contact points are in the piston valve reliefs (eyebrows). Either they are not wide enough from the use of larger valves, or they are at the wrong angle. This happens when you use aftermarket heads, larger valves in OEM heads, or you are using heads that have been angle-milled. With the information you have learned from checking the piston-to-valve clearance you will be able to tell your machinist what is needed. If you are unsure of what to tell him, simply leave the clay on the piston(s) during disassembly and take them into the machine shop for a more experienced inspection by your machinist. Remember, taking any material off the pistons will lower your compression ratio. Taking too much can cause other problems (piston top gets too thin, etc).
After you get your items back from the machinist you will need to re-test your piston-to-valve clearance. Go through the same procedure listed above.
MONEY & TIME-SAVING TIP: It is best to have the machinist setup and machine only one piston assembly. This will save you a lot of grief and expense and then once you have double-checked your clearances you can have the machinist copy this to the other pistons and have your reciprocating assembly balanced "after all test-fit procedures are completed".
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