EngineLabs Blueprint Series: Verifying Piston-To-Head Clearance

Enthusiasts sometimes balk at the perceived high cost of a custom-built performance engine. True blueprinting, however, demands that every clearance in the engine be checked – and if necessary, modified – to fit the desired parameters. In this episode of our Blueprinting series, we’ll look at piston-to-head clearance.

In a simple world of rebuilding a stock engine, new pistons are installed on the original rods, the crank is ground 0.010-inch-under and everything bolts together nicely. It’s rarely that simple when it comes to juggling a stroker crank, longer rods, custom pistons, and a skewed deck that is anything but square.

Piston-to-head clearance is a little like that old variety act where the artist runs around the stage trying to keep eight plates all spinning on the top of narrow wooden poles. There are literally over a dozen variables that all affect piston-to-head clearance. We’ll start with the simpler ones and then move to some of the more complex conditions.

Feeling The Squeeze

Besides the fact that tightening the piston-to-head clearance improves compression, this move also has an effect on the quench area. On a wedge design cylinder head, the quench is the flat portion of the chamber that is matched by a similar flat area on the piston. As the piston reaches top dead center (TDC), the air and fuel in this area are “squished” or pushed outward into the active portion of the chamber. Tightening this piston-to-head space to less than 0.050-inch improves combustion efficiency by more actively mixing the air and fuel which also has the positive effect of making the engine less sensitive to detonation.

Gasket thickness plays a big role in determining piston-to-head clearance. For example, a small-block Chevy with pistons 0.025-inch down in the cylinder could benefit from a set of rubber coated 0.015-inch shim head gaskets, like those sold by Fel-Pro, as a way to tighten the piston-to-head clearance.

The easiest way to check for piston-to-head clearance on a flat top or dished piston is to install the crank, piston, and connecting rod combination with the appropriate bearings where the clearance has already been set. The piston doesn’t need the ring package installed. Rotate the piston to close to TDC and mount a magnetic base dial indicator on the block and zero the gauge to the block deck surface.

Now swing the dial indicator plunger near the wrist pin centerline at the edge of the piston and slowly rotate the crank through piston TDC. The piston’s maximum travel will indicate a range of zero through perhaps as much as 0.025-inch below deck. We’ve measured pure stock 400ci small-block Chevys with their pistons 0.040-inch in the hole before.

In the opposite direction, production LS engines often extend the piston above the deck. This does not mean the piston will contact the cylinder head since there is also the head gasket thickness to consider. But for the sake of this discussion, we’ll assume the piston is below the deck. While some may assume that this would represent the piston-to-deck clearance for all eight pistons in a V8, the reality is quite different.

Checking piston-to-head clearance is easy with a flat top piston and should be checked at the wrist pin centerline to minimize the effect of piston rock. A deck height tool is a handy way to accomplish this task. In this mockup before the block has been decked, the piston is 0.020-inch below the deck (left). Checking piston rock is generally done at 90 degrees to the wrist pin with the piston at TDC. Measure both extremes of the piston rock movement and then calculate the average to determine piston-to-head. Sometimes this has to be done because piston dome location does not allow checking at the wrist pin centerline (right).

Sample Size Matters

Few production engine block decks are truly square to the crankshaft centerline. At a later date we’ll get into why merely decking a block is not as good as machining the deck relative to the crank centerline. In most cases, the block is not “square” so checking piston-to-deck should be done at all four corners of the block to indicate how close the deck is parallel to the crank. Let’s assume that for a small-block Chevy that our deck height check reveals that the Number 1 cylinder is 0.010-inch below while Number 7 checks 0.008-inch below, Number 2 is only 0.004-inch below and Number 8 is 0.010-inch shy of the deck.

Since the tightest number is 0.004-inch below deck, that could be our zero point, allowing our machine shop to mill the decks on both sides to create a 0.004-inch below deck final spec. Of course, we could also just have the block machined for a zero number just as easily. This would depend upon exactly how much compression we wanted to run and would also depend heavily on the head gasket thickness. If we are after maximum compression, then a zero deck would be advantageous. For the purpose of this discussion, we’ll put the piston at 0.004-inch below the deck.

Our piston-to-head clearance would then be determined by the addition of the piston-below-deck distance added to the compressed head gasket thickness. 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.

This is a worthwhile test to perform and can be done very easily. Position the piston in the bore at TDC and place the dial indicator at 90-degrees to the wrist pin. Then rock the piston by pushing on the top of the piston on opposite sides. The dial indicator will generally show movement of around 0.010- to perhaps 0.015-inch. Let’s say our piston rock numbers are 0.020-inch and 0.010-inch. Averaging them would give us a piston deck height of 0.015-inch, but this also means the piston could come to within 0.010-inch of the head plus the head gasket thickness.

Never assume that the block is square. Big-block Chevys are especially prone to sloping decks that can be off by 0.010-inch or more front-to-back or side to side. One way to quickly determine if the block surface is “square” to the crank centerline is to measure deck clearance at all four corners. The correct way to minimize this issue is to machine the block square to the crankshaft centerline.

Dealing With Domes

So far, we have only dealt with a flat top or dished pistons, but plenty of engines are built every day with rather large domes in search of compression. While a piston deck might miss the head with sufficient clearance, it’s possible that the dome may strike the head so this clearance must also be verified. The easiest way to do this is to install a piston in the cylinder and place small pieces of clay around the dome where the piston may contact.

Then install the cylinder head and head gasket and torque the head in place. Rotate the engine gently by hand over at least four rotations to ensure the piston has rotated past TDC at least twice. Then measure the thickness of the clay impression to determine the piston-dome-to-chamber clearance. This should be roughly 0.050-inch in order to compensate for piston rock, which is amplified with a dome that is a greater distance from the wrist pin.

The length of a piston skirt combined with its clearance plays a major factor in the amount of piston rock in the cylinder at TDC. A shorter skirt and a 0.005-inch piston-to-wall clearance will generate more piston rock than a longer skirt hypereutectic piston running a tight 0.0015-inch piston-to-wall clearance.

Another way to do this is illustrated in Reher-Morrison’s engine assembly book. This involves painting the cylinder head deck surface around two adjacent cylinders with blue machinists dye. Then mount the head on the block and scribe a line on the head using the bore as a guide from underneath. Use a wrist pin to attach two pistons together and lay them on a cylinder head on the bench placing the pistons inside the bore scribe lines. If the piston dome touches the chamber before the quench flat on the piston contacts the head, then this indicates where the dome will contact the head and will need massaging. This may not necessarily be at the dome’s tallest point.

This should be performed on each cylinder if the piston-to-head clearance is run tighter than normal. The builder cannot assume that each piston will clear, just because one piston indicated adequate clearance.

That shiny mark on the piston occurred because the edge of the chamber contacted the edge of the dome on this big-block in the same spot on two pistons. This occurred because these were cast pistons and slight variations caused contact. This clearly indicated these pistons pushed ever-so-slightly past the piston-to-head clearance limit.

Another issue to consider can be illustrated by this example: During disassembly of an engine, we used the old set of pistons to establish deck clearance because the new pistons had not yet arrived. The block was decked based on those measurements. Then we discovered that compression height was taller on the new pistons, placing them 0.010-inch taller in the block, protruding 0.006-inch above the deck.

This required us to use a thicker head gasket to compensate. This worked, but illustrates that we should have waited for the new pistons and checked this critical dimension with the pistons that we would be using. Lesson learned.

As you can see, there are plenty of variables when it comes to determining piston-to-head clearance. Checking all clearances thoroughly is the only way to know that the engine will live a long, prosperous life. And that’s an excellent goal for any engine builder.

Aluminum rods will require more piston-to-head clearance than steel rods since aluminum expands almost twice as much as steel when heated.

Piston-To-Head Clearances

These are generic recommendations for a typical street engine. Tighter clearances can be achieved but the builder does so at his own risk.

CONNECTING ROD MATERIAL PISTON-TO-HEAD CLEARANCE RANGE
Steel 0.038 – 0.045 in.
Aluminum 0.050 – 0.065 in.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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