Blueprint Series: Measuring And Setting Deck Height And Finish

Performance engines thrive on proper assembly, which is what blueprinting is all about. It means pre-assembling an engine several times to ensure that every spec agrees with the engine builder’s plan. If a spec isn’t right, then steps are taken to correct it.

One seemingly innocent yet critical dimension is deck height. This term is often used in two different ways. The first defines the actual distance from cylinder head mounting surface to the crankshaft centerline. As an example, a big-block Chevy’s deck height is 9.800 inches.

The other commonly used measurement is the difference in height from the flat portion of the top of the piston relative to the block’s deck surface. This can be where the piston is either above or below the block’s deck surface and what is referenced when the assembly is said to be “zero decked” — that is, the piston is perfectly even with the deck surface at TDC.

While this sounds simple enough, because we deal with production engines and assembly-line parts, actual measurements don’t always agree with the factory specs. For example, it is common to find the deck surface on the block is not parallel to the crankshaft centerline.

If the block has been align-bored, it can change the distance from crank centerline to the deck, adding to this wandering of specs. Add in the variables of connecting-rod length and changes to the piston’s compression-height and it’s not unusual to find deck height variations of 0.005 inch or more.

To measure deck height, first zero the indicator on the deck surface. With this near-flat-top big-block piston, we positioned the indicator at the wrist-pin centerline to minimize the effect of piston rock. This piston measured at 0.020 inch below the deck.

Measuring Instead of Guessing

The only way to truly know this dimension is to measure it. On an engine already assembled with the heads removed, it is an easy procedure. For a rebuilt engine, it requires pre-assembly for several reasons. First, we want all the pistons to have as close to the same relative deck height as possible. This has a direct effect on the engine’s compression ratio, and that needs to be as consistent as possible.

Piston deck height also has a critical impact on piston-to-head clearance. So you can see how a flat and parallel deck surface affects several engine performance areas, especially when pushing piston-to-head clearance to the minimum in order to maximize compression.

Common OEM Deck Heights

Ford Chevy
289-302 8.206 283-350-400 9.025
Boss 302 / SVO 8.201  / 8.210 LS Gen III/IV/V 9.240
4.6L – 5.0L Modular 8.937 396-454 MK IV Gen V/VI 9.800
351W 9.480 – 9.503 Chrysler
351 C Boss 9.206 273-318- Poly 9.600
351C, 400-2V 10.292 – 10.302 273-318-340-360 LA 9.600
FE 332-428 10.170 Chrysler LA Race 9.560
429-460 “385” 10.300 – 10.322 361-383-400 B 9.980
5.4L Mod 10.079 413-426-440 RB 10.725
426 Hemi 10.725
5.7L- 6.4L Hemi 9.240

The best way to determine the relationship between your rotating assembly and block is to mock-up at least the four corners. Many performance piston sets now come as full-floating kits where the wrist pin is not an interference-fit to the connecting rod. This makes it easy to mock up one piston and rod.

That is what we did on a recent 540ci big-block engine: We supported the crankshaft with the front and rear main bearings and then pre-assembled a single piston and rod assembly. We then installed this assembly in each of the four corner cylinders. If you really want to be thorough, you could assemble all four — or even all eight — pistons and rods for each of the holes.

If it is not possible to measure piston deck height at the wrist pin centerline due to piston dome obstructions, you can pick an appropriate place and then rock the piston back and forth to measure the “swing” in measurements. Add the two together and divide by two to obtain an approximation of the deck height.

Measuring deck height was the easiest part of this process. We used an inexpensive deck bridge and a dial indicator. Place the bridge on the block and zero the gauge, then set the dial indicator up as close to the wrist-pin centerline as possible. This minimizes piston rock across top dead center (TDC).

If the piston is a pure flat-top, it’s best to measure directly in the center of the piston, but it’s usually easier to measure toward one side. If you are an accuracy freak, you can check both sides of the piston just to verify the numbers.

Here, we are measuring the piston height directly in the center of the piston. Checking at both ends established the same dimension in this particular application at .000 inch deck height.

If it’s not possible to find a flat portion of the piston near the wrist-pin centerline (as is sometimes the case with large dome pistons), find a suitable flat portion on the piston at TDC and rock the piston back and forth noting the two indicator numbers. These figures might range between 0.004 to 0.012 inch, as an example. Add these two numbers and divide by two to obtain the actual deck height. For this example, it would be 0.008-inch.

Measuring the 540ci big-block project, we found piston deck height numbers with the piston between 0.021 and 0.024 inch in the hole — that is, below the deck’s surface at TDC. We recorded these numbers and took the block to our favorite machine shop – Jim Grubbs Motorsports (JGM) in Valencia, California.

This is a brand new Gen-V iron block and JE Pistons assembly where we discovered a 0.023-inch-below-deck position. That required removing 0.018 inch of material from the deck to establish the desired 0.005-inch in-the-hole deck height.

At the Machine Shop

Ryan Peart, JGM’s owner, found his measurements concurred with ours (within 0.001 inch) and decked the block to produce our desired deck height (we opted to place the piston 0.005 inch below the deck). He also discovered that our block was tapered slightly from top to bottom on both deck surfaces by 0.003 inch.

While that taper isn’t a huge issue, it reveals why engine building must take in all the variables. This discrepancy was eliminated by the first cut but does point out that if not decked, this would also affect other variables such as valve angle relative to piston valve-reliefs.

In this photo, JGM’s new owner, Ryan Peart, is using the BHJ Blok-Tru fixture to accurately position the block for machining. The fixture uses both the crank and camshaft bores to locate a crankshaft centerline. The flat portion of the fixture is used to establish an accurate deck angle and height relative to the crankshaft.

JGM uses the BHJ Blok-Tru fixture to establish the proper block orientation. This fixture locates the block using both the crank and camshaft bores along with a perpendicular surface marked with an accurate distance. Measuring from that reference point to the top of the block will produce the actual deck height from the crank centerline.

Comparing measurements front to rear will establish if the block deck surface is truly parallel to the crank centerline. If the machine shop merely machines the deck “flat,” there is no guarantee it is parallel to the crankshaft centerline. Since that measurement is an important part of the engine build, it might be worthwhile to ask your machine shop if they use a fixture to measure the deck to the crank centerline.

Once the block is properly positioned in the fixture and the actual heights are measured, then milling can begin. The last cut usually removes a small amount at a slower rate to create a smoother deck surface. This photo is of an iron LS block being machined.

Smooth As Glass

Another variable worth investigation is the final deck surface finish. The engineers at Fel-Pro tell us that this finish is expressed as the average of the microscopic peaks and valleys — abbreviated with the term Ra. This surface finish directly affects the sealing ability for different gaskets. We’ve included a chart that Fel-Pro recommends depending upon the style of head gasket you are using. As you will note, MLS gaskets require a far smoother finish than older style gaskets.

We recently ran across a video posted on the internet that showed how they “surfaced” an aluminum cylinder head by gluing some sandpaper on the work surface and pulling the cylinder head across this 60-grit and later 150-grit surface until the head was “flat.” If you’re living on a deserted island and have to get the engine running, this might work. Please don’t do this unless your name is Barney Rubble.

In the case of that 540 big-block, with the block back in the shop, we again pre-assembled the engine to recheck our deck heights and were rewarded with more consistent deck height measurements within 0.001 inch. Now our piston-to-head clearances will be far more consistent, as will each cylinder’s compression.

MLS gaskets, like this Fel-Pro small-block gasket, require a smoother surface finish for optimal sealing. Different gaskets have different surface finish requirements.

With the previous combination, our piston-to-head clearance would have been 0.060-plus inch, while now it’s closer to 0.046 inch. An ideal number would be 0.040 inch for a street engine with steel rods. Aluminum connecting rods would require additional deck height clearance closer to 0.060 inch.

Tightening up the piston-to-head clearance is a wise move since this makes the quench portion of a wedge head more active. Tightening the quench to around 0.040 inch still allows sufficient piston-to-head clearance but creates a more active combustion chamber that promotes more even and complete combustion, and can decrease an engine’s sensitivity to detonation. Compression on this particular 540 engine also improved from its previous 9.71:1 to its current 10.1:1.

Proper engine assembly is as much about taking care of all the little details as it is making sure all the fasteners are tight. While a flat deck may not add 50 horsepower, the engine will be happier and be far more responsive to small changes. If nothing else, you will know you did it right.

Fel-Pro Deck Surface Recommendations

Cast Iron block and heads 60 – 100
Aluminum block and heads 50 – 60
OE MLS gasket 20-30 or less
Fel-Pro Perma-Torque MLS <60.

Article Sources

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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