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What is Deck Height? How to Calculate and What it Means

October 18, 2018 / by Jeff Smith

Deck height is a vital engine measurement that dictates rod length, crankshaft stroke, piston-to-head clearance, and so much more. Here, we define deck height, how to measure it, and its impact on your next engine build. 

It’s all part of the art of building a performance or competition engine. The details separate the exceptional from the also-rans. Some specs like rod and main bearings receive a majority of the attention, but ignore something as simple as deck height and you could find a piston smacking the head at high rpm is not a good way for reciprocating parts to become acquainted.

Deck height is simple: the distance from the crank centerline to the deck surface of the block. A standard deck small-block Chevy is 9.025, but there are plenty of variations when it comes to aftermarket blocks and used cores.

This really isn’t a critical dimension if a standard rebuild is the goal. But if you’re a performance engine builder and stroker cranks, longer rods, shaved decks, and custom pistons are your thing – then block deck height is an important dimension that demands attention. For the record, deck height is the distance between crankshaft centerline and the top of the block.

Deck machining and align boring can affect the overall deck height of any engine. Especially with older cast blocks that have been subjected to previous machine work, this demands that the deck height be verified.

Deck height dictates many engine dimensions. It limits stroke, rod length, and piston compression height. This last dimension is a critical part of the rotating assembly equation and is defined as the distance from the wrist pin centerline to the piston deck. For the record, piston domes often protrude past the deck into the chamber, but compression height establishes the distance to the deck.

A simple way to check deck height variation is to measure the piston position relative to the deck using the same piston and rod on all four corners of the block. While perhaps not accurate to 0.001-inch, it is an easy way to determine whether the deck is square.

All of this is important if you want to add stroke to the crankshaft to increase displacement. Let’s use the classic small-block Chevy 350 as a case study. Our example uses a 9.025-inch deck height, along with a 3.48-inch stroke and a 5.7-inch connecting rod.

This leaves the last variable as piston compression height. The simple math calls for dividing the stroke in half (3.48 / 2 = 1.74 inches) and then adding the connecting rod length and compression height. This is then subtracted from the engine’s deck height to find the deck clearance, which can be positive (above deck) or negative (below deck).

We looked up a typical 350 piston with a 5.70-inch rod and found compression height listed as 1.560 inches. Adding 1.56 + 1.74 + 5.70 = 9.00 inches. With a small-block’s deck height listed as 9.025, this appears to put this piston 0.025-inch below the deck. This is not unusual because many (but not all) production engines place the piston deck below the block surface. We added the qualifier because, for example, the new generation LS engines typically push the piston deck around 0.005 to 0.008-inch above the block deck so it pays to know the specifics of your engine package when making changes.

Stock engine piston position can vary, especially with older engines. The early ‘70s 400 engines had notoriously shallow pistons. We checked this bone stock 400 and discovered all eight pistons were between 0.035 and 0.040-inch below the deck. This one measures 0.038-inch. There are numerous reasons why this is a bad idea including low static compression and virtually eliminating the quench area.

Each piston manufacturer establishes a specific compression height that allows the engine builder to work around this number. Where things get sticky is that not every block measures up to its specs. Because of production tolerances, and decks that may have been cut to reestablish flatness during previous engine rebuilds, block deck heights will vary. Big-block Chevy production blocks, for example, are especially noted for their dimensional instability.

The combination of crank stroke, connecting rod length, and piston compression height must add up to a number very close to the block deck height. This iron 6.0L LS engine pushes the stock pistons about 0.008-inch above the deck, which helps compensate for the 0.053-inch MLS head gasket thickness to create a reasonable 0.045-inch piston-to-head clearance.

The proper way to deck a block involves using a mandrel placed in the crankshaft main webs to simulate the centerline of the crankshaft. Then measurements are taken from the outside diameter of the mandrel to the block deck surface at multiple positions. This establishes whether the deck is parallel to the crank centerline. Often, this is not the case. We’ve seen production deck surfaces tilted front to back by 0.015-inch. We recently witnessed a small-block that was oddly bowed in the center by 0.008-inch.

Given these inequities, you can see that simply leveling a block in a fixture and then milling the deck “flat” could in fact make a bad situation worse. A simple way to evaluate an engine’s deck alignment is to use the same piston to check piston-to-deck heights at the engine’s four corners and compare the numbers. If all the deck heights are within 0.002 to 0.003-inch, consider yourself lucky. This still leaves plenty of opportunity for the block to be “out of square” but at least it will be close.

This illustration shows what deck height is actually measuring.

In our 350 Chevy example, if the block actually measured 9.025-inches with the pistons 0.025-inch below deck, the engine builder would have the option of machining the block which would reduce the deck clearance and also increase compression ratio. Besides the effect on compression, another issue is determining the piston-to-head clearance (also known as quench). This is established by the piston’s position relative to the deck combined with the compressed thickness of the head gasket. As an example, if the piston was located 0.005-inch below the deck surface combined with a 0.039-inch head gasket, this would create a piston-to-head clearance of 0.044-inch.

Here is a cutaway illustration of a piston at top dead center to show deck height in action.

This dimension has a significant effect on the static compression ratio as well, so it’s up to the engine builder to determine both the minimum piston-to-head clearance as well as the static compression ratio. All of these factors must be included when machining and assembling the engine.

Stroker crank packages have the biggest impact on block deck height.  We’ve included a quick example of the math necessary to calculate the compression height.

 

How to Calculate Compression Height

Block Deck Height – Conn Rod Length – (Stroke / 2)

Example: Small-block Chevy with 4.00 stroke and 6.00 inch rod

9.010 – 6.00 – (4.00/2 = 2.00)

Compression Height = 1.010

As shown in the chart example, a K1 4.00-inch stroke small-block Chevy crank for a 4.155-inch bore, 400 block with a 9.010-inch deck height demands a very short 1.010-inch compression height piston when used with a 6.00-inch rod. This would create a thumpin’ 434ci displacement small-block using a near-stock deck height with a flat top piston making over 12:1 compression with 64cc heads.

If big displacement is your goal, there are often also tall deck versions of the classic engine deck heights. Dart, for example offers iron tall deck versions of the small and big-block Chevy blocks that can be used to accommodate longer stroke versions. The tall deck 9.325-inch small-block Iron Eagle block, for example, offers the ability to build a small-block out to 460 inches.

So there’s plenty to think about when it comes to deck heights. None of this is rocket science, but it does deserve attention to detail.

Deck Heights of Popular Domestic V8 Engines

Chevy

Block                                                  Deck Height

283-350-400 SBC                            9.025

LS Gen III/IV/V                                 9.240

396-454 MK IV Gen V/VI  BBC       9.80

348-409                                            9.60

366-427 Truck Tall Deck BBC       10.20

Ford                          

Block                                                  Deck Height

289-302                                             8.206

Boss 302 / SVO                               8.201  / 8.210

4.6L – 5.0L Modular                        8.937

351W                                                 9.480 - 9.503

351 C Boss                                       9.206

351M                                                  10.297

Ford Racing 351W                           9.206 – 9.503

351C, 400-2V                                   10.292 – 10.302

FE 332-428                                       10.17

429-460 “385”                                   10.300 – 10.310 – 10.322

4.6L-5.0L Mod                                  8.937

5.4L Mod                                           10.079 

Chrysler

273-318- Poly                                 9.60               

273-318-340-360 LA                      9.60

Chrysler LA Race                            9.56

361-383-400 B                                9.98

413-426-440 RB                             10.725

426 Hemi                                        10.725

5.7L- 6.4L Hemi                              9.240

Buick

400-430-455                                     10.57 

Oldsmobile

400-425-455                                     10.625

Pontiac

326-350-389-400-428-455             10.20

Topics: Theory and How To, ENGINE TECH, featured, ENGINE BUILDS, CHEVY

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Written by Jeff Smith

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