What I Learned Today With Jeff Smith — Deck Height Disaster Averted

What I Learned Today With Jeff Smith — Deck Height Disaster Averted

We’ve had unexpected deck height issues bite us twice, delivering several lessons along the way. The first was a few years ago and we were in a hurry to assemble a 468ci big-block Chevy for a friend. We had planned to have the engine completed days earlier but several setbacks ate up all of our extra time and we had to get the engine assembled that night. We had all the parts and with the short-block assembled, we discovered to our disgust that the new pistons from a different manufacturer were a taller compression height.

These new pistons now sat 0.010 inch above the deck rather than the -0.005-inch deck height we had anticipated. This was going to be a problem since now we could not use our planned 0.041-inch compressed-thickness head gaskets because that would place the piston-to-head clearance at 0.028-inch on the tightest cylinder. This was entirely too close. This tightest cylinder was because the deck was not completely square with the crankshaft centerline, something that is not unusual with production big-block Chevys.

Our normal procedure after the engine is bored and honed is to bring the block back to our shop and pre-assemble the engine with pistons in all four corners of the engine. Then we can quickly measure the piston position at TDC on each corner and this would help us determine how much to have the deck milled. In this case, we skipped that step only to discover the problem after we had the short-block assembled.

measuring piston deck height

We prefer to measure deck height in the middle of the piston in line with the wrist pin. This tends to minimize (but not eliminate) piston rock as part of the measurement.

The solution was to go with a thicker 0.053-inch thick MLS head gasket that would increase the piston-to-head clearance back to a safer 0.040-inch on the tightest cylinder. Several companies such as Cometic and Fel-Pro offer these head gaskets in thicker applications for just this emergency and we were lucky enough to have a set of Fel-Pro gaskets on hand to solve this problem. Cometic also offers a series of multi-layer steel (MLS) gaskets in a variety of thicknesses that can be a lifesaver for projects that tend to go a little sideways.

We’ve also experienced situations with a small-block where the piston was much farther down the hole than anticipated. In one situation, we discovered a 355ci small-block with several pistons as much as 0.032 inch below the deck. If we had used a typical 0.041-inch compressed thickness gasket, this would have killed 0.6 points of compression ratio (9.0:1 instead of 9.6:1), which was wholly unacceptable. These deep pistons really hurt combustion efficiency since this makes the piston-to-head clearance (often called quench or squish) distance over 0.070 inch, which is just plain ugly. An ideal quench distance for a steel connecting rod street engine is closer to 0.038 to 0.040 inch.

We planned on using iron Vortec heads on this engine, so we decided on a Fel-Pro steel shim head gasket (P/N: 7733SH1). We sprayed it with some aluminum paint, since we didn’t have any copper paint (the classic recommendation) although I’m not sure it really needs any help. If this were an aluminum-headed engine, Cometic makes a 4.060-inch bore MLS gasket in 0.023-inch thickness that would help with the piston-to-head clearance and still do an excellent job of sealing cylinder pressure. This can be found under P/N: C5245-023. These gaskets are a little pricey but certainly less expensive and less hassle than completely disassembling the engine and decking the block. Sometimes there are shortcuts worth taking.

gasket deck height

With this big-block, our replacement pistons were a little taller than the originals pushing the new pistons above the deck. A 0.053-inch thickness MLS gasket solved our dilemma and saved our bacon.

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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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