# “What I Learned Today” With Jeff Smith — Thin Is In For Quench

When building any engine, it’s important to pay attention to the little details like piston-to-head clearance and quench as a way to improve engine power and efficiency. In the case of a small-block Chevy, for example, let’s say we’re building a mild street 350 with a set of aftermarket 70 cc chamber heads and a set of flat-top pistons. Let’s also say that after the block has been lightly milled, you discover the pistons are still average about 0.018-inch below the deck.

Most casual engine builders would just add a pair of composite head gaskets that measure about 0.041-inch compressed thickness and be ready to bolt the heads on. But let’s look at this a little more carefully. By adding an 0.041 gasket, it places the cylinder head (0.041 + 0.020 = 0.061-inch) which is almost a 1/16-inch between the piston and the head.

Attention to the small details like head gasket thickness can make a measurable difference in engine performance. Sometimes a thinner head gasket can offer significant improvements in compression but you have to do your homework first to make sure all the clearances will be within spec.

Let’s compute our compression ratio based on a 4.030-inch bore, 3.48-inch stroke, along with the 70 cc combustion chamber volume, a 6 cc value for the four valve reliefs, and the aforementioned 0.020-inch deck height. All these factors compute to a 9.2:1 compression ratio. Many people would be content with this and just assemble the engine and be done with it. There’s nothing wrong with this approach. However, there are options we could explore to optimize compression and the quench area.

Quench is the flat area in a wedge-style combustion chamber that is adjacent to the matching flat area of the piston. Generally accepted tight clearance of piston-to-head in this area is around 0.035-inch. In our example with the 0.041-inch gasket, this clearance for our theoretical small-block is 0.059-inch.

This means we could tighten up the piston-to-head clearance with a thinner gasket. As an example, Fel-Pro makes a steel shim gasket with a very thin rubber coating that can be used with either iron or aluminum heads. This gasket is only 0.015-inch thick.

If we use this much thinner gasket, this creates a 0.035-inch piston-to-head clearance which is nearly ideal. But more importantly, this also drastically improves the static compression ratio from 9.2:1 to a far more optimized 9.73:1. With the optimized quench, this will improve mixture motion in the chamber and would likely improve power while also possibly reducing the amount of total timing the engine requires.

Of course, a good engine builder would probably have milled the block to reduce the piston-to-deck clearance to something around 0.008-inch and then could go with an MLS gasket from Victor-Reinz that is 0.026-inch thick which would produce a 0.034-inch piston-to-head clearance. This would also produce a similar 9.75:1 compression ratio. There are various other options available as well.

Knowing how much a piston sits in (or sticks out of) the cylinder is an important component of figuring quench. Ideally, you would have reasonable measurements for all of the components involved, and not lean on any one component to make up all of your clearance.

Cometic, for example, offers an MLS gasket with a tighter 4.060-inch bore and a 0.023-inch compressed thickness that would also work well for an engine with a piston about 0.010-0.012-inch below the deck. The slightly smaller 4.060-inch bore would also increase the compression compared to a 4.125-inch bore gasket but that 0.060-inch bore difference is only worth about a tenth of cc in terms of volume.

We’ve listed a few of the gaskets, thicknesses, bore diameters, and their part numbers in the following chart that may be of some use. Of course, this same technique can be used on any engine. The only limitations are the breadth of gaskets available from the manufacturers. But it pays to do your research ahead of the final engine assembly.

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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