Cylinder sealing is a topic that should be at the top of any serious engine builder's mind. After all, pressure is power, and letting precious cylinder pressure slip past the rings is a waste of potential power. Many components add up to achieving the goal of cylinder sealing at the cylinder wall, including the pistons, rings, and bores. These are broad areas, and each can be analyzed down into subtopics. With rings, we might investigate twist, taper or face profile, back clearance, width, materials, coatings, hardness, tension, and a laundry list of other nuances. In contrast, the cylinder bores seem like a simple aspect of the sealing system, but such an impression is definitely mistaken. There's much more to effective bores than having eight holes you can stuff some pistons into.

Here's some news for anyone who has slid a dial bore gauge up and down a bore, and then marveled at how true and distortion free it seems. True bore distortion cannot be measured with a dial bore gauge. Basically, a bore gauge will indicate length between two points, but there is no axis of reference for the measurement. This can cause a serious misconception about how good a bore is--a serious defect in bore shape can easily be missed. In its ongoing research, Sunnen uses an Inner Contour Meter, a highly accurate device that measures the true bore shape, reading the position of the cylinder wall every 5 degrees around the bore and along its full length. The need to intricately evaluate a cylinder bore is essential for a company like Sunnen. Sunnen relies on such information and testing as a basis for continual refinement of equipment, tooling, and procedures. This information is passed on to Sunnen's customers in the performance engine machining industry via improved processes and equipment, and ultimately we all benefit from the development.

The Goal: Minimize Bore Distortion
An ideal bore will come as close to perfection as possible in key geometric qualities, including straightness, roundness, and cylindricity. Simulating the loads and stresses that may affect the bore shape in operation through mechanical bore distortion compounds the difficulty of achieving this goal. Some of the key factors contributing to mechanical bore distortion include torque plate design (see sidebar: Maximizing Torque Plate Performance), head or torque plate fastener type, torque sequence and lubrication, and the head gasket type. Other easily overlooked sources of mechanical distortion include the machining techniques, stack-up of tolerances, and external loads like the engine support system or mounts. There is value in identifying the contributing sources of mechanical bore distortion, and either attempt to minimize them, or compensate for their influence during the cylinder bore honing operation.

Surface FinishWe have concentrated our discussion on bore shape, but surface finish is another characteristic vital to engine sealing and life. The most basic measure of surface finish is the Ra value, which is the roughness average. The Ra is a specification denoting the range from the highest peak to the lowest valley in the surface, and takes an average of that surface finish. Rmax and Rz show a little more detail--showing total height to valley, over a certain segment, but it still isn't enough since these measures do not indicate how much area the ring has to ride on vs. how much of the surface is valley. Bearing area will help fill in the picture, by adding information on how much peak is available for the rings to ride on. These are just some of the parameters in surface finish looked at by Sunnen in creating finishing procedures.

A variety of processes result in varied surface finishes. The most common of these processes may be described as a single-grit finish, multiple-step finishes, and a plateau finish. A single-grit finish is the simplest, basically where you bore the block, and then hone it with one grit of abrasive. A multi-step finish is where the walls are brought smoother and smoother in steps, such as going from 220, 280, 400, to 600 grit. A plateau finish is characterized by a comparatively rough finish followed by a very fine finish. This produces effective valley areas, in conjunction with flat peaks that provide significant bearing area. For high-performance engines, the plateau technique is still considered one of the best for surface finish.

There is no doubt that getting into the real science behind cylinder bore shape and finish is beyond the means of most engine builders. We are grateful that research in this area is ongoing thanks to the work of companies like Sunnen, a company that is always raising the bar. With continued analysis and refinements of equipment and techniques, it's worth paying attention to your Sunnen rep if your goal is raising your game.