Three views of a typical dynamic bore distortion scenario. (Photo Courtesy MAHLE Clevite)
Even though you may think that an engine block is a massive and strong component, it does experience movement that can and does affect the geometry of the cylinder bores. Regardless of the type of engine, it’s vital that you simulate, to the best of your ability, the assembled stress that the block experiences before the cylinders are final-honed to size.
Engine development researchers often take advantage of sophisticated laboratory equipment that allows them to “map” a cylinder bore in order to obtain a clear dimensional picture of how that bore is shaped, from top to bottom. APAT gauge system (inner contour meter) travels vertically through the cylinder bore centerline. It has a sensitive probe that monitors the cylinder bore, which provides a dimensional view of the entire bore. This provides an overhead radial view and an isometric view at various angles
Since most engine shops don’t have access to this level of equipment, bore diameter checks are made with a calibrated bore gauge before, during, and after cylinder honing. Bore diameter measurements are taken from top to bottom of the bore in four height locations, and at four clock positions (12, 6, 3, and 9 o’clock as viewed from overhead). If you measure a cylinder with the block in a relaxed state (no deck plate), and then take measurements at the same locations with a deck plate installed and fully torqued, it is very common to find different readings. This is clear evidence that the stresses caused by the cylinder head can affect cylinder bore geometry. Once you realize this, it should become clear that you should always hone cylinder bores with a deck plate installed.
An additional factor that can affect cylinder bore shape relates to the cylinder head. Depending on the hardness level of the cylinder head material, different levels of stress can be placed on the block, although they are beyond your control. This becomes important when swapping heads (for example, changing from cast iron to aluminum) because it could result in a change, however slight, to cylinder bore geometry.
Cylinder bore shape can and does change when bolts are tightened and during engine operation. You can minimize the effect of cylinder bore shape changes by prestressing the block during cylinder honing. In addition, always follow the same routine with regard to component installation. For instance, the same torque level and the same tightening sequence/pattern should be followed every time a cylinder head is installed. This includes the installation of the deck plate for honing, and every time the cylinder head is installed to the block. Maintaining this consistency of head bolt tightening helps to minimize changes in stresses to the block.
A deck plate should be installed and torqued to the required spec for both cylinder banks before a hone is performed. On a V engine, that means installing a deck plate on both banks. From a practical standpoint, however, most engine shops may only have one deck plate for a given type of engine. Using the same deck plate for both banks is common practice and certainly is acceptable. But if two identical deck plates are available, it’s a good idea to install both at the same time.
Also, always install a head gasket along with the deck plate. This further simulates the stress that the block sees when assembled. Be sure to use the same type of head gasket that will be used during final assembly, whether composite or multi-layer steel (MLS). Preferably, use a head gasket that has already been crushed (a used one). It doesn’t “settle” as much as a new gasket.
The type of head gasket is a consideration. MLS gaskets offer greater consistency of clamping load than a composite gasket with a wire ring “crush zone” around each cylinder bore. Although an MLS gasket typically relaxes by fewer than 10 percent after initial installation, a wire-ring type of gasket can relax by as much as 15 percent or so. In other words, MLS gaskets provide greater consistency in maintaining the clamping load between the head and block.
Two types of MLS gaskets are available: active and stopper. An active MLS cylinder-head gasket has a series of metal layers with gaps between the layers and an elastomer layer that allows the gasket to compress and seal with a slight spring effect. This helps to reduce block and head deck distortion. Stopper MLS gaskets include a “dead-stop” layer and rigid cylinder bore seals, which provide additional sealing under high cylinder pressure conditions. However, the stopper MLS gasket places more distortional stress against the cylinder head. When deciding between the two styles, it’s best to follow the recommendation of the gasket manufacturer for your specific application.
I’ve already discussed the importance of prestressing the engine block prior to honing the cylinders. The importance of mechanically stressing the block with a deck plate is paramount. Always hone the cylinders with a deck plate installed. With that said, you can also consider the stress imposed as a result of heat. Although the block experiences stress that affects cylinder bore shape when assembled (primarily as a result of cylinder head installation), cylinder bore geometry is also affected by engine operating heat.
In the pursuit of optimal shaped cylinder bores during operation, some builders use a process called “hot honing.” In simple terms, the block is connected to a heater that circulates hot water at a predetermined temperature. Cylinder bores are then honed with a deck plate and with hot water that elevates the block to a temperature that approximates operating water temperature. In theory, this more closely mimics the stress and temperature that the block experiences when it runs. The practice of hot honing is embraced by some engine builders and viewed as unnecessary by others.
As engine temperature increases, a cylinder wall commonly tends to assume a slight barrel shape as a result of thermal expansion. This expanded area has inconsistent high and low spots. The result is a cylinder that is slightly out of round and no longer uniform. However, the engine’s pressurized cooling system tends to partially counteract this as the water jackets that surround the cylinder try to push against the cylinder. If the builder plans to use a hot-honing process, the coolant must not only flow through the block, it must also be pressurized.
Damaged lifter bores can be over-bored, fitted with bronze liners, and then honed for proper lifter oil clearance. In addition, lifter bores can and should be corrected to attain the proper centerline and angle by using specialty indexing fixtures or by CNC machining. OEM blocks typically require these procedures because sloppy factory machining tolerances are common in mass production. But high-quality aftermarket performance blocks already have correct geometry, and may require only final-honing for oil clearance.
If you’re dealing with an old OEM block, pay attention to each lifter bore. Just because the lifter bores don’t look scored doesn’t mean that they’re all the same size. When an OEM block is machined to correct lifter bores, all bores are likely machined at the same time using multiple tooling. On occasion, one or more lifter bores may have been machined slightly oversize (to correct tooling flaws), in which case the factory may have installed oversize lifters in certain locations.
A CNC cutter spot faces lifter bore roofs in preparation of lifter bore centerline correction. With a programmed CNC machine, no indexing fixtures are needed for lifter bore correction.
Measure each lifter bore for diameter. If you have oddball sizing, your best solution