Engine Block Deck Surface Explained

Engine Block Deck Surface DiagramThe deck surface on an engine block is one of the most critical areas outside of the cylinder bores and main bores. The deck surface allows cylinder heads to be fastened to the block, between a head gasket, and an improper seal can result in coolant leaks that contaminate the engine's oil. In the image to the left you will see a 350 Chevy bare block, which had the deck surface machined in a CNC machining center. If you click on the picture, you can enlarge the photograph to see a close-up view of the deck.

As you can tell by the picture above, the deck surface has a smooth machined finish. The surface finish is ordinarily measured by its RA (roughness average measured in micro-inches) using a profilometer. However, some high performance applications also measure RP (maximum peak height) and RV (maximum valley depth) microfinish values as well. But for all intensive purposes, the average engine builder will be looking at the RA value that is needed for their customer to run his/her desired cylinder head gaskets. Those using a graphite head gasket on a cast iron engine block typically need a 60-120 RA microfinish. However, aluminum blocks require an even smoother finish at 20-50 RA for the same graphite gasket. Customers who intend on running a multi-layer steel (MLS) cylinder head gasket typically need the deck surface on the block to have a 15-30 RA at a minimum.

High performance applications, such as those for professional racing, street rods, etc., may also require that the deck be finished at a specific height. For example, to achieve a specific compression the preferred piston location may be at zero deck at its highest point. Deck height is calculated from the centerline of the main housing bore to the deck surface. Once this is determined, the appropriate amount of material may be removed during the milling process to achieve the desired finished deck height.

Detailed Deck Surface ViewAn engine block's deck also contains other important features that allow cylinder heads to perform their job. In the image to the right (click it to enlarge) you will see some important areas pointed out. First you will see the deck dowel pin holes. These holes contain dowel pins (removed from the picture for surface milling) that allow engine builders and automotive repair technicians to easily locate the cylinder head gaskets and heads onto the block. There are also many coolant passages on the engine block's deck, which allow coolant to flow into the cylinder heads so that they stay cool as the engine is operating. Finally, the deck of the block contains threaded holes to fasten the cylinder heads with either bolts or studs.

Milling and Resurfacing

Because an engine block's deck is not a wear surface, they often need no machine work when an engine is being rebuilt. However, heat can cause the deck to warp or an improper microfinish may not allow the vehicle owner to utilize the type of cylinder head gasket that he or she prefers. To refinish the deck surface of the engine block, the machining process used is called milling but is also referred to as resurfacing, decking and squaring. Milling is often performed using a rotary broach or preferably with a CNC machining center. The advantage of using a CNC machining center is that the proper angles can be maintained on the deck surface, which automotive machinists refer to as "square decking."

To achieve a proper microfinish on the engine block's deck surface, the appropriate cutters must be used in the milling process. Carbide cutting bits are often used in rotary broaches, with shallow cuts and a slow feed, that are fine for cast iron engine blocks that will be assembled with graphite head gaskets. Automotive machinists prefer to use cubic boron nitride (CBN) inserts for cast iron blocks or polycrystaline diamond (PCD) inserts for aluminum blocks as they allow the milling machine to take larger cuts at much faster feeds than standard carbide bits. Regardless of the cutter used, additional consideration must be given to those engine blocks that are using cylinder sleeves of a different material than the block they are installed inside of. In nearly all cases, a final finish cut of .001" to .003" is preferred to provide for a smooth finish that is free of chatter and/or grooves.

Deck Surface Repairs

Helicoil Thread Repair KitA problem commonly seen on engine block decks are stripped cylinder head bolt holes. Regardless if the engine block is made of cast iron or aluminum, helicoils may be used to repair the threads. If the short block is assembled, and a bolt or stud will not torque down when the heads are installed, drilling for the helicoil must be performed while the pistons, rods and crankshaft are installed. Obviously it is not good to expose these components to chips produced by drilling/tapping for the installation of a helicoil. Therefore, it is smart to fully inspect all of the head bolt holes prior to building the short block assembly.

Engine blocks do crack on the deck surface and their weak points does vary by the manufacturer and engine model. Cast iron does not weld easily and cracks are often repaired by pinning. Using a drill bit, holes are drilled into the crack and special tapered pins are installed. Each pin slightly overlaps the one previously installed to create a crack repair that will maintain its seal under high heat and compression. Aluminum cylinder blocks may be welded, and most automotive machinists prefer to use a TIG welder with matching filler rod to perform the crack repair. When cracks repairs are performed on aluminum engine blocks with a weld, the crack is first slightly underground first so that the weld has an additional bonding area for added strength.

Now that you know more about the engine block deck surface, we encourage you to visit our engine blocks information page to learn more about the other components of an engine block. If you are shopping for a new engine block, please also consider visiting our Chevy, Ford and Mopar pages to locate blocks suitable for all applications.


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In this section of our website you will find important tips that will help you to identify and purchase the best engine block for your application.
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Deck Surface Facts