A magnetic deck bridge is an indispensable tool that belongs in the toolbox of any engine builder intent on performing precision engine assembly. A deck bridge is a basic comparator tool that allows you to easily read dimensional differences between adjacent surfaces like the deck surface of your engine block and the top of your pistons. It is most commonly used to check deck height (either positive or negative) with the accompanying dial indicator. The tool is a bridge shaped aluminum bar with legs on either end that allow it to directly span the distance across most cylinder bores up to 4.5-inches. Deck bridges use a dial indicator (usually sold separately) mounted in the center of the tool and there are usually provisions to mount the indicator off-center to either side if required. High quality units like the ProForm tool shown here have magnetic inserts on each leg so the bridge will remain snugly in place and provide consistent readings. The magnets are strong enough to hold the bridge in position, but you can still slide it around to the most desired location with minimal effort.
To check deck height you set the tool on the deck surface of the block adjacent to the cylinder bore so the dial indicator stem rests on the same surface as the deck bridge. Then rotate the dial to zero the indicator. Now whenever the indicator is reading zero it indicates the level of the block deck surface. Next move the bridge so the indicator stem is above the piston top. Bring the piston up to TDC (top dead center) and determine the highest point of travel by observing the dial to see exactly where the pointer stops traveling and reverses direction. At this point you can check the deck height which will be either positive or negative depending on whether the piston is slightly down the bore or protruding slightly above the deck surface of the block. When checking deck height keep in mind that it must be checked against flat surfaces on the piston top. Domes, dishes and valve reliefs will not provide accurate deck measurements. The comparison you are making is between the flat surface of the piston deck and the deck surface of the block.
Most instructions tell you to check the deck height adjacent to the minor and major thrust side of the piston and take the average to allow for piston rock. Certainly an acceptable method, but this is usually accomplished with pistons that don’t yet have rings installed. Piston rock is less with the ring pack installed so we like to check deck height along the pin axis at both ends of the piston. This yields absolute deck height and you can judge from there how much piston rock might affect piston to head clearance. If it’s too close, the piston may indeed acquire a witness mark if it slightly kisses the head. This unloads the ring and bleeds cylinder pressure so you want to avoid it. One thing you can do is to check absolute deck height and also piston rock and compare the two to make a final judgment. Some builders also rig their deck bridges with dual dial indicators mounted in the outboard holes so they can check both sides of the piston at the same time. In this case you must remember to zero bother indicators carefully to yield precise results.
The dial indicator reads in .001-inch hash marks so it delivers very precise results. If you work carefully you can also use it to determine exact top dead center for the purpose of degreeing a camshaft. Using the deck bridge with a degree wheel is easy. Bring the number one piston up to approximate TDC and install the degree wheel with a suitable pointer adjusted to indicate zero. Then install the deck bridge and zero the indicator. Rotate the engine in reverse a short distance and then slowly rotate it forward toward TDC. Watch for the exact point where the indicator stops moving and note the reading on the degree wheel. Then continue rotating until the indicator just starts to move downward. Note the degree wheel reading again. This will normally cover a range of several degrees. Once you determine this range, loosen the degree wheel and rotate it half the distance of your observed travel. It works the same as a piston stop except that you’re using piston movement starting and stopping points.
You can also modify your deck bridge to provide an indicator for CC’ing cylinder heads. Locate a length of aluminum rod similar in diameter to the dial indicator mount. Cut a three inch piece and grind one end to a fine point. It does not have to be perfectly centered. The tool will now allow you to CC combustion chambers without using a plastic deck plate to cover the chamber. Set up the cylinder head and level the deck surface with a small level. Then set the deck bridge on the cylinder head deck surface and install the pointed rod in the center hole allowing it to drop down until it also rests on the deck surface. Tighten the set screw and move the deck bridge over the chamber so the pointer is approximately centered. Slide your Burette in from the side and slowly fill the chamber until the checking fluid just touches the pointer. This indicates the chamber is full and you can read the CC’s on the burette scale. It’s less messy than using a CC plate smeared with grease and it’s easier to suck out the checking fluid with a syringe when you’re finished.
As a final note, you can also use the deck bridge with a set of dial calipers if you don't have an available dial indicator. There are three screws on on one side of the deck bridge. They are used to secure the dial indicator in the center or offset toward one side of the piston if desired. They can also be used to clamp a dial caliper in place to accomplish the same measurements. A digital dial caliper will also work from this position, providing you with a poor man's stroke checker as the dial caliper typically has a range of 6 inches or better; more than enough to check the stroke on an assembled or partially assembled engine if you so desire.
Specialty Auto Parts, USA, Inc.
P.O. Box 306
Roseville, MI 48066