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World Guide To Blueprinting

You're Probably As Tired Of Wondering What "Balanced And Blueprinted" Actually Means As We Are Of Hearing It, So We Bring You

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Rod side-clearanceMeasuring rod side-clearance is a simple matter of inserting feeler gauges between assembled pairs of connecting rods to determine the amount of clearance between the rods and the sides-or "cheeks"-of the crank journal. Generally, on V-8s we're used to 0.010-0.020 inch as the norm for steel rods (more for aluminum rods), but as with most specs, the ideal amount can vary between manufacturers. The potential trouble here is that since the rods must be assembled to take this measurement, it is often first dealt with during final assembly, at which point finding too much clearance, which is quite possible when dealing with reconditioned cranks and rods, is a difficult situation to remedy. This is just one example of why it's a good practice to assemble the engine to measure specs prior to final machining if possible. The upside is most builders seem to feel that a little extra side-clearance is not a real problem. On the flip side, too little definitely can be. When assembling your rods, particularly during a mock-up phase when the pistons may not yet be mounted, make sure to keep the chamfered side of the rod facing out toward the crank's cheek. If the unchamfered side of the rod is installed against the radius between the journal and the cheek, it will likely push the rod toward the center, resulting in a false clearance reading; installing the rods this way during final assembly can cause the rotating assembly to bind, resulting in damage if the engine is started. Note that some engines don't have radiused rod journals and therefore may not have chamfered rods. Another tip: Don't measure side-clearance at only one point between pairs of rods-gauge the gap all the way around, as imperfections in machining can leave high spots on rod sides.

If you find too little rod side-clearance, the rods can be filed or sanded to provide more. Professional machinists will often use a belt sander, but this requires an experienced touch as it is easy to remove too much material. Some builders like flat files, others prefer emery cloth wrapped around pieces of flat steel. The objective is to keep the side of the rod flat and parallel to the beam.

Deck Height/QuenchThe difference between an engine assembler and an engine builder is little details like checking the relationship between the piston and the block deck. Generally, most domestic engines place the flat portion of the piston below the block deck for piston-to-head clearance, often as much as 0.050 to 0.080 inch down into the bore. If improved combustion efficiency and more torque and power are your goal, a good way to get there with a wedge-type (non-Hemi) combustion chamber is to tighten the quench, which is the area between the flat portion of the cylinder head and the top of the piston. For steel rod engines, this can be as tight as 0.040 inch or even less if you're brave. This clearance also includes the head-gasket thickness. A tight quench area effectively squishes air and fuel into the chamber as the piston approaches TDC, improving combustion activity and generating a more efficient combustion process. This also reduces detonation sensitivity.

The best way to measure piston deck height is to mock up the crank, connecting rods, bearings, and pistons (without rings) and place each piston at TDC to measure the difference between piston and the deck. Bring the piston up to TDC and then use a deck-height mic or a deck bridge and dial indicator to measure the differential. Measure all eight pistons to get an accurate map of how much the deck surface will need to be milled to create your ideal deck height.

All The RestThis blueprinting guide is hardly complete, especially when you consider that Rick Voegelin has written a much more extensive guide, The Step-By-Step Guide To Engine Blueprinting from SA Design, that has recently been updated. We've purposely skipped topics such as blueprinting oil-pump clearances, assembling cylinder heads, pushrod length, port-matching, and degreeing a cam. Many of these items have been covered in past Car Craft issues, but if there is enough demand we can certainly present the information with all the latest updates. Let us know what you want to see.

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