Blueprinting has become an abused term. For those who build their own engines—or at least want to know how to check clearances like valve-to-piston on an existing engine—we will offer some techniques that you can follow. What we’ll look at in this two-part series are the major processes that are among the dozens of small details that—if ignored or improperly executed—can destroy an engine. We know; we’ve been there. The days of just slapping an engine together with Plastigauge should be relegated to the past. It takes patience, a certain amount of skill, and a few specialty tools to perform these tasks. We’ll show you a few of the tricks and techniques to get the job done correctly.
Even with a two-part story, we can’t begin to cover all the details involved with blueprinting an engine. For example, we will not cover degreeing a camshaft, as much has been delivered on that subject. But we can offer a couple of hints. The biggest is to double-check that TDC is accurate. If the TDC position is inaccurate, every other measurement will be in error. It pays to be meticulous.
So take a spin through this introduction to the art of engine blueprinting. The purveyors and patrons of fine arts museums will never understand, but any aficionado of high-performance internal combustion engines will acknowledge that a true performance engine builder is every bit an artist.
Measure Bearing Clearance Now that we are firmly into the 21st century, there is no excuse for not measuring actual bearing clearance. Tolerance stack-up is a reality when dealing with production parts, and that new engine deserves more respect than to just stick the bearings in and hope for the best. To do this job correctly you must invest in or borrow an accurate micrometer and a dial bore gauge. This is the only way to do this job correctly. Both must be able to measure down to 0.0001-inch. That’s one-tenth of 0.001-inch.
Use the micrometer to measure the main and rod journal diameters and then use the same mic to set the dial bore gauge to the journal size. Next, set the dial bore gauge to measure the inside diameter of a rod or main with the bearing in place. The reading displayed on the gauge will be the bearing clearance. The accepted clearance rule that works very well for street engines is 0.001-inch for every inch of journal diameter. So for a small-block Chevy with a 2.100-inch rod journal, you would shoot for a bearing clearance of 0.0021 inch. Typically, a 0.0023–0.0025-inch figure for the rods and 0.0025-0.0028 for the mains on a small-block Chevy are acceptable clearances. Always measure bearing clearance in the true vertical.
We’ve also found that measuring all eight rod bearings will create a range of clearances that might span 0.0005-inch. Swapping bearings between the loosest and tightest can sometimes narrow the range of clearances. This idea can also be used on main bearings. Of course, substituting half bearing shells is another common way to bring the clearances closer to the desired spec. For example, the builder can compensate for a slightly oversize standard crank journal with a 1X bearing that offers more clearance. By adding one shell rather than both, this will increase the clearance by 0.0005 inch. Most performance bearings companies now offer bearings in -0.001 and + 0.001-inch sizes along with 0.009, 0.010, and 0.011-inch sizes to allow the builder to customize the clearances.
Crank Thrust Clearance This is a blueprinting step that many backyard engine builders do not check. Over the past few years we’ve noted that nearly all of our engines have required sanding the main thrust bearing in order to create the proper clearance. So this really hammers home that you should always measure this on any engine fitted with new main bearings.
The procedure is simple. Install the front and rear main bearings in the block, drop in the crank, and lightly install the front and rear main caps. Before torqueing the rear main bolts, tap the crank rearward and then forward with a rubber mallet to align the thrust bearing surfaces. Now torque the mains and set a dial indicator on the crank snout.
If the clearance is too tight, remove the bearings from the engine and clamp the two thrust bearings together in their proper orientation using a hose clamp. We sand the forward side of the bearing on a thick, flat aluminum plate using 400-grit wet-dry sandpaper doused with light machine oil. We measure our progress with a dial caliper. Of course, you must surgically clean the bearings once the task is completed to be sure that all of the abrasive material on the bearings has been removed. This is a time-consuming, but important, task.
Rod Bolt Clearance
The most popular stroker swap in human history is the 3.75-inch stroke crank in a 350 Chevy block. This swap instantly adds displacement, but also requires some careful clearance considerations. This swap moves the rod journal—and rod bolts—very close to the block near the pan rail and also swings the top of the rod bolt up near the camshaft. Both of these clearances need to be verified before final engine assembly. Block clearance grinding must be done carefully since there is a water jacket directly behind the area needing attention, so remove the absolute minimum necessary. A clearance of 0.050-inch is all that is necessary. Aftermarket blocks like Dart’s Sportsman, for example, are designed with extra clearance so grinding is usually not necessary.
For 383s with a big cam, the best solution is to use stroker rods that offer more clearance, but often on big cams it may be necessary to use a reduced base circle camshaft. Always check the clearance after the camshaft has been degreed as moving the cam might alter the clearance to the camshaft.
Piston-to-Head Clearance This is one area that isn’t critical ... unless you’d prefer your engine be more efficient. Minimizing piston-to-head clearance tends to increase compression slightly but the real benefit is creating more turbulence in the chamber. This increased mixture motion generally results in reduced ignition timing requirements and improved combustion efficiency. The best time to test piston-to-head clearance is before final engine block machining. Mock up a piston and rod for each corner of the block and measure the distance of the piston below the deck. Many LS engines actually put the piston above the deck, so keep that in mind.
This measurement will establish the piston-to-head clearance and indicate how the block should be machined. To set this deck height properly, make sure your machine shop is measuring from the crank centerline as opposed to just making the deck surface flat. Piston-to-head clearance is the distance the piston is below deck added to the compressed head gasket thickness. As an example, let’s say our small-block piston is 0.008-inch below deck and the gasket compressed thickness equals 0.039-inch. This would give us a piston-to-head clearance of 0.047, which is slightly more than ideal. Decking the block to bring the piston to a zero, or near-zero deck height would be the ideal change.
One thing that affects piston-to-head clearance is piston rock. At top dead center (TDC), the piston has a tendency to rock back and forth. The best way to minimize this effect is to measure the piston depth directly above the wristpin. Sometimes a piston dome can get in the way of this process. If necessary, you can measure both the high- and low- piston rock at TDC and then average this number. For example, if the piston measures 0.020-inch at the lowest and 0.006 at its highest point, then the average would be 0.013-inch (0.020 + 0.006 = 0.026 / 2 = 0.013). Measuring this movement also offers a clear picture of how much the piston could move at TDC and why a 0.038–0.040-inch value is important. This clearance is for steel rod engines. Aluminum rods require more clearance because of expansion. We’ve listed recommended clearances in an accompanying chart.
Rod Bolt Stretch
The reason it is important to measure rod bolt stretch rather than merely using a torque wrench is to perhaps save you the loss of an engine. We lost an engine about 20 years ago to an under-torqued rod bolt that loosened up and destroyed an otherwise pristine small-block. Now that we’ve hopefully garnered your attention, a rod bolt stretch gauge should be the first precision tool you buy right after a good set of micrometers.
By measuring rod bolt stretch, this eliminates a raft of variables that contribute to either under-torqueing or over-stretching a rod bolt. Both of those scenarios can spell disaster. Not all rod bolts offer a stretch number, which is a really good reason for not using them and investing in a set of quality rod bolts like those from ARP that will always give you a very specific stretch figure. Install the rod on the crank, set the stretch gauge to zero for each bolt and then tighten until you achieve the stretch figure.
This chart is intended as generic clearances for small- and big-block Chevy and LS engines. These are basic clearances for a naturally aspirated, pump gas street engine. These numbers can be used as a starting point for a basic street engine, but certainly can be modified to fit individual needs. Certain clearances, like stroker rod clearance for example, are minimum clearances. CHP
|Application||Small-Block Chevy*||Big-Block Chevy*||LS Gen III/IV*|
|Roller cam endplay||0.004-0.010||0.004-0.010||0.002-0.010|
|Valve-to-piston – Int.||0.070||0.090||0.070|
|Valve-to-piston – Exh.||0.100||0.100||0.100|
|Stroker rod clearance||0.050||0.050||0.050|
|* all measurements in inches|
Photography by Jeff Smith