The Long (Over) Haul

Part III: Balancing and pre-assembly of the short block

Dave Verna - February 21, 2013 10:00 AM


For Your Information:


1 Neal McGrory takes all the measurements he can with his micrometer. The crank is being checked on the rod journal. We also checked the stroke, out of round, and made sure the oil holes were drilled the entire way through.


2 The Compstar rods were checked for size, length, and uniform weight. All rods were within two grams of each other, averaging 600 grams. Seven of the rods were .0002-inch large and one of them was .0003-inch large.


3.31-inch Stock stroke setup - Stock Piston (CH 1.81) - .984 pin - Stock 340 rod - total weight – 1,508 grams


4-inch Stroke setup - KB Piston (CH 1.46) - .984 pin - SCAT 6.125 rod with 2.125-inch journal - total weight – 1,216 grams


5 The crank’s counterweights keep things from vibrating too much when your engine is running. The black belt spins the crank to the desired rpm. In our case, 400 rpm provided us with adequate information to balance within a gram.


6 The 170 grams we had to remove from each side of the counterweight was removed slowly and rechecked often to assure we were making progress in the right direction.


7 Here is a Ford crank that is being serviced. The funny thing about this crank pictured is that it comes with several weights offered when purchasing, so this apparently was a quick fix to an incorrectly ordered crank.


8 Starting out with a complete brush set you need to get into every single hole in the block and clean them to loosen any and all dirt.


9 The tool shown is a dial bore gauge and is a very sensitive piece. An experienced machinist will have the feel needed to obtain repeatable results every time.


10 With the main bearings where we needed them, we put the BPE crank in and torqued all the mains.


11 You can see from this shot that the out of the box ring has a pie cut at the gap, we are looking to achieve a .016-inch gap and a nice parallel cut on the rings when complete.


12 Ring gaps differ depending on application, and we are going for a street strip setup. A ring squaring tool sets the rings down about a half inch from the top of the bore.


13 This ring tool cuts the sides as well as maintains a square cut.


14 A feeler gauge gets pulled out towards the center of the bore, not up or down.


15 We can now check for end play, or the amount the crank moves fore and aft.


16 Good tools don’t need to be expensive. Cable ties can get into spaces where a traditional feeler gauge cannot.


17 Two sets of feeler gauges are needed to properly install a set of rods on the crank.


The Long and Short of Rod Selection

The rod, piston, and pin setup we are using is 27 percent lighter than a stock set-up. With almost 25 percent more stroke, we are adding a lot of cubes, and a lot less weight is being slung around. This is easier on the block as it swings the longer stroke but there are concerns about piston speed since we are moving the piston faster with the additional stroke. One thing to consider is that the added stroke lowers the power band of the engine. The 340 set-up shown here would have to spin to almost 7,000 rpm to achieve the same power we are going to make in the 5,000 rpm range.

This lighter, faster, stronger set-up will have less work to do to achieve the same power. This equals less wear and tear on the entire engine, and especially the valvetrain. Expense is also another consideration because it is easier and cheaper to maintain a lower rpm valvetrain. There are many opinions on adding this much stroke to the design but this path should result in excellent results on the dyno and eventually in elapsed times.

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As you can gather by this third installment, we are taking our time and detailing the process so you can get a deeper understanding of what it takes to go from unpacking parts to a running engine.

After we get you spun around on balancing, we’ll start down the pre-assembly process. One thing to keep in mind is that this step is not to be taken lightly. You need to check and double check everything in this step to ensure that the assembly process has no issues.

All of your problems need to rear their ugly heads now, so you don’t get in a bind during the assembly process. Why, you might ask? For one thing, the assembly process needs to be like surgery, absolutely surgical clean and no exceptions!

Balancing is, more or less, one of those voodoo items when it comes to the process. Most people recommend that you balance a rotating assembly but things can get a little vague in terms of information after that. What do you balance it to? Is within +/- five grams good? How about a half a gram? What’s the right way to remove weight when you need to? One thing is certain: balancing will provide better durability and more power when done correctly for your application than choosing not to do it.

With everything weighed when we blueprinted them, the rods, rings, bearings, pistons, pins and locks were added up in the balancer. There are factors for other variables such as oil that will be slung around and the split weight of the rods (big and little end). Our total bob weight came right in at 1,749.6 grams. That number might not mean much to some, but that is a pretty light rotating assembly considering most four-inch stroke small-block Mopars come in at around 1,900 grams and most big blocks (any make) are in the 2,500 gram range – this engine should rev fast, like a dirt bike! With a properly balanced assembly, these modern lightweight and high strength parts will last much longer and operate with precision.

With the bob weight calculated, the scale can now be loaded with the bob weights. These take the place of the items we weighed earlier. The counterweights on the crank offset this weight and therefore create the balance we are trying to achieve. Too much weight on the crankshaft counterweights and we will have to remove some, not enough and we will have to add mallory metal to the crank to get it in a happy place.

The assembly is spun and a sensor is measuring the vibrations it feels at any given area of the crank’s 360-degree rotation. A report will be generated that shows you if you have to add or remove weight at any given angle. In our case, the crank was heavier than we needed, so we have to remove around 170 grams per side.

That’s a lot and we could have just left it on the balancing machine and drilled a few large holes in the crank, but 170 grams required a one-inch diameter hole almost ¾-inch deep. Sometimes the weight is taken out of the crank by drilling the counterweight to the max. This method works, but is not recommended. As the weight you remove starts to get away from the point you need to remove, and when you drill and check over and over, you will find the weight moves around because you are not drilling in the required areas.

Instead, the BPE crank made its way over to the shell mill for a haircut and a shave. After the weight was close, we only needed a few taps with the drill to get the crank dead on where we needed it.

Now that the rotating assembly is balanced and ready to go, we are ready for the pre-assembly stage. The block needs to get a good cleaning as if you are going to assemble it; you don’t want to assemble a dirty block with your new parts. After a shot in the jet wash the block can now be pressure washed. The nozzle needs to get into every hole just like the brushes. The idea is to wash the block from the top down and have any and all dirt leave the premises. When this stage is complete, compressed air will finish the job and the block will be as clean as possible at this point. The last thing you have to do now is a quick shot of light oil on the machined surfaces to prevent flash rusting.

With everything cleaned, the block can start to be assembled. The mains and bearings get torqued down to see where your bearing clearances are with a dial bore gauge. With a well thought out and careful layout, the mains should be at .0025-inch. We put the BPE crank in and torqued all the mains. During the pre-assembly stage, you might be taking this crank in and out several times to assure things are correct. The time killer for us was the angle studs on the main caps, as they do not have any means to tighten them down like the larger main studs do.

The rings that came with the kit from BPE are a file-fit type. This allows us to cut the rings to exactly where we want them. Ring gaps differ depending on application, and we are going for a street/strip set-up. A ring squaring tool sets the rings down about a half inch from the top of the bore. Whenever possible, you can use the same torque plate to load the block. We checked this X block and there was little to no distortion. On a factory stock block, some distortion may occur, and if you want things to be correct, you need to torque the mains and heads down to achieve the exact cylinder wall loading.

Cutting the rings might seem like a piece of cake, but in reality it takes a lot of time to set each ring to each bore. You need to label them as you go, since they are fit to specific bores, and you must cut a bit at a time to get the ring end gap exact. Your leakdown test will show results of the hard work you put in now. Since the bores are perfect, the rings will be gapped correctly, and power will be optimal.

When measuring ring gap, the feeler gauge gets pulled out towards the center of the bore, not up or down. Going up or down will move the ring location and distort the readings. The ring gap determined best for our engine is .016-inch on the top ring and .018-inch on the second ring. There is much discussion on gapping rings and it really depends on your application. Neal likes to open the second ring up a tad (from .014 to .018) to allow any blow-by to escape without causing ring flutter. He also tightened up the top ring (.018 to .016) to achieve a proper ring seal. Again this is for a street/strip setup with our bore diameter, so your results may be different.

We can now check for end play, or the amount the crank moves fore and aft. The amount the crank moves on the thrust bearing is right at .005-inch — right where it needs to be, which is another testament to the quality parts and machining we have in this build.

Next, we need to make sure we have at least .060-inch clearance around everything rotating so something flexible, like a cable tie, is exactly what you need. The ability to check counterweight clearances all around the block as well as cam lobes to connecting rods, etc. is the benefit of a cheap cable tie, as they can get in to spaces where a traditional feeler gauge cannot, making sure you have no issues later in the build.

Once you have checked all of the crankshaft clearances, you can proceed with installing the pistons and connecting rods. With the feeler gauges set at .016-inch, the purpose is to hold the rods square to each other, as they can get crooked and not seat correctly. With the rods properly seated, centered and torqued, we are all set now to get the top end together and continue the pre-assembly process.


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