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Engine Rebuild - Bottom End Mockup - Step 2

By Ryan Ballou

In the first stage of the build up we covered the very first step of a ground up engine build, case prep. Assuming that any and all issues with your case have been squared away, the next step in the process is to begin mocking up the bottom end. What this means more or less is you want to make sure your crank spins freely in your case, has the correct tolerances for both the main and rod bearings, then finally hang the rods on the crank.

Checking the bearing tolerances on the mains requires the use of a dial bore gauge. Unfortunately this piece of equipment is quite cost prohibitive for many, even having your local shop take the measurements for you can prove to be an expensive undertaking in labor charges. What this means by skipping this step, is if you run into a problem you may have to backtrack and take a closer look at things and perhaps enlist the help of your machinist. So what I recommend doing is just trying the crank out and cross your fingers, after all this is only a mockup, not final assembly. So we won't ignore problems if they arise. However by proceeding we will only know if a problem exists, we won't necessarily know what that problem is.

Set all the bearing dowel pins in the case saddles and install the main bearings with assembly lube in their respective locations either on the crank or in the case halves. The crank should still be relatively bare at this point, meaning no rods. You may not even want to install the gears yet in case you run into a problem. Now set the crank into the case paying very close attention to the dowel pins and their locator holes in the bearings. I like to lay the crank down with the bearings set close to where they need to be. Then starting at the #1 bearing (flywheel side), use a finger in the gland nut hole to gently lift the crank, well not really lift, but keep the weight off the bearing. Now use your other hand to spin the bearing until it drops in over the dowel. Move on to the #3 bearing and supporting the weight at the pulley end, do the same. Now repeat for the #4 bearing and you're set. You should be able to put a little weight on the crank and the bearings will press down into the saddles all the way. If they don't seat into the case or if you detect the crank rocking in the bearings, you probably have a dowel not seated into the bearing properly.

With the crank in position, set the other case half in place and tighten the nuts on the six main studs by hand. Remember this is a mockup, no sealer yet. Once the six mains are as tight as you can manage with your fingers, try to spin the crank with two fingers on it's nose (pulley side). If it doesn't spin freely then you probably pinched a bearing on a dowel pin and you'll have to start over. This is why you only go finger tight at first as it greatly reduces the chance of damaging the bearing.

If it spins freely, then you can start tightening the mains down. Do the middle two first, then the outer 4 in a crisscross pattern, but only to about 15 ft*lbs. Now try to spin the crank again. If it still spins freely, then go to 20 ft*lbs, spin again. All good, then go to 25 ft*lbs, this is final torque for the six large mains. If the crank still spins freely, then torque the nuts on the two 8mm studs at the nose of the crank, the #4 main, to 15 ft*lbs. Now then, if the crank still spins the same as when the nuts were hand tight, then you're good to go. If it got progressively harder to turn with each step then you've got a problem. Problems may be with the crank, the case, the bearings, or any combination of the three. Problems with the crank are easier to find and fix so look there first. Whatever you do, do not proceed until the crank spins freely or you may suffer bearing failure as a result.

Once the crank is spinning freely, you can now do your final cleaning of the crank and install the timing gear and distributor drive gear. The preferred method is to heat the gears on a stove, then drop them on the crank. Save yourself the headache and only do one at a time. The cam gear will require the most heat; the distributor drive gear won't need as much and should be handled with case due to the soft nature of brass. It's best to keep a brass drift nearby in case the gears need a little coercing to go on all the way. Many people will even use a section of pipe that fits over the crank nose to apply even pressure to the gears. Once the gears are set you can install the spacer and C-clip.

Now that the crank is dealt with we can focus on the rods. I like to mic the width of the big ends before starting and label them from the thickest to thinnest. The thinnest will go in the #3 location, next thinnest #1, next #4, and the thickest at #2. This will help to promote more splash oiling to the hotter cylinders. Now pop all the bearings into the rods, smear some assembly lube on them and one by one bolt them into position. Stock rods have a hump on one side (top/bottom) that always goes up, this is because they are offset to help eliminate piston slap. Aftermarket rods are not offset and can be hung anyway you see fit, but the most widely accepted method is that the bearing tangs be facing down.

With the rods bolted into place you first want to make sure they spin freely. If you set the crank in the flywheel on a table, you should be able to swing a rod and have it make at least one full rotation. If that checks out, then you can start measuring side clearances. Use feeler gauges to measure the play between the side of the rod and it's relative crank journal. Factory spec is .004"-.016" with a wear limit of .027". I actually like to be around .016"-.020" for performance engines. Again, this helps get more oil to the cylinders to aide in cooling, however anywhere in this range is acceptable. Excessively large clearances can only be dealt with by using a different crank and/or rods. Smaller clearances can be enlarged by having a machine shop shave the rods. In this case, it's better to have too much clearance than not enough.If all the side clearances are within acceptable limits, then use a sharpie marker and label each rod so you know where it goes for final assembly. If you do not own a micrometer, then use the side clearances you measured and try swapping rods around until you get the clearances where you want them.

You can now remove the rods and clean the bearings and crank journals. Checking the bearing clearance of the rods is easy due to their split shell design. You can use plastigauge to check them all. Plastigauge should be available from any performance automotive store and even some of the chains like Napa will have it. Factory spec is .0008"-.0027" so buy the appropriate sizes of plastigauge and check the rods one by one. If you ask around, most people prefer their rods to be in the .0015"-.002" range, but unless you have some extra time and money to adjust this, just be sure it's within factory spec. If all is well, clean the rods thoroughly, lube the bearings and bolt them into the positions you labeled them each with. Make absolutely sure you torque the rod bolts correctly. For stock rods, this means light oil on the threads and under the cap of the nut, then torqued to 25 ft*lbs. For aftermarket rods, follow the manufacturers instructions to the letter. A rod bolt failure is something that no one should ever have to live through; it can be very ugly and expensive.

As far as the rotating assembly goes, you're done for now. I suggest using a new, clean trash bag to protect the crank/rod assembly until they are needed again.


1-A bare case half with the #2 bearing shell installed and ready to test fit the crank.

2-Ignoring the cam, this is how this is how the crank is set to ensure it's spinning freely.

3-With just the crank installed, you should be able to grab it here on the nose and spin it with only two fingers.

4-You may laugh, but this is how I keep track of my rod positions and bolt lengths. Many aftermarket rods use APR brand bolts. With these, it's not the torque that's important; it's how much they stretch.

5-The easiest way to measure bolt stretch is with a stretch gauge, however it is possible with a micrometer provided the anvils are correct and you're a little more patient. I still use a torque wrench to get them close, and then slowly creep up on the correct stretch numbers measuring often.

6-Finally we have a completed crank assembly to set aside for future use. You may notice that I'm doing this all in the kitchen, quite frankly this is the cleanest place for me to work, no carpet or furniture to spawn dust. A dedicated workshop would be nice, but we make due with what we have.

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