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Do-It-Yourself Projects
Engine Assembly, Part 2: The Bottom End
By Wayne Scraba/autoMedia.com
Printable version
Degree of DifficultyModerate
Moderate
Estimated Time240 minutes
240 minutes
Check the Clearances
Using an inside micrometer or dial bore gauge, measure the bearing inside diameter. Much like the crankshaft, we tend to measure the bearings (within the bore) in several different locations. Subtract the crankshaft outside diameter (measured previously) for journal number one from the bearing bore diameter. That resulting figure is the bearing clearance. Check the figure against manufacturer specifications. If the bearings are out of spec, you’ll have to juggle bearing halves (you can buy slightly under and oversize bearings for popular engines) to come up with the appropriate clearance figures.
 
Repeat the entire process for all of the main bearings and caps. Once complete, remove all of the caps. Keep each cap and bearing intact. Leave the lower bearings in the cylinder block.
 
Installing the Crankshaft
Depending upon the engine you have, it can be equipped with either a one-piece or two-piece rear main seal. No matter what format, it must be installed next. In either case, install the seal so that the lip faces inward (toward the engine). Clean the seal groove with a shot of brake cleaner and a fresh shop towel. The groove must be clean and oil free for the sealant to work properly. Apply a small amount of RTV silicone sealer on the seal groove in both the cylinder block and the main cap. Wipe up any excess (a wet finger works perfectly). Install the bottom half of the seal, or in the case of a one-piece seal, gently tap into place over the crankshaft (you can use a seal driver, but most seals easily tap on).
 
Apply motor oil (the same SAE 30 oil works) to the main bearings. Alternatively, you can use engine assembly lube (shown in the photo). It sticks with more tenacity than oil, providing more protection during the initial start up. Apply a small amount of engine oil or assembly lube to the main seal lip. Lower the crankshaft into place.
 
Reinstall the number one cap and the thrust bearing cap only. Seat the caps (using a soft face hammer). Install the bolts by hand, but don’t tighten.
 
Checking Thrust Clearance
Using a soft face hammer (plastic dead blow or brass), tap the crank nose (moving the crankshaft rearward). Install a dial indicator to read on the crank flange or nose of the engine. Using a large (clean) screwdriver or prybar, move the crankshaft backward. Zero the gauge on the dial indicator. Pry the crankshaft forward and check the reading. Record the measurement. Next, torque the caps to specs and repeat the process. Compare the measurements. If the second reading is less than the first, there’s a chance the rear main cap shifted and the thrust surfaces are misaligned. Shift the cap and recheck. By the way, this doesn’t regularly occur, but if it does, you might have to check and shift the rear main cap a couple of times to get it right. Compare your final thrust clearance figure to the manufacturer specifications. Finally, install the balance of the caps (and bearings) and torque to specifications.
 
Checking the Rod Clearances
Beginning at the front of the engine, use your micrometer to check the overall diameter of each of the connecting rod journals on the crankshaft. The process is exactly the same as we used to check the main bearing clearances. Check each journal in multiple locations and record those figures.
 
You can now check the rod bearing clearances. Use the same process we used for the main bearing caps: Install the bearings with the tabs aligned. You can match the numbers on the rods or check to ensure the chamfers are all on the same side and install the caps. The bearings (and caps) are aligned tang to tang (not offset). Using engine oil as the lubricant, torque the cap bolts to the recommended figure and measure the bearing ID with a bore gauge.
 
Subtract the journal diameter from each of those figures to determine the clearance. Repeat the process for all of the connecting rods. Like the case with the main bearings, if the clearances are out of spec, you can often correct the dimensions by juggling bearing inserts.
 
Once all of the rod clearances have been checked, you can loosen the rod bearing cap hardware, but don’t completely disassemble. You need to keep the rods and their respective bearings in order. Wrap the engine in the plastic storage bag. You’re done with this segment.
 
In the next part of ours series, we’ll show you how to file-fit piston rings to each cylinder, how to assemble rods onto pistons and ultimately, how to install all of the pistons and rods in the engine shortblock.
 
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