Before the crankshaft is installed for good, the bearing-to-journal clearances must be measured with Plastigage.

Once verified that the main bearing clearances are acceptable, you can install the new crankshaft with assembly lube.

Installing Main Bearings:

Inserted the new main bearings in the engine block, after to wipe the bearing pocket with a paper towel sprayed with brake cleaner.The main bearings were marked "upper" and "lower". The upper bearing shells go in the block, the lower bearing shells go in the caps.

Note how the tang on the bearing shell aligns with the groove in the casting (arrow).

After all 4 upper bearing shells were installed, carefully set the new crankshaft in place.

pushed the lower bearing shells into place in the main bearing caps.

Checking The Bearing Clearances:

placed a small piece of Plastigage (red arrow) on each main bearing journal before installing the cap.

installed the main bearing cap and tightened the bolts to 77 foot-pounds

Then removed the bearing cap and compared the width of the squashed Plastigage to the scale on the package.In this case, the Plastigage had flattened out a bit wider than the scale for .002 inches, but narrower than the scale for .0015 inches. That meant the clearance on that bearing was a bit less than .002", perhaps .0017 inches or so. All four of the main bearings had very similar clearances.

The Haynes book says that the GM 4.3 liter V6 engine, for model years 1996 to 2000, should have main bearing clearance between .0011" and .0023". Except for the number 1 main bearing, which should have clearance between .0008" and .0020", which is three ten-thousandths of an inch tighter.

Installing A New Rear Oil Seal:

Ideally, the rear oil seal would be installed when the new crankshaft is set in place, but I didn't do it that way, as explained below.

To remove the old rear oil seal,  placed the metal housing on two blocks of wood to support the edges, and  used a small pin punch and a hammer to pound the seal out.

cleaned up the oil seal housing with mineral spirits. used carb cleaner for the stubborn oil stains, then  rinsed it with brake cleaner, which leaves no residue.

installed the new rear oil seal by tapping on the edge of the seal with a block of wood and a hammer. tapped all around the perimeter to make sure the seal went in straight, not crooked.

removed the crank and scraped off this old gasket that goes behind the oil seal housing.cleaned up the mating surface on the engine block.This gasket removal should've been done earlier, but I missed it.

This plastic ring came with the gasket set.This ring is folded into a "C" shape and inserted inside the rear oil seal to keep the seal lips spread apart during assembly Like this

But installing the oil seal at this point didn't work for me... I could not keep the seal on the end of the crankshaft and get the seal housing over the stud on the back of the engine block.I had to wait and install the rear oil seal when I could get the engine off the stand, which meant when I was ready to install the engine and had rented a hoist.

Installing The Crankshaft, For Real:

applied a bit of assembly lube to each main bearing.Then I set the crankshaft in place.

applied more assembly lube to the crank main journals, and installed the bearing caps, starting at the rear.tightened the bolts snug with a large ratchet, then   torqued each main cap bolt to 77 foot-pounds.After installing each main cap, turned the crankshaft back-and-forth a bit, to make sure the crank was not binding.

The crankshaft after installation.If there is a lot of assembly lube oozing out, that's okay.The crankshaft should turn freely at this point, although the highly viscous assembly lube gives the shaft a "sticky" feel as it's turned.