416ci LS3 Engine Build - Part 1 - Bottom End

Assembling The Beast from the Bottom Up

Jeremy D. Clough Dec 18, 2013 0 Comment(s)
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One of the things that was eye-opening to me was the labor and tooling required to build an LS properly. Hailing from the custom gunsmithing business, I was under the impression that engine building was mostly just assembly. I was wrong. The LS isn’t the old-school SBC, and to assemble one correctly requires a great deal of precision measurement and fitting on virtually all of the components. The specialized tools required to measure the various small, but critical, dimensions cost more than many complete engines.

The first step was machining the block and preparing it for assembly. Since LS3 blocks can vary considerably in bore diameter, ours was machined so it would be the correct 4.070 inches after final honing. The total height of the rotating assembly was measured, and the block deck machined to leave the piston 0.010-inch “in the hole.”

Decking the block ensures a smooth surface for cylinder-head mounting as well as the correct compression ratio. It also establishes a “true deck,” with both sides of the block cut to the same deck height; this eliminates the problem of uneven forces acting on each cylinder bank. Otherwise, as Scott pointed out, the engine will run, but the question is how well—and for how long.

With that done, the cylinders were honed to remove any taper and to meet the finish diameter. While some shops simply mount the block in the machine and go to town, Grimes uses a torque plate to simulate the pressure of having heads installed. Otherwise, while the bores may be round when honed, they can distort when the heads are torqued down, leaving the cylinders out-of-round. Since this engine is a stroker, the block also had to be clearanced with a little U-shaped notch near the base of each cylinder, to make room for the longer stroke of the connecting rods.

Once the bores were done, it was time to measure and hone the main bearing caps. Since one of the caps was a little large, giving too much clearance, it had to be shortened at the bottom mounting surface and then re-honed. The bearings were then installed in the caps and measured for crank clearance.

We should note here that GM assembles engines using “torque to yield” bolts. These are torqued to a set value, then turned an additional distance, measured in degrees, to stretch them. Needless to say, they’re a single-use proposition, which gets pricy considering the number of times a bottom end should be assembled and disassembled during blueprinting. Accordingly, we opted to ARP fasteners throughout, along with studs on both the bottom and top ends.

With that, the block was done, and it was time to turn to the rotating assembly. While we ordered the crank with the 58-tooth reluctor wheel installed, Grimes checked the wheel’s position to make sure it was indexed properly, and used a dab of weld to make sure it stays in place. The crank itself proved to be within the NASCAR tolerance from the factory—a strong showing for Lunati—but it had been balanced with reciprocating parts a little heavier than the ones we were using, necessitating a rebalance.

The other parts—the pistons and connecting rods—were numbered by cylinder with an electric pencil, then weighed and prepared. The pistons, crisply machined and carefully deburred, came out of the box looking too beautiful to be consigned to an existence where they’ll likely never seen again. The passage where the wristpin passed through the piston was honed, while the I-beam connecting rods were measured and cleaned. The rods also had their caps repeatedly removed and re-torqued to check for both the proper amount of bolt stretch and the roundness of the rod ends.

The assembly was relatively straightforward. With the block on a rotating stand, the pistons and rods were laid out in order of installation. First, the crank was washed and installed in the block, and the thrust (the forward-and-backward play of the crank in the block) was set by measuring and adjusting the clearance on the thrust (center) bearing. While it may seem counterintuitive, race engines require more clearance, to allow for greater expansion due to heat.

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