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Ruthless Pursuit of Power: The Mystique of the C6 Corvette LS7 Engine - Page 8 of 26


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Ruthless Pursuit of Power: Lucky Seven Edition: The Mystique of the 7-Liter, 7000-RPM, LS7 - Page 8 of 26

Click on the images for expanded views — it's much easier on the eyes. 
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by Hib Halverson
© May 2013— Updated:  November 2014
No use without permission, All Rights Reserved

An LS7 piston pin, or "wrist pin"--arguably one of the most highly stressed parts in a high-performance engine--is made of a gas-nitrided, chromium-molybdenum-vanadium steel meeting the 31CrMoV9 specification. This is a more robust material than normally used in GM V8 pistons. it allowed the wall thickness of the pin to be less and the inside diameter to be tapered to reduce mass but still enabled the pin to meet GM's abusive fatigue life tests. The piston is phosphate-coated, the main purpose of which is to improve the reliability of the pin bores during the break-in period. The pin locks are circlips, but they're made with 1.8-mm rather than 1.6-mm wire to increase the circlip's tension. During development, according to then Small-Block Chief, Sam Winegarden, the LS7 engineers learned that, at high RPM, the loads on the 1.6-mm circlip at top-dead-center and bottom-dead-center can deform it and pop it right out of the pin lock groove. Going to the more robust circlip solved that problem.

Again, looking at the underside of the piston, to increase strength, the top portion of the wrist pin bore bosses, indicated by the shorter arrows, are wider than the bottom portion.

Image:  Author

Again, looking at the underside of the piston, to increase strength, the top portion of the wrist pin bore bosses, indicated by the shorter arrows, are wider than the bottom portion.

Additional mass reduction comes in shortening the piston pin, but to do that and preserve the ability of the pin bosses to carry the load, the pin bores were moved closer together and the tops of the pin bores curve inward to further strengthen that area. To clear those parts of the pin bores, the small end of the connecting rod is formed with a pronounced taper with the top being more narrow. Another reason for a shorter pin? It provides clearance between the crankshaft reluctor wheel and number eight piston.

An '06-'11 LS7 piston/rod assembly. The '12-'13 unit is identical except for a polymer-coated, bi-metal rod bearing. The four valve reliefs are a welcome feature of the piston top for those wanting to go to an aftermarket cam profile.

Image:  GM Powertrain

An '06-'11 LS7 piston/rod assembly. The '12-'13 unit is identical except for a polymer-coated, bi-metal rod bearing. The four valve reliefs are a welcome feature of the piston top for those wanting to go to an aftermarket cam profile.

The piston top has four valve reliefs which, according to Aaron Dick, are not necessary with the stock LS7's valve lift. They exist because, after the design was finalized, GM wanted slightly less compression, so four valve reliefs were added. An unintended, but sometimes welcome consequence, is that these reliefs provide adequate valve-to-piston clearance for some aftermarket camshafts, such as Katech's "Torquer" series of LS7 cams, having more aggressive profiles without having to change pistons. The ring grooves are machined with a slight upward tilt which counteracts the rings' tendency to flex downward under operating pressures and temperatures. The top ring land is hard anodized to prevent microwelding on the flanks of the ring groove.

The top ring is filled with moly as an antifriction measure. The LS7's Napier second ring was developed for the 2002 LS6 and later used on all Small-Blocks. A 'Napier ring' has a distinct shape that enhances oil control by scraping oil off the cylinder walls as the piston moves down in the bore.

Image:  Author

The top ring is filled with moly as an antifriction measure. The LS7's Napier second ring was developed for the 2002 LS6 and later used on all Small-Blocks. A "Napier ring" has a distinct shape that enhances oil control by scraping oil off the cylinder walls as the piston moves down in the bore.

The LS7 ring package starts with a 1.2-mm, moly-filled, steel top ring. It is "coined" to give it an upward twist which flattens under combustion pressure improving ring seal. The second ring is, also, 1.2-mm, but is made of ductile-iron and has a Napier-face for enhanced oil control. The oil ring is a 2-mm, 3-piece unit consisting of two gas-nitrided rails and an expander.

During engine assembly, techniques similar to those used in the engine shops at Katech or Hendrick Motorsports are used to install the pistons, including an awesome ring compressor which the author insists would look excellent in his tool box.

Image:  Mark Kelly/GM Powertrain

During engine assembly, techniques similar to those used in the engine shops at Katech or Hendrick Motorsports are used to install the pistons, including an awesome ring compressor which the author insists would look excellent in his tool box.

Most of this cutting-edge piston technology is aimed at reducing mass but, also, increasing durability. Aaron Dick's closing statement says it all about the level of technical sophistication in the piston assembly: "it's a very, highly-engineered piece for a specialized application."

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