Most Pontiac V-8s were originally fitted with cast-aluminum pistons, and they operated quietly and consistently. Forged-aluminum pistons were specified to improve the reliability and durability of Pontiac’s very high performance engines such as the Ram Air IV and Super Duty 421 and 455.
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The cast Pontiac piston is a durable design that offers reliable performance throughout the lifespan of a typical engine. Its wrist pin is biased toward the thrust side to lessen cylinder wall loading and promote quiet operation. Large valve pockets allow maximum piston-to-valve clearance. A dish was machined into the crown of some to reduce compression ratio in certain applications. Though suitable for everyday driving, cast pistons are susceptible to failure when exposed to very lean conditions, extreme heat, and/ or detonation over long periods of time.
Pontiac sourced forged-aluminum pistons from Mickey Thompson for the early Super Duty engines while TRW provided the forgings for the R/A IV and SD-455. Forged-aluminum pistons are far more forgiving than cast units and the TRW pieces are quite durable. Using the R/A IV design as the basis, TRW began offering a wide array of stock-replacement forgings for most Pontiac engines in stock displacement and popular overbore sizes. Several of the high-volume offerings remain available under Federal-Mogul’s Speed Pro line.
Myriad stock-replacement cast pistons were produced throughout the years, and a variety is still available on the aftermarket at a very reasonable cost. A hypereutectic piston is made from a modern silicon-aluminum alloy that’s often used to cast pistons, and it’s generally considered a step better than aluminum. While pistons constructed of either material can certainly provide a long service life in a naturally aspirated engine, operating conditions must be ideal at all times or failure can result. With that, I am not comfortable using cast or hypereutectic pistons in any high-performance engine.
I consider forged-aluminum pistons a must in any performance engine, especially if nitrous oxide or forced induction is planned or could be used in the future. Forgings are generally much more tolerant to high-combustion heat, momentary lean conditions, and/or detonation. While no piston should be expected to survive indefinitely if such conditions aren’t quickly corrected, there’s much less chance of failure with a quality forging when compared to a cast piston.
Most Pontiac vendors can supply you with a set of high-quality forged pistons at a price ranging between $400 and $600. JE Pistons, Keith Black, Probe, and Speed Pro are just a few companies that produce off-the-shelf forging that fit the Pontiac V-8 without modifications. Options can range from a stock-replacement design to a lightweight forging better suited for high-RPM applications.
JE Pistons offers several high quality flat-top pistons for 400, 428, and 455 engines in its SRP line. Constructed of high-silicon aluminum alloy, its pistons are fully machined to a limited number of specific overbore sizes and feature a valve pocket displacing 5 cc. The wrist pin hole is bored to accept a .980-inch-diameter pin for use with stock-length Pontiac connecting rods for certain applications or .990-inchdiameter pin for stroker engines using 6.8-inch rods. The wrist pin hole is grooved to accept lock rings for floating pins.
The ICON line by Keith Black contains a number of high-quality forged pistons for original 400 to 455 Pontiac blocks, and at least one large-bore aftermarket Pontiac block dimension. Available in myriad standard and overbore sizes, and in a flat-top design with a valve pocket displacement of 4 to 5 cc, or a dished crown ranging from 10 to 30 cc to reduce compression, ICON pistons are constructed of a low-silicon alloy aluminum for maximum endurance and reduced drag. The wrist pin hole is machined for floating pins and accepts .980- or .990-inch pins depending upon the application.
Federal-Mogul offers several high quality forged pistons in its Speed Pro Power forged line. Constructed of extruded aluminum alloy, an updated skirt design reduces drag and is moly coated to resist scuffing and reduce side clearance for quiet operation while the 5/64-inch top ring provides maximum reliability and cylinder seal. Considered a high-performance stock replacement for select 389 to 455 engines in limited stock and overbore sizes, the crown area is very thick for maximum durability, and can also be machined for custom applications. Because the valve pocket is positioned for valves in the stock location, some modification may be required when using certain aftermarket cylinder heads and/or larger-diameter valves. The wrist pin bore is offset as on a Pontiac original, and is machined for a press-fit .980-inch wrist pin only.
If your rebuild uses a stock-stroke crankshaft and stock-length connecting rods, then stock-replacement forgings are an excellent choice. Custom pistons are sometimes required when using a long-stroke crankshaft, aftermarket connecting rods, specific overbore sizing, or for various other reasons. Diamond, Ross, and Venolia are a few companies that produce high-quality custom forgings. When designing a custom piston, the wrist pin bore, ring grooves, and valve pockets can be located in most any position on the piston.
Some Pontiac builders regularly stock specially designed custom pistons for their proprietary stroker kits. Special characteristics may be required if custom pistons are required for your particular Pontiac. Expect to spend more than $600 for a quality set. If you plan to tear your engine down regularly, you might consider ordering one or more extra pistons. Having spares can save you time and money.
One custom piston feature that’s somewhat as popular with other makes, but not always popular for Pontiacs, is gas ports. Gas ports are a series of small holes that are precisely drilled into the head of the piston and through to the ring groove. A gas port allows a small amount of cylinder pressure to pass behind the top ring, pressurizing it and improving cylinder seal. It can be beneficial when using very narrow ring packs and/or high ring positions, which are commonly associated with very high RPM engines. If you plan for such operation with your Pontiac and will be using custom pistons, it may be a feature to consider.
Piston rings are intended to keep cylinder pressure from entering the crankcase while maintaining a very light film of oil on the cylinder walls to lubricate the piston as it travels up and down. Most Pontiac pistons utilize a ring pack consisting of three separate piston rings. The top two are compression rings, which serve to keep cylinder pressure from entering the crankcase while scraping excess oil off the cylinder walls to reduce oil consumption. The bottom ring is designed to keep a majority of the engine oil that’s splashing about the crankcase (and onto the cylinder walls) below the compression rings, thoroughly lubricating the piston skirt in the process.
Generally speaking, improved piston ring seal equates to greater horsepower and torque. Modern technology has improved piston ring design. Available materials and cylinder-wall finishing processes have improved the performance and longevity of an otherwise typical piston ring set. Compression rings can be made much thinner, significantly reducing operational friction without compromising cylinder seal, and applying this technology to your Pontiac has benefit.
When selecting a piston ring package, you will find that most aftermarket forged pistons require 1/16-inch-thick compression rings, and a 3/16-inch-thick oil-control ring. Most conventional ring sets are purposely left a bit tight and must be file-fit by hand to achieve the proper amount of end gap for a particular combination. Some are even designed to run gapless. Be sure the ring type that you select is compatible with the cylinder wall finish left after honing.
I have used various Speed Pro ductile iron rings with moly-face coatings with excellent results. Total Seal offers a variety of readymade piston ring kits for many popular off–the-shelf Pontiac pistons, and it can quickly produce custom rings in any width or thickness for highly specialized applications, such as those running nitrous or forced induction. I highly recommend discussing your piston ring options with the piston manufacturer and/or your Pontiac builder to ensure you’re making the best selection for your particular application.
Piston Ring End Gap
A specific amount of compression ring end gap is required during assembly. It allows the ring to expand controllably in the cylinder as the engine reaches normal operating temperature. Too much end gap can allow an excessive amount of cylinder pressure to pass into the crankcase a performance-compromising condition referred to as “blow-by.” An end gap that’s too tight can cause the ring to bind with the cylinder bore, scoring the cylinder wall, and breaking the piston ring and/or ring lands.
In years past, it wasn’t uncommon to find end gap specifications wider for the top ring than the second. Generally speaking, modern ring packs are designed to operate with about .004 inch of top compression ring end gap for each inch of bore diameter, and .005 inch of second compression ring end gap for that same measurement. The oil-control ring gap simply needs to be more than .015 inch in nearly all applications.
Preparation and Inspection
There are many excellent piston options. Once you’ve made your selection, it’s wise to closely inspect each new piston before sending the entire set to your machinist for proper weight balancing along with the rest of the rotating assembly. For me, that process typically includes taking very fine sandpaper (1,000 grit or more) that’s been wetted to the valve pocket area in the head of the piston. A red scouring pad also suffices.
The idea is to gently remove any sharp edges that could otherwise induce detonation if the fuel octane is ever marginal for the engine’s compression ratio, or if its operating temperature is ever unusually high. I run my finger over the ring lands and carefully use the same wetted sandpaper to remove any burrs that might scratch the cylinder wall finish and/or prevent the piston rings from properly sealing. I also recommend inspecting the supplied wrist pin to be sure it fits tightly and can easily be pushed from its bore
Assuming that sufficient cylinder wall clearance was present at all times and there were no lubrication issues or major failures, you will likely find the pistons in excellent condition during routine tear downs. If you hope to reuse the pistons, the first step should always include measuring the dimensions with a micrometer. If any of them is not within the manufacturer’s stated tolerances, it’s best to replace the entire set.
If your pistons and block measure correctly, then there shouldn’t be any issue with reusing the pistons once properly cleaned. Any carbon built up on the head can be removed chemically. It’s best to ask your piston manufacturer for its recommendations on what to use. It isn’t unusual to find some very light scratches on the piston skirts during the inspection process, particularly on the thrust side.
You might be tempted to reuse pistons that are slightly loose, but in otherwise good condition. This, however, can lead to excessive piston skirt noise (piston slap), wrist pin noise, and/or oil consumption. In the past, knurling was a popular way to reuse slightly-worn pistons. The knurling process gathers material around the skirt area and expands the piston’s dimension to compensate for the wear. I feel this practice has no place in a modern performance engine.
Engine component coatings are popular in today’s performance world, and pistons are likely the most commonly coated engine component. Some coatings, such as a ceramic applied to the head of the piston, act as a thermal barrier that rejects heat. It can increase the amount of combustion heat a particular piston can withstand, while preventing the heat of the combustion process from escaping, potentially improving power output. Other coatings, such as a dry film moly-based lubricant applied to the skirt, serve to reduce friction and wall clearance while improving acceleration without skirt damage.
Certain Pontiac pistons, such as the Speed Pro units from Federal-Mogul, come with a light moly-based coating that’s factory applied to the skirts. It reduces friction and provides quieter operation when compared to non-coated examples.
Written by Rocky Rotella and Posted with Permission of CarTechBooks