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Piston Slap: Super Duty Piston Slap?
Tony writes:
I remember seeing a factory sticker or notice somewhere that piston slap is normal with the Super Duty (SD) 455 Pontiac engines from 1973 to ’74.
Was I dreaming, or was that a fact—and if so, why was that the case?
I bought a ’73 455 T/A too late in the year to get the SD engine, so I ordered the SD parts “over the counter” with the help of my brother who worked in the parts department of a local dealership. While they cost more than the price of the car, the result was well worth the effort. I had no issues with piston slap, by the way.
I still own the T/A, plus a 1972 and a 1980 Pontiac Grand Prix SJ. The ’72 is the beneficiary of the original TA 455 engine parts! The ’80 Grand Prix now has a 400. (You are a sneaky, sneaky guy, and I love it! — SM)
Sincerely, your faithful Hagerty member, Tony.
Sajeev answers:
Tony, let me start by thanking you for this question and for your commitment to Pontiac’s iconic small- and large-journal V-8 engines. I recruited Pontiac enthusiast and Hagerty Price Guide editor Greg Ingold to assist with our answer, as he’s owned many an impressive Poncho.
“Sajeev, this is probably the sticker Tony is talking about. While I don’t recall the specific issue of piston slap, let’s remember that the 455 SD was essentially a race spec engine from the factory. You must accept some compromises when you buy an engine like that.
These motors used forged pistons. It is not uncommon for bore tolerances to be a tad more generous on forged-piston engines to account for expansion rate when the metal heats. That’s not the case with every engine, but until the metals warm up, an amount of piston slap might occur.”
That makes sense, and those forged pistons were significantly different than the cast units found in non-Super Duty applications. The design variances seen in the forgings, plus the engine’s unique compression and oil-control rings, are enough to suggest that piston slap is a distinct possibility.
In this case, piston slap is a feature and not a bug. Modern engines in modern cars, and their owners, don’t live in the same space that owners of Super Duty Pontiacs did over 50 years ago. Today we want the fuel economy present in low friction engines, but they should not be “loose” enough to cause piston slap as the motor ages. We didn’t pay extra for that noise, like we could have back in the era of the Super Duty Pontiac.
That said, I was unaware of just how unique those Super Duty motors were in the world of large-journal Pontiac V-8s. I am glad that Tony isn’t experiencing piston slap, but if it happens, that is a mark of pride that’s worthy of bragging rights at the local car show.
After all, how many manufacturers had Pontiac-levels of stones in their engineering departments?
Who else had the nerve to put race-worthy, low-tension, reciprocating parts in a vehicle you could buy at your local dealership?
And that’s precisely the joy of Pontiac ownership, and one of the few times when this series will extol the virtues of an engine possessing piston slap.
Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com—give us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.
I don’t have a dog in this hunt, but I curious in the cast vs. forged aspect of the pistons. I spent 40+ years in the manufacture of pressure vessels and rotating machinery and in almost all cases our customers specified forgings over castings for critical and/or pressure-retaining components. The forgings were considered more uniform in internal structure and having better mechanical properties. Interesting. I guess one does learn something new everyday.
Been around a number of SD engines as well gorged Pontiac pistons and piston slap was never a problem unless it was assembled wrong.
Here is a hood story on the engines details, also you may want to contact Herb Adams retired Pontiac power train and suspension engineer. He would have the best answer for this since this was his project and quite because Pontiac killed it.
Much of the engines owner was better combustion chambers and larger intake and round head ports. It was an evolution go the ram air 5.
https://www.hemmings.com/stories/the-guts-inside-the-glory/
Thanks for the link. Cool info in there.
60’s Pontiac engines assembled with forged pistons had a “Cold” light on the dash to remind the driver to go easy on the throttle until the engine warmed up.
Never saw that. Got a photo or example?
My 1963 Chevrolet Impala had one of these. It was a four barrel 283 high compression motor from the factory. The carb was a Rochester 4jet, a predecesso to the Quadra jet.
Mt 64 Impala had a cold light also, and I can assure you it didn’t have forged pistons! GM used this light and a two-way temp sensor on many models for several years during this period. I guess they eventually decided the extra cost wasn’t justified. I always thought it was an excellent idea because many people tend abuse cold engines with hard acceleration and high revs.
Was in metal casting and forging for 20+ years. Forging heats the metal to a ‘plastic’ condition where it can be forced into shape by hammers or dies. It retains the original grain structure of the metal without the breakdown of the alloy due to melting, as well as the inclusion of air bubbles in the casting process called porosity which decrease strength. Forgings have a homogeneous solid structure which is why they are used in the most demanding applications. The down side is you do not get a very exact shape so it requires much machining. Castings, particularly die castings, can be ‘net shape’ (used without machining sometimes). But no other forms of shaping metal are as metallurgically strong or consistent as forgings. Loved the business, too bad it left me for China years ago.
Piston slap very common with any performance engine with forged pistons from 60’s-80’s. And even on some newer engines as well. Usually only lasts minutes and is gone. The denser aluminum takes longer to thoroughly absorb heat and expand than thinner cast pistons.