Norris had finished second and Piastri fourth before post-race checks found the planks on both cars below the 9mm minimum.
The lost points cut Norris’ championship lead to 24 and brought Max Verstappen level with Piastri heading into Qatar.
Team principal Andrea Stella said the problem was evident almost immediately.
“From the early laps of the race, it was clear from the data that the level of unexpected porpoising would be a concern,” he said.
McLaren was able to track Norris’ car via telemetry, but the job was “made more difficult on Oscar’s car, after we lost one of the sensors we use to establish the level of grounding.”
As porpoising increased, skid wear levels climbed rapidly and both drivers were instructed to adjust their driving. But those efforts, Stella said, achieved little.
“Unfortunately, we also saw that, because of the car operating window and the circuit characteristics, most of these actions were not effective enough in reducing porpoising,” he explained.
Stella insisted McLaren did not gamble on ride height, saying the team added margin after practice. But the conditions in Vegas and the limited dry running created a misleading picture.
“The safety margin was negated by the unexpected onset of the large vertical oscillations, which caused the car to touch the ground,” he said.
“The porpoising condition that the car developed in the race was also a difficult one to mitigate, as even a reduction in speed – an action that, in theory, should increase clearance to the ground – was only effective in some parts of the track but in others was actually counterproductive.”
McLaren accepted the penalty after verifying the FIA’s measurements — Norris was 0.12mm under the limit, Piastri 0.26mm — but Stella noted the rules leave stewards no discretion.
“The measurement of the skid thickness was correct,” he said.
“Even if the excessive wear is relatively minor and in only one location, the regulation is very clear.”
He welcomed the FIA’s stance that McLaren’s breach was unintentional and that future technical penalties may need more proportionality.
Neither driver was surprised by the verdict.
Piastri said in Qatar on Thursday he “knew we were in trouble when we were called to the stewards,” while Norris admitted the ruling “hurt”, but said he moved on quickly.
Stella said he expects no repeat of the issue in Qatar.
“The conditions we experienced last weekend and which led to the onset of porpoising and excess of grounding, compared to what was expected, are very specific to the operating window of the car in Vegas and the circuit characteristics,” he said.
“We have a well-established and consolidated way of setting up the car and we are confident that this will lead us to an optimal plan for the coming races, starting from the Lusail International Circuit.
“Nevertheless, we learn from every lesson and the one in Las Vegas has been able to provide some useful information about the operating window of the car and the porpoising regime.”
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