The Williams Formula 1 team says a wheel-retention issue caused George Russell's Russian Grand Prix retirement, and reiterated a lack of spares led it to stop Robert Kubica's sister car.
Russell crashed on lap 27 of 53 at Sochi under virtual safety car conditions, shortly after making a pitstop.
Williams then decided to call Kubica in and retire him a lap later, citing the need to conserve parts, but this was queried by Kubica's sponsor PKN Orlen.
However, in explaining the cause of Russell's crash, Williams stressed on Thursday that following Russell's race-ending clash with Romain Grosjean in Singapore the subsequent Sochi incident left it concerned about having enough spares for next weekend's Japanese Grand Prix.
Williams added it did not anticipate a repeat of the issue that sent Russell off in Russia.
"We found an issue with the wheelnut retainer on George's car, which led to the front-right wheel not sitting perfectly," said senior race engineer Dave Robson.
"This caused a lock-up under braking.
"The design is very mature and well-proven. The remaining inventory will be inspected thoroughly, and we do not anticipate a repeat of the issue.
"Unfortunately, we were forced to retire Robert's car due to the amount of accident damage we have sustained in the Singapore-Russia back-to-back races in order to protect ourselves going into the next events.
"The team has worked extremely hard to ensure race quantities have improved ahead of Japan and the final races."
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