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Your motor mount is probably 4130 steel in the normalized condition. 400°F won't affect the temper of that metal. However, depending on the specific material and heat treat condition of the aluminum (7076-T6? 6061-T6? 2024-T3?) a 400°F soak could degrade the material properties quite a bit. 20 minutes might not hurt, but do you want to experiment?
BTW, the fact that your motor mount hasn't failed in 35 hours is essentially meaningless data. Try to understand the difference between high-cycle fatigue properties and ultimate tensile properties.
Jack Kane
EPI, Inc.

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