I found this explanation online:
After the Ford Pinto was recalled in 1978, the fuel tank was modified with the following changes:
- Plastic shield: A polyethylene shield was installed across the front of the fuel tank.
- Fuel filler pipe: The fuel filler pipe and seal were replaced with a longer pipe and improved seal.
- Gaskets and seals: New gaskets and seals were installed.
- Fuel filler cap: A stronger fuel filler cap was installed.
- Bracketry and hardware: Bracketry and hardware were installed.
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The Pinto's gas tank was vulnerable to puncturing or rupturing in rear-end collisions because it was positioned behind the rear axle and close to the rear bumper. The lack of reinforcements or shields between the tank and rear bumper allowed direct contact with the tank.