[glabifrons]

As I understand it, the bolts were pointing forward towards the fuel tank which caused the puncture and (the threads caused) the spark, but this was only in the Pinto wagon, a far less popular variant of the car. The hatchback most people think of as the Pinto did not have this design flaw (the bolts were perpendicular to the car instead of aiming forwards).

So, you had to rear-end a Pinto wagon (not hatchback) with a nearly empty tank so the bolts entered at the fume level for any chance of fire or explosion.