Mine measures just about 4.5", certainly not any more than that.

As far as start-up amperage, I would think that 100 amps for a fan would be very high. That's 1200 watts. The fan in my roof top a/c unit at the house runs on less than 400 watts, and start up on that couldn't be more than double. At any rate, that's what your battery is for, so your alternator doesn't see that kind of hit. In any case, the start up load would only be momentary.

A source for an engine oil cooler fan might be from a motorcycle junk yard. Let me know what you find.

John B.