What might have caused this?

I want to say there should have been a round reinforcement plate that sits inside the pulley?

It has happened to others on this forum. Ford released a replacement that had a doubler on the hub.

Metal fatigue.

perhaps some time ago hit was lightly hit, received a small fracture. Then broke with you

I would guess over-torque of the bolts in combination with crappy metal.

Most of the pulleys I've seen that broke like this were the result of either overtightened belts or running without a fan.

Metal fatigue happens , but not as catastrophic
, as it was on the empennage of the deHavilland Comet.

"empennage"......I had to look that one up..!

My original one did the same thing.
Got another one off a 289 that had been parked 30 years.

Happened to me too...definitely know now why Ford changed the design.

Brent G said:

Just curious if it’s happened to anyone before. ****ty metal? Belts too tight? Dumb luck? Water pump seems fine. Not loose or anything. View attachment 897071

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I've had that happen too. Got a ride home and then dug out another one to fix. Had it going in an hour or so. I have gone with the billet pulleys now.

Exact same thing happened to mine not long after I got the motor going. I chalked it up to a defective part. Was lucky as was in the driveway but scary as I have no hood or windshield and had some shrapnel flying a ways. Imagine if your face is buried down in there checking something....

kechke said:

Metal fatigue happens , but not as catastrophic
, as it was on the empennage of the deHavilland Comet.

Click to expand...

this story turned movie was written by a comet engineer warning about metal fatigue on the infamous vomit jet. Wait for it at the very end of this really great performance by James Stewart.

I saw a rear frame rail on a 65-66 mustang cracked right above the axle housing. The Martin 202 was made of low fatigue 70?? aluminum and came apart. The Lockheed L188 electra had prop whirl mode that fatigued the outer wing and they broke off.

the good movie ↓

Woodchuck said:

Most of the pulleys I've seen that broke like this were the result of either overtightened belts or running without a fan.

Click to expand...

A snub nose fan spacer heps.

Good to know. I had no idea about this common part failure and was wondering why there were two different designs.