Likely not enough lubrication happening. Pity, that engine would have been perfectly good if it had been treated a bit more carefully, and a matching numbers 914 would be worth something today. They're a very interesting car, mid engined lightweight sports car that handles a lot better than you would expect from something that old.

There was a six cylinder version of this as well (super rare) and they made a prototype 914/8 which was never offered for sale, afaik only two were made (one for Ferdinand Porsche, one for Ferdinand Piëch).