Belt flew off last 2 out of 3 drives. I'm assuming it just needs to be tighter, but are there any other potential causes?

How does it sit in the pulley's. Flush, down or up? 1/2" slack in the belt when squeezed?

I believe it sits down in the pulley.

I had a more experienced friend tighten it the second time, and I think the 1/2" slack when squeezed is what he was going for.

Would it be possible that the alternator seizes when hot?

When the belt was off, did you spin the alternator by hand to see if there was binding or bearing sounds?

That happened on mine...the pulley turned out to be cracked.

Pulley alignment causes this. Check all the mounting brackets/bolts first to make sure nothing is loose or broken.

An old customer bought a 914and drove it for 2 years with no problems. One day it threw the belt and I put a new one on and it came off the next day. The pulley was on backwards and out of align. It took that long to throw it off. I've also seen one with a Ford pulley. As for yours, its probably loose and misaligned

Ok, I do think the pulley might be a bit sloppy, I'll check it out.

I just found my belt on the side of the road, split. So next question, how long can I drive without the alternator charging the battery?

Alignment is very important. I powder coated all of my engine parts including a washer that acts as a spacer. It kept throwing belts. I left it with McMark and he had to realign the pulleys.

New pulleys over on theSamba.com under 411/412 Parts $40

I had this happen to me. The bracket had been loose for some time. It actually damaged the fan housing. The movement of the loose bracket wore the fan housing so when tight, it would not be aligned properly

Depends on how fast you’re going, whether you’re turning the engine on and off, and the condition of the battery. Probably not more than an hour of normal driving.

Here’s a pic of my cracked pulley (and the incorrect belt the PO installed)
Click to view attachment

How often should the belt be changed? I have no reason to think mine needs replacing other than it's probably been on there for 20 years.

Contact 914Sixer, Mark, and see if he has anymore of the correctly sized belts. I bought 2 of them. Cheaper than at my FLAPS with shipping included. I figure I'm set for life.

Check that alignment like most have already mentioned. Did you happen to recently reinstall your engine tins? Make sure your alternator is not bottoming out on the tin that's on the back side of the cylinder head. I just had this issue when I reinstalled the tins and had to remove and reinstall that piece correctly. Luckily I caught it before starting up the engine.

Yeah, I had the whole engine apart last summer...

Yes, I have 5 more correct belts left.

I had this same problem years ago. It was the pulley that had a small fracture in it. I took it apart several times changing the belt and looking at alignment. When I finely studied the pulley I found the crack.

Looks like I'll be needing one. How much?

My pulley actually looks to be in good shape.

As for the alternator, I spun it by hand with the belt off. It spins freely and smoothly, but it does have a squeak. Probably not a good thing.

Update on this very exciting topic:

I put the old chewed up alternator belt back on and was only able to tension it very loosely.

Then I drove out of town to an autocross event, ran all day, then drove it home. Belt never came off.

I am thinking that the new belt I bought was too long, or otherwise wrong in some way.

Correct belt size is 9.5 mm X 965 mm

Mcmark did a very thorough test to determine the best available belt to use. Here’s a link. The contitech he recommends is perfect.

http://914world.com/bbs2/index.php?showtopic=312624