So did someone booger up the threads?

 

Yes, sorry about that, there are other types of thread chasers, here is a set of alternative ones, their threads go nearer to the end of the tool.

 

Possibly, or it could just be corrosion or crap in the threads.

People argue for days about the tap vs thread chaser for fixing damaged threads. It's pretty funny reading some of this stuff online. People have very strong feelings about it apparently. I'm in the "don't light your smoke with a blowtorch" camp. It'll work but you might get burned. I used to always use taps because I didn't want to buy another set of something and figured they were the same thing but with my jeep the metal is much older and weaker and I totally hammered some threads chasing them with a tap. They might have got hammered either way but I think they probably would have been ok with a chaser.

 

It sounds very much like a previous owner has done.

I have encountered a similar thing here in the UK, a customer of mine once ordered a set of 5/16-18 stainless steel hardware for his Sportster, I duly sent out his order, he contacted me to inform me that the bolts didn't fit his machine.

It turned out that a previous owner had retapped the threads with an M8 isometric coarse tap, it is possible that he couldn't get hold of 5/16-18 hardware, so he decided to buy the more common metric bolts and he retapped the holes to fit them.

 

I'm guessing the previous owner may have, or they are just rusty and dirty in there, Who knows how long there were no bolts in there.

 

The bolts that HD supply for the brackets are a type of thread cleaner with a section cut out of them.

The most important thing whichever way you go is feeling that the device, tap, bolt, what ever actually goes in straight.

HD #3214 - Harley-Davidson 3214 for sale online | eBay

 

Thread cleaner or tapping bolt? I have a couple of these. It appears the threads at the start of the mounting block are fouled, but look okay going further. Will I be able to use a thread chaser if the start of the threads are the part that is messed up? It's a blind hole so I can't come through the other side.

 

It could be that Harley or a PO used threadlocker of some sort, your thread chaser should clean that out. FWIW another common problem is when someone used too much silicone seal and it gets into a blind hole, that can prevent the bolt from tightening fully and if you keep going you can crack whatever you are working on. I had to use a thread chaser (from ARP) for that recently and it worked great. I also used mine in your situation where the first thread or two was damaged, it fixed that too and starting it was no problem as due to the taper it doesn't engage the first threads until you have screwed it in a bit.
The ARP thread chaser was pricey, but worth it, especially since I was working on an aluminum Evo engine case