After putting in a 96" S&S engine into my FXR, it started breaking coils (at the mount) about every 2000 miles. Luckily I had two of them warrantied at no charge. Aftermarket didn't really offer any solutions.
To get advice for something like this, you want to talk to someone has repaired countless high mileage FXR's that were beaten on daily. That person is Tim Wallingford, who works for the City of Garden Grove. Tim repaired FXRP's for a living and is also factory trained, so he's a grip of information.
"Yeah, we had some bikes that broke coils regularly. Just 80" FXR's that had a tendency to break coils just like how you're describing."
Okay, I was onto something. At least I knew I wasn't alone. I wanted to ride to work the next day, so I put something together that I thought would hold up for sure and it did, for at least 5000 miles or so.
This morning, on my commute to work, one of the grade 5 bolts broke. The coil itself was already broken, so I just sandwiched it back together with the bracket.
I'll put another coil on tonight (a new one) and use grade 8 this time to see how it goes. I used grade 5 bolts at the time because that's what I happen to have. So far, one year and 9600 miles on my new S&S engine and 2 broken coils... one semi-broken home made coil bracket. Not complaining, just saying!
Oh, I always wondered why I seen a lot of bikes in Japan with coils hose-clamped to the frame. They must've been breaking coils too. I can't imagine what else I would possibly break if I actually would sometimes GET ON IT!
I'll be doing a short 10,000 mile write up on this engine here soon too.