[barenekd]

I used a 3/8" grade 8 bolt on mine. I needed a longer bolt for the rearsets. I found a 12" bolt at Home Depot, I think, or some other local hardware store. It was just the right length for the rearsets, but would be a bit long without them. You could either use washers to make up the extra length or cut the threads farther down with a die. The bolt I put in there never had any problems. The softer grade bolt that comes in it is probably being overtorqued for the tensile strength of the bolt.
Finding a bolt that long in metric sizes is tough, but it it seems like I found some on McMaster Carrs website.
I'd be a bit reluctant to use an all thread rod. That stuff is pretty low in the tensile strength category.
Bare
Bare
Bare

[FiRE Comms]

yeah the B7 was not grade 8 but still better than a grade 5...  I'm hoping for the best and will keep an eye on it... http://www.nvmetalworks.com/steel-inventory/steel-properties-comparison-chart

[AussieDave]

It must be a high stress point as that is where my G5 frame developed a crack awhile ago . I guess it's under a lot of tension - the amount of flex in the frame with the motor out is impressive!

[no bs]

I got tired of breaking the ace hardware all thread every few hundred miles, and was looking around the shop at work for some suitable roundstock and found a fireplace poker I got out of a used car years ago that was just the right diameter to run a 10x1.25 die on each end. seemed to be fairly springy and tough, so on it went. so far,so good...

[rep_movsd]

I've replaced almost every fastener and stud I can with HT allen bolts and SS threaded rod. Got tired of stripped threads and inability to torque to spec.

[mattb]

I'm going to get a high tensile bolt (12.9) and replace this on my new B5 as a preventative. Are there any other bolts I should replace while I'm at it?

[wildbill]

I have not had one break on any of the 5 bikes I have previously owned. although the furtherest one was ridden was  around 10k or 6000 miles.
maybe you are jumping the gun. just keep an eye on them and if it happens - give em all a workout!