Might been a few months ago this thread by still want to leave my experience /extra info for the next visitors or troubled cyclists:
In my fgfs setup, I have 2x Kings 36H, the bolt broke during jibs and it got me pretty bolt-paranoid.
Not the fact that broke but due the fact HOW IT BROKE and is scary looking at the small circular contact area that binds the head of this bolt and it's threaded rod bit (sorry screw nerds I am not using the right terms, I am not a engineer nor a mechanic nor a bolt wizard).
So after moving one bolt from a different wheelset I have that also has 2x Kings laced, I realised that this hubs (that are 3years old VS the 1year old ones where the breakage happened) they came with different screws quality. Is a different product/make or at least they were made differently.
The most noticeable visible aspect is: the screw thread doesn't go all the way up to the head of it and also the AllenKey fitting is not really well finished inside the head of the bolt.
I rode it for a while and after researching, concluded:
Titanium bolts are a no - in case some of you think this would be an upgrade. Source: BMX forums, MTB/downhill forums - correct me if I am wrong, but for what I understand Ti bolts tent to "snap" like ice, let's say instead of have a certain level of flex? Again, I am not a eng. or bolt wizard.
This new screws' threads is american standart I was told (maybe the pitch) - makes sense maybe because they are a copy of Profile Hubs (USA)? Just assuming.
Managed to get advised in one of my bolt-hunting adventures that my place to go was Clerkenwell Screws Ltd in 109 Clerkenwell Rd, London EC1R 5BY.
I took the broken one, explained that I needed replacement, explained the application and the very knowledged man found what I needed.
They look better quality than the ones who came with the Hubs, he told me the specification of them that is something like UNF15 (if I can remember well), and also said that is a American standart, my usual 8mm fits nicely snug in the bolt head.
Paid £6 for 4 of them just to try them out...
BLB sells A PAIR for £6
(on THIS image: on the left is the broken one that came with the Hub.
ON THE RIGHT the right, 2 the new ones, one vertical and one on the table so you could see inside the head.)
They look and feel more solid.
Scaled each one of the new ones and pretty consistent weights too with MAX margin of difference of 0.03g on a scale with 0.01g sensitivity. and scaled the old ones that have been corroded and used , so doesn't matter much but some had a 0.1g difference to each other.
As you can see in the photo, they are ready to go in, greased and hope they hold ok.
Hope this info helps someone.
Any questions just shout.