Again with my other post I found no offerings of this kind of advice whilst searching and this website pops up a lot so I am going to post my experience here!
Now FSA have a cretenious system where they decided that the best thing for their bolt patterns to be would be three bolts in a 86BCD (bolt circle diameter) see this link for 'advantages' which is claimed to be better because you can get down to a 26T chain ring (because shimano have really struggled, right?).
This proved a problem when I decided I wanted to run 1x10 on my Boardman HT Pro (which uses a bb30 bottom bracket therefore cheap cranks can not be bought anywhere!). After e-mailing FSA with my problem they suggested, helpfully, that I buy their FSA afterburner cranks with the chain rings for a delicious £260. Not going to happen.
Another option was to buy a bottom bracket converter to something sensible like GPX, these cost ~£70 and then I would have to buy new cranks on top of that so that was also a complete NO GO.
I also decided that I didn't want to get a custom chainring shipped from america from warhawk industries who seem to be the only people I could find at the time.
I then decided (being an engineering student) to make an adapter to fit one to the other, this consisted of the drawing below (just incase anyone feels brave enough to do this as well, I would read everything before you jump ahead and do it!)
I then had this laser profile cut out from steel as no one actually cuts from aluminium apparently. The bolt holes all lined up very well and everything looked like it would run smoothly.. How wrong I was!
Once I had aligned the 386 bolt holes I then noticed that the cranks interfered with the 104 bolt holes so I then took a file to them to make it fit. Once this fit I then thought job done!
Wrong again. Then came the issue of trying to fit the chain ring, this required a hack saw, measuring multiple times and a file. Once cut down to size and filed properly I had to use a hammer to get the chain ring bolts, chain ring, crank and adaptor to sit together. Not a fun experience at all!
Below are the close ups of the cranks from inside and outside, below them are the before/after of the bike.
I personally think it looks very neat and tidy now that it has actually been fitted. It works well and doesn't flex even a little. I have abused it around sherwood pines in the highest gear I have and stomping around the course, it is being raced tomorrow and I have every faith in it.
If you are having a similar problem you are more than welcome to use the drawing above but I would advise against it unless you know your way around a work shop and are happy riding butchered (but completely one off and custom) bikes. If I were to do it again I think I would buy a warhawk chain ring or buy some silly cheap cranks off of ebay second hand, not quite the same though!
Costs - £20 for the chain guide from superstar, £30 for the guide ring from e-thirteen, plate £0 (I know a guy).