Reddit mentions of Nut & Bolt Thread Checker (Inch & Metric)

http://www.amazon.com/Bolt-Thread-Checker-Inch-Metric/dp/B003FJW0GK

$30 on Amazon, no work required.

http://www.amazon.com/gp/product/B003FJW0GK/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

There are others, but this one works very well.

Nut & Bolt Thread Checker (Inch & Metric) https://www.amazon.com/dp/B003FJW0GK/ref=cm_sw_r_cp_tai_4lnUCbTMAM5TT

I have this and it's pretty good.

If it was critical, a professional would probably have that bolt called out in a drawing somewhere. If not, these come in handy - essentially an organized version of trying different screws until you get the right one. Completely legitimate method - just be careful to not cross thread.

yeah, since asking, I've realized I will likely need to just do that.... also: thread checkers... holy cow. something like this... I could keep this in the shop no problem. at first, I thought you were talkingg about the boards I've seen - hadn't seen one like this before... too cool.

Long thread incoming

We're working on resorting our hardware now, but the goal is:

• 2 racks with 4 drawers each containing our most frequently used hardware (SAE Grade 8 1/4-28, 5/16-24, and 3/8-24 mostly, all of varying lengths, plus rod ends, bearings, and of course, nuts and washers).
  
• We have an "EE corner" with all our electrical stuff, and they have a few (neatly sorted) bins of electronics connectors and fasteners.
  
• Engine-related hardware will be stored with our engine parts after we do a rebuild this spring break and document all the fasteners.
  
• Everything that doesn't fit into the above categories will be individually bagged, labeled, and stored in an overstock box. If this box becomes huge, I'm considering getting myself a storage unit and donating a chunk of it to the team.
  
• As for electronic inventory, I'm almost certain we'll have our overstock inventory stored in an Excel spreadsheet. The problem with having your common fasteners digitally inventoried is keeping tabs on it (not everyone will adjust quantity on hand when they take some parts). A solution could be to have cycle counts generated weekly, and you could script your Excel/Google sheet to automatically warn you of low quantity on hand. I'm rambling now, but the solution would be to get a hardware sponsor and have them do VMI (Vendor Manged Inventory), which isn't out of the question......
  
  If you're looking to sort your current hardware, it will take a while. This took me two weeks of 3-4 late nights in the lab after finishing my work for the day, and that was just our random bin. Still have washers to sort, as well as the fasteners already in our rack of drawers.
  That being said, I'd highly recommend getting a thread checker to accurately distinguish between different fasteners. BoltDepot.com has a lot of info on fasteners as well as printable fastener sizing charts (really useful for sorting washers).
  
  Best of luck!

If you often play guess the thread one of these is pretty handy.
https://www.amazon.com/Bolt-Thread-Checker-Inch-Metric/dp/B003FJW0GK/

If not, go to the hardware store and get some metric and see fasteners in that diameter range to test fit.
Most likely a coarse thread.

I'd make the dealership deal with it since they fucked it up in the first place.

But if your dead set on figuring it out yourself head to your local hardware store. I'm talking like your local joint like an Ace or something small along those lines, not a big box store. Pair up with one of the staff and take their thread checker out to the vehicle and figure out what size threads your looking for. Alternatively bring in an existing dash bolt that fits and check it in store. Then buy the hardware you need.