The Stovebolt Site Forums The Shop Area Engine Shop Figuring Out the Bolt Thread Count?

I spent part of the weekend trying to figure out the thread count on some bolts on the truck.

I know there must be an easier way than determining the bolt diameter then just guessing if it is fine or course thread and trying different nuts to fit at the store.

Any suggestions that will make this easier?

There is a gauge you can buy, has multiple blades with teeth of various spacings. Match the blade up to the bolt you have and it will tell you number of threads per inch. I got one of these years ago with a tap and die set. Also you can buy a plate with threaded holes for every size of bolt and thread pitch, metric and SAE.

go to Sears and get yourself a thread gauge.they come in various sizes and you can get s.a.e and metric

a thread pitch gage. it has individual fingers ill call them like a feeler gage in a way with the the thread pitch number on them instead of the decimals on them.there are little pocket size charts as well that have thread sizes and tap sizes as well.anyone that sells machinist tools should have them.the chart and gage go hand in hand.the pitch gage has the thread shape right on it lay the right one on the bolt and it will tell what thread it is . pretty straight forward.the traded pich gage /plate works well too as mentioned.

There are two basic thread pitches for every commonly-used bolt diameter. Once we get into metrics, it's different, but the only place you'll find metric threads on a Stovebolt is spark plug threads.

U.N.C.- - -Unified National Coarse

U.N.F.- - -Unified National Fine (Same as SAE threads)

1/4" UNC---20 Threads per inch
1/4" UNF---28 TPI

5/16" UNC---18 TPI
5/16" UNF---24 TPI

3/8" UNC----16 TPI
3/8" UNF----24 TPI

7/16" UNC---14 TPI
7/16" UNF---20 TPI

1/2" UNC----13 TPI
1/2" UNF----20 TPI

Metric bolts and nuts are measured in millimeters of diameter, and the distance from one thread to the next in millimeters. For instance: 6 X .75 X 20: 6MM diameter, .75 MM between threads, 20 MM long. An inch-size bolt might be 1/4" X 20 X 1 1/2": 1/4" diameter, 20 threads per inch, 1 1/2" long.

Bolt head size is usually standardized, also, at least for fractional bolts:

1/4" bolt: 7/16" head
5/16" bolt: 1/2" head
3/8" bolt: 9/16" head
7/16" bolt: 5/8" head
1/2" bolt, 3/4" head

Hope this helps!
Jerry

Jerry,

Thanks for the great information. This makes everything easier. I'm sure others will appreciate it as much as I do !!!!!!!!

Bolt gauges can be a help for some. Your local hardware store should sell something like the first one at this link. http://www.atlanticfasteners.com/tech_help/gauges.html

I grew up with a huge advantage in this department. Dad would fill his pockets out of the scrap bin at work at an aircraft company. Every few weeks I would sit on the floor with a bucket of hardware with fasteners of all styles and sizes, sorting them out. At the age of 5 I could tell the details of a vast number of bolt sizes and styles, including jo-bolts, hi-loks and huck bolts.

Now, if I could only find a way to use that talent to make money.