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Imperial and Metric Bolts

2316 Views 2 Replies 3 Participants Last post by  sedonix
Hi Folks,

As you probably all know, for better or worse the Advanced DC motors use imperial measurements for their bolts and thread sizes.
I'm just wondering if someone could translate one of those strange imperial measurements like eighteen-three-hundred-and-seven-thousandths-of-a-hexidecimal-octogoglet-of -an-inch into English for me.
Ok, that was a made up example obviously. The Imperial Octogoglet hasn't been used for almost 3.17 Tundrilibes now.

Ok, I made all that up. Imperial frustrates me and I took it out on you lot. And considering most of you are American and more than happy with Imperial measurements, I think I'm about to get served as they say.

The reason for this (now ridiculous) post is that my FB1-4001A motor's tailshaft has a threaded hole which is labelled as a "1/4 2OUNC 2B" thread.
Now that sounds unnecessarily complicated as imperial often is, so if I go to my local hardware store looking for a bolt to fit it, how would I explain it the guy at the counter?
Can anyone translate what that thread code means exactly?
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Without making it overly complicated, you want to ask for a 1/4" - 20 bolt. That's 1/4" shank diameter with 20 threads per inch, which is considered "coarse" threads. "Fine" threads would be 28 threads per inch.

You may want to ask for bolts with a greater tensile strength than simple mild steel. Hardware-store variety bolts will be "Grade Three", and will have no distinguishing markings. "Grade Five" bolts are hardened, and will exhibit three hash marks on the head, at 120 degree spacing. Very hard bolts are "Grade Eight", which show five hash marks.

1/4"-20 is a very common size here, and I assume that it won't be too difficult to find on your side of the planet.
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UNC stands for UNified Coarse thread, there is also UNF for UNified Fine thread, among other more arcane thread sizes. 1/4 is the nominal hole size in inches of course and 20 is the number of threads per inch implied by the course name. I've attached a .pdf file of my wHOLE Chart that gives all the decimal equivalents, thread sizes and the drill sizes for them. Also has metric sizes for you. Personally, after using both systems for years, metric is nice for doing calculations, but when doing the hands on work I find the fractional system more convenient.


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