Dimension wise ?am I right in say this is around m8 size . The carriage lock bolt broke off today so I used my thread gauge to findicate out the thread pitch ,it's a new gauge and I don't really understand the size it's giving . 18G 5/8 ? that's it .
Sean
Hi Sean,
I'm not really sure what you are looking at.
A thread gauge will tell you the pitch of the thread, not the diameter of the fastener, as there are multiple sizes that use the same pitch thread (M6x1, M10x1, 1/4-20, 1/2-20 etc). You need to measure the diameter of the male thread to establish the correct size. It may be a little undersize, which can cause confusion. For example, a M8x1.25 bolt may have a diameter between 7.78mm and 8.00mm. This is where having the correct thread gauge is important, as that will generally tell you whether to expect an inch or metric fastener (not always though...).
I'm guessing that your lathe will be using UN (60°) or British Whitworth (55°) threads.
18 tpi (threads per inch) 5/8 is 5/8 UNF
I believe that Andrew is correct but your comment about it looking like a M8 bolt and stating it is Whitworth made me wonder. If I am correct, UN and Whitworth (British Standard Whitworth [BSW]) major diameter and tpi were the same until they got to 1/2" and above. 1/2" UNC is 13tpi where 1/2" Whitworth is 12tpi.
British Standard Fine (BSF) is (as the name suggests) the finer version of Whitworth threads and I believe that 5/8" Whitworth is actually 14tpi, as opposed to the 5/8" UNF 18tpi. There was also a British Standard Cycle (BSC) but that is best left alone!
I'm guessing this is the type of thread gauge that you are using:
![[Image: 11288.jpg]](http://www.myford.co.uk/acatalog/11288.jpg)
If so, I would still confirm the major diameter of the bolt with a micrometer or caliper. Even a drill index will work. The reason that I wouldn't simply assume that it is a standard (if there is such a thing) fastener, is that there are so many variations. I think I have 5/16" bolts in at least 5 different thread pitches.
Pictures of what you are using and what you are measuring would help.
Someone posted a document on identifying threads in a yahoo group I'm in. First chance I get I'll make it a PDF and post it.
Wow. Just checked it out and that file's alot bigger than I thought.
*had to reword that, no tellin how much fun Darren woulda had with that one!*