Thread: Acorn headed bolts

Originally Posted by superquag

Piffle !
That depends on being able to see tiny things clearly... at our vintage... Das olden Lookenpeepers nicht verk too vell....

Imperial works basically the same as metric.

you will have a bolt that is so many fractions of an inch (or inch + so many fractions) long that is a fraction of an inch wide (or inch + fraction) by a number of threads per inch.


both types of bolts ( and no I am not getting into BSP, BSPT, Withworth, BSC, BSE, British conduit, and yes they all appear on landrovers of various vintages and in some cases on one major sub assembly) come in a variety of thread setups

For metric you have (for most fittings) a fine, normal and coarse thread. in the case of M8 that might be .75 1 and 1.25mm

for imperial you generally have UNF and UNC. one might be 18TPI for coarse and 24tpi for fine.

Generally you can pick the measurement type (metric/Imperial) and tensile strength of a bolt by its markings. Metric will have a number that indicates its hardness and imperial will have a series of radiating lines.

Higher numbers and more lines indicate a higher tensile strength this mean the bolt is harder and stronger but it becomes more brittle this is a good and bad thing. If you go with a both thats too high in tensile strenght it can chew out the threads on whatever you install it into or in some applications it will shear under side loading. They also have less "feel" when you install them. where a low tensile strenght bolt might only handle say 100nm of torque you'll start to feel it yeilding and it will still hold from 80nm through to 120nm (I just made those numbers up stay with me) before it lets go. A high tensile bolt might hold as you torque it up to say 400nm but it might only give you the same 40nm range when you can feel it yielding and yielding wont be over a half of a turn it'll be 1/12th of a turn.

put simply, low tensile strength bolts are very plastic and will stretch and bend and when they fail they fail in a fairly calm manner, much like when you pull play dough apart. High tensile bolts shatter, like when you smack an ice block with a hammer.

Ahhh.... I see.....

But alas, these appear to be 8mm acorn heads... and 5mm Allen-key.