I've spent my life working on Model T and A Fords, as a rule anything the went into a cast iron threaded hole was course, everything else was fine thread. Bob
@Boones Quoting Oliver Hardy wrongly (Laurel and Hardy)......................... "this is another fine mess you've gotten us into"!
Well, I'll pitch in with; When I was rebuilding the drawbar on my car float I asked the local auto engineer (no, he doesn't drive trains) what bolts to use for the coupling. He surprised me by specifying grade 5 bolts because in his opinion you could check periodically for stretch in the bolts whereas grade 8 would just snap and the subsequent shit storm occurs. By the way, he is the main engineering signatory for motor vehicles and trailers in N.S.W. so he knows his stuff. I had that trailer for 10 years and covered a lot of miles without a drama and it was an unsprung trailer at that. I sold it to a local scrappy and he's still using it. Grade 5's haven't moved.
I have always understood that fine threads gave superior holding capability. ie, less likely to come loose. the quoted comment above tends to confirm this, but as we all know, once you tighten the nut on any bolt, the bolt is in tension, so why are there even coarse bolts available ?
I guess I will give my 2 cents here. I believe the pitch on a fine thread is less than the coarse thread making it easier to get the bolt tighter and for the bolt to stay tight. Also the coarse bolt threads are deeper than a fine thread making it better for cast iron as some have suggested. This is my story and I'm sticking to it.
You are lucky you didn't say you're nutz. I would have had to respond what about 'em. Once you get beyond the basic rule of thumb (introduction to structural design) there are volumes written on fastener selection and use alone. Way too many variables.
The design of screw threads and the 'art' of specifications is far too advanced for an old car forum so allow me to offer that, as noted, there are materials, like cast iron, where fine threads could not be properly formed due to inherent qualities such as grain structure. In order for any thread to do its job it must be in contact with the receiving threads and some materials do not provide the surface finish required for 'fine' threads to engage. Coarse threads being much deeper then capture more of the receiving material and allow for some fairly high torque loading. And yes, due to the shallow thread angle, fine thread requires more turning to move the same distance as coarse threads and when tightening it 'feels' like it is easier to torque fine threads, even with a box end wrench. No doubt someone else will opine with other observations but really, this topic is usually covered in level 300 classes and it is difficult to condense it. .
In engineering terms they like to refer to the point of a breakage on a high tensile bolt as "brittle" . What the term describes is the fact that there is little elongation before breaking and a somewhat cleaner break when it does reach its yield point. What it doesn't address is the fact that it may never reach its yield point because of the additional strength engineered into it. Its a poor choice of terminology because to the vast majority of society, the definition of something being "brittle" means something that will "shatter" "easily". A high tensile bolt does not shatter easily and thats why its the choice for use in suspension systems. Not many things take much more abuse than the components used to absorb suspension shocks. These continual shock loads.....especially in drag racing don't "shatter" these high strength bolts. They do on occasion manage to overload bolts and break them because the bolt wasn't sufficient for the job. Would you suggest that the suspension in a funny car would be better served by using Grade 5 bolts to harness all that power and torque? While I understand that engineers like to use that term, it gives the wrong implication that the bolt is not as good as its weaker bretheren. Thats just my opinion. Maybe I can explain it better this way. In scientific/engineering terms Work is defined as W=Fs (work=force x distance) Work is only done if the applied force moves something a distance Merriam Webster says:"to perform or carry through a task requiring sustained effort" which says nothing about distance. So if I tighten a nut onto a bolt to hold two things together, since the nut was moved a certain distance by the force I was applying......I performed WORK both in engineering definition and societal definition. Once the bolt and nut have been tightened and are holding something in place, a force is still being exerted but scientifically no work is being done. Another way of demonstrating it is for someone to lift a barbell over their head and hold it there for 5 minutes. When he lifts it he is doing WORK. Once he has it over his head and is standing there holding it in place, he is by scientific definition "not performing work". So the next time someone is laying under a vehicle with a transmission they are holding in place......they should not get tired because they really aren't performing any WORK,scientifically. My point here is that scientific/engineering definitions are often not descriptive of what is really happening and can give a false impression. To me, something is "brittle" when ONE sharp rap will cause it to break "easily" rather than something that takes quite a bit of force to break it. This is what I consider "brittle" Jellyworms
Going back near 40 years I was a new tech at a company, I asked the engineer what the term was when a spring was over stretched and wouldn’t come back to form. I thought there was some fancy name for it He told me “it’s no good”. Back to regular scheduled programming
One-third of the posts are on-topic, discussing fine vs course. The other two-thirds are split between off-topic thread grades and thread puns. This is not a passing grade (pun not intended), fellows. I don't have the science or the engineering, but I think maybe see a pattern that when precise torque specs are involved, fine thread is chosen. Course thread seems to be the choice for general, down and dirty fastening. Let's agree to never discuss metric threads, which have fine, medium, and course for some reason. They hide the fact by using decimals and such to fool ya. But I think it may have been a major cause of the revolution in 1776.
Over here UNC and UNF fasteners are much more expensive than metric, so I tend to use metric anywhere that Imperial is not essential. Standard metric thread pitch is somewhere between coarse and fine. Sent from my moto g(8) power using The H.A.M.B. mobile app
Would this not be due to increased contact area of a fine thread? A fine thread would have more surface area of thread face, thereby creating more friction and more holding capacity against coming loose. This, however, would have nothing to do with tensile or shear strength.
Because much of Rod frame & suspension work offered choices of grade 5 or 8, I went w/grade 5, as the tight/torque range had a window, vs the narrow range for a grade 8. Also, even primarily, grade 5 had a strengthening quality, if tightened close, to 'plastic' region, before it ultimately parted. Grade 8's are stronger for given dia., But failure can be sudden, by cracking. In over 100K miles, had no fasteners fail.
I own Carrol Smith's book on hardware, and it's even in the stack by my side of the bed. Thanks for this thread to everyone who contributes to it. Lots of important info here. Thanks, HAMBers.
Well you got me wondering...Will a fine thread bolt, torqued to the same value as a coarse thread bolt, have greater clamping force due to the closer to perpendicular angle of the thread? Does that make sense?
We Aussies are a little slow, and made the mistake of only becoming a nation in 1901. By that time it was too late. Metric was being used in parts of Europe. Our English heritage brought with it the joys of Whitworth, British Standard Cycle and British Association threads. A pipe fitting could readily be British Standard Pipe, British Standard Brass, National Pipe Thread or Gas thread. Our GM, Chrysler and Ford history brought with it UNC and UNF... and on a bad day, NS. We have slowly metrified... though they come in standard, fine and superfine too. Our first locally made GM vehicle (1948) was a mixture of metric and imperial dimensions, UN and metric bolts and NPT fittings. Coarse versus fine? Most days I'm lucky to find a nut and bolt that work together at all Cheers, Harv
Makes perfect sense to me. The 'clamping force' on a fine thread would be more perpendicular to the thread face, as opposed to at more of an angle on coarse thread. Combine this with increased thread surface area and you can see why fine thread is probably more likely to stay torqued. The trade-off is that fine thread needs a stronger material to thread into. The softer the material, the more 'pronounced' and deep the thread needs to be, necessitating coarser thread. This is just how it works in my head. Please correct me if I have it wrong.
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