Deciding Coarse Or Fine Thread ?

dave2176 said:
Don't want to hijack the thread but I don't understand something on this subject. If fine threads are superior for alignment on tightening and resisting loosening under vibration why would engines (automotive) be built with coarse threads in critical locations like heads and bearing caps? Just hoping to understand.
Dave
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Coarse threads start better when using air tools or automation, and are more forgiving of imperfections in cast metals.
I seem to remember my old Buick 460 had fine thread rod caps, can't remember the other American iron I've owned. I'm pretty sure all the Euro stuff I have taken apart used fine threads on rod caps. Mains are into cast, so coarse is better. I do recall American v-8's used coarse on rocker studs, but also used crimp nuts. Head bolts and mains bolts tend to be really long as well, giving them the thread holding comparable to fine. I remember one of the Japanese engines had fine thread head bolts, and they would pull the thread on the aluminum block more than half the time. It was probably Honda, they had a lot of issues with their castings, and general material failure.
 
The quality of the thread and the material used will also play a roll. Most bolts used on engines for example will be rolled thread and most likely be a high tensile bolt. Rolled thread will be much stronger than normal machined threads. So the pitch is not the only thing to look at
 
Thanks for the replies - seems unanimous - finer thread it is.

The 10tpi nut goes back in the "maybe-one-day" box.

With respect to hijacking a thread - I've often benefited from "accidental learning" from threads that wandered around a bit - I'd encourage topics to diverge, happens often in face to face conversations.

Bill
 
I haven't good experience with fine thread in aluminium...
 
I was always taught that when a fastener is going into a weldment, especially cast, then coarse threads are stronger. When the bolt is passing thru a weldment and will be retained by a nut a fine thread is more preferred. That would definitely explain the head bolts of an engine being coarse yet the rod bolt threads being fine.

Mike.
 
I was taught if a bolt was subject to vibration to use a fine thread over course thread.
 
Fine is fine by me, for most things.

CHuck the grumpy old guy
 
Handy calculator. You can key in any diameter and any thread count.

Yes its good to calculate yourself from first principles but once youve done it a few times and understand the formulas something like this might be quick and handy.

I am aware that one formula or one calculator cannot satisfy all situations.

http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx
 
You can get more torque with fine threads. I understand that some vise screws use fine threads so you end up with more torque using the same amount of elbow grease.


Sent from my iPad using Tapatalk
 
Fine is better, course is cheaper
 
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