Course vs. fine thread

[macrisel]

So, here a general question for all you machinists out there.....

What is the benefit of using a fine thread bolt over a course thread bolt?

I can see using a fine thread on torque sensitive engine parts, but why use them on body parts?  It seems Willys used them quite a bit.  

I just installed my spare tire bracket and started wondering.  Also during my tear down I got to the point of HATING fine thread bolts as it seemed they were the ones that always snapped off or refused to break loose.  It also seems on newer vehicles fine threads have seen a decrease in usage or been replaced by metric threads.

[mud pie]

The theory behind fine thread (or so I've been told), is that when you tighten down a fine thread, there is less chance of it working itself loose.

Whenever possible, I've always used a lockwasher, regardless of thread.  If I want to make sure I never have to jack with it again, I also slop on some Loc-Tite.  

[Carls_jeep]

Lock washers have nothing to do with staying tight. A fine thread will take more torque for the same size bolt. The accepted method is to use a fine or coarse thread in steel with the thread depth 2 times the diameter of the bolt. A nut is normally as wide as the diameter is and most the time the bolt thread will strip before the nut thread does.

Most the time what causes bolts to come loose is migration of the metal and/or stretching of the bolt

In aluminum you get better strength with a coarse thread than a fine thread. In aluminum it's recommended 2 to 3 times the diameter of the bolt for thread length. Cast iron normally has coarse threads in it and thread depth is 2 to 3 times the bolt diameter.

I have read that lock washers don't help much for retention and even star washers are questionable. I still use lock washers and star washers if only because it makes me feel better.

Rod and main bearing bolts only use washers to hold them and they take a beating. Head bolts only use washers and they suffer expansion and stretch from combustion more than any others. Those bolts and washers are special hard items but think about the forces they take and stay tight WITHOUT lock washers.

I asked an engineer once why most hydraulic fittings use 12 tpi.�He said 12 tpi is the strongest thread pitch there is. Using any size bolt with the pitch close to 12 tpi will give you the strongest thread for your job.

If your having trouble breaking bolts find out what torque your tightening the bolt to and what class bolt it is. You may have to use less torque or a harder bolt. Lock washers have no holding power over plain washers. Most washers are soft steel so you may have to buy hard bolts and washers and nuts to solve a loosening problem. �They make hard washers and nuts.

[macrisel]

Very interesting!  I always thought course threads would withstand more torque than fine threads.  In my mind, the threads look deeper on a course thread....which would translate to more holding power.  I guess the increased number of threads on a fine thread lets it withstand higher torque.

Any ideas why Willys used fine thread for body mounts?