Course vs Fine threads: how do I know which to choose?

[robroy]

Good evening,

I've noticed that most fasteners seem to come with either course or fine threads.

If I'm installing a new bolt with a new nut to go along with it, when should I choose course threads and when fine? What are the pros and cons of each?

Thanks!
-Robroy

[oldschoolrods]

For the most part I just use coarse thread. However (somebody correct me if i'm wrong) but I believe some suspension parts use fine thread bolts because fine thread gives a little more clamping force. Best thing to do is probably put back in what you took out.

[robroy]

Hey Oldschoolrods! Well, I took out a rivet so I can't put that back easily.

I've also heard that fine threads have more clamping force, but I'm curious to hear what other folks here think.

Thanks!
-Robroy

[oldschoolrods]

Yeah its hard to reuse those rivets! Honestly, I don't think It'd hurt either way fine/coarse, as long as you get a strong enough bolt, I use grade 8 for pretty much everything.

[fordwannabe]

And when I was in TSC on Saturday the grade 8 bulk stuff was $3.99 a pound. Alot of bolts in a pound of hardware. Tom

[Wes]

Royboy, I am assuming you removed rivets from a frame component ? Personally I would put a high quality grade 8 (lawson, kimball, dorman) fine thread fastener the same diameter as the rivet, with 2 flat washers and use a metal lock nut, tighten to prevailing torque. Our fleet f450's are tearing the rear spring hangers off the frame continually, we drill the rivets out and fasten them as described above. So far none of the repaired hangers have been redone.

[robroy]

Hey Oldschoolrods, Tom, and Wes, thanks for the excellent replies!

Wes, I actually removed the two rivets that hold the funny clutch fork bracket to the bell housing. Here's a photo of the bracket with the two rivets that I removed:



I think your suggestion's a great one, thanks. I'm actually using countersunk allen head bolts here since they fit the funny indentations in the bell housing, and I was thinking of just using regular strength bolts since that's all I could find locally, but I have the option of ordering "Class 10.9" bolts from McMaster instead. They don't fit as nicely in the hole since they're metric, but the "Class 10.9" part will let me make them a lot tighter than I could make the no-grade bolts.

Thanks!
-Robroy

[Wes]

My suggestion was general clamping 2 structural pieces together. The allen head bolts should be fine. I would think that 5/16 grade 3 fasteners would be plenty for that bracket. Are there threads in the bell housing, or is there room for nuts and washers ? If threading into the bell housing be sure to a flat washer and Locktight (blue) on the threads. If nuts either use Locktight, or metal locking nuts. You don't want that bracket coming loose, it would be a royal pain to repair and could cause a lot of damage if it should. You might check for a military surplus store in your area, they almost always have hardware, and normally all mil spec.

One thing I forgot to mention in 1st post the torque specs for fine vs coarse is normally higher.

[Ranchero50]

Robroy, metric 10.9 is another animal all together. Metric grading is different from standard, 8.8, 10.9 and 12.9 vs grade 2, 5, 8.

For that application, drill and tap whatever is closest to the rivit size 5/16 or 3/8 and just use some grade 5 or 8 with some blue locktight. Maybe a dumb question, but why did you remove the rivets?

Jamie

[robroy]

My suggestion was general clamping 2 structural pieces together. The allen head bolts should be fine. I would think that 5/16 grade 3 fasteners would be plenty for that bracket.

Hey Wes! I agree that it seems like it shouldn't take much to hold that bracket in place. I think my concern there is based on the consequences of the bracket coming loose! I think that wouldn't be a pretty picture.

Are there threads in the bell housing, or is there room for nuts and washers ? If threading into the bell housing be sure to a flat washer and Locktight (blue) on the threads. If nuts either use Locktight, or metal locking nuts.

There is indeed room for nuts and washers! Since that's what I'll have on hand, I'm planning on using a combo of Nord-Lock washers under all metal pinch nuts.

You don't want that bracket coming loose, it would be a royal pain to repair and could cause a lot of damage if it should.

I can imagine!

You might check for a military surplus store in your area, they almost always have hardware, and normally all mil spec.

I never thought to check a military surplus store for hardware! That's interesting. It think the nearest one is around eighty miles away, but I could be wrong.

One thing I forgot to mention in 1st post the torque specs for fine vs coarse is normally higher.

I see! So does that mean that for the same size fasteners, you can put more torque on the fine threads than you can on the coarse threads?

Robroy, metric 10.9 is another animal all together. Metric grading is different from standard, 8.8, 10.9 and 12.9 vs grade 2, 5, 8.

Ooooh okay! I was wondering about that. So metric class 10.9 corresponds roughly to grade 5?

For that application, drill and tap whatever is closest to the rivit size 5/16 or 3/8 and just use some grade 5 or 8 with some blue locktight.

That sounds like good advice to just tap the holes instead of messing around with nuts, but I'm inexperienced with making good threads so nuts are a safer route for me at this point. I'd rather practice thread making on something less critical than this.

Maybe a dumb question, but why did you remove the rivets?

I removed the rivets because they were loose, which made the whole bracket wiggle back and forth. I didn't want it to introduce additional play in the clutch mechanism, nor did I want it to come completely loose (although in the state it was in, it was pretty far from that).

Thanks for your most excellent advice Jamie and Wes!
-Robroy

[thebannister8]

fine threads are primarily for finer torque settings and they also are less likely to back out at low torque settings

[AlleyCat]

i've heard their primarily for finer torque settings and they also are less likely to back out at low torque settings

Fine threads are also more resistant to vibration. Fasteners that are used for things like flywheels and connecting rod bolts are always fine thread to prevent them from backing out due to torsional vibration caused by changes in RPM.

When I have to replace a rivit in something like a spring hanger or crossmember I use fine thread flange head bolts with matching lock nuts. Often refered to as frame bolts by truck mechanics these are used by most U.S. truck manufacturers to assemble frames and other structural components. Flatwashers can flatten and mushroom when the parts they are used on are subject to twisting. Flange headed bolts help to eliminate the problem of bolts coming loose.

Have you ever seen a big truck twist up like a drag car when the driver pulls out with a heavy load on a steep hill? The frame and crossmembers flex like tree limbs.