The Stovebolt Site Forums The Shop Area General Truck Talk Bolt thread pitch

Taking apart the frame on my 46 1/2 ton pickup I’ve noticed a lot of the bolts are fine thread. Is there a particular reason for this? A few of the parts include welded nuts so I guess I’ll have to case them down as my supply is all coarse thread. For the application where Nut and bolt are used does it make a difference if I replace with coarse threaded bolts?

Just wondering if there’s a critical reason for this.

Many times, fine thread bolts are Grade 8 versus Grade 5, although Grade 5 are also available in fine thread.
Frame bolts would also likely be Grade 8 for strength in critical areas. I would be reluctant to replace a fine thread bolt with a Grade 5 coarse thread. Unless you can be sure, I would treat all coarse thread bolts as Grade 5.
Check to see if there are head markings on the bolts you remove to determine the the grade.

The science of nuts and bolts is a lot more technical then I originally thought. Probably the reason for the fine threaded bolts is that they can be tightened a lot snugger then a course threaded bolt.
Also they need to match up the right grade of bolt and nut. A mismatched bolt or nut will create a weak link.
I am sure others will come along with more information.
Good luck on your project.
Jim

Not sure what "frame" bolts you mean? Isn't your frame riveted? Shackle/spring eye bolts may be fine but what else? Possibly other suspension hardware like shock bolt? Steering adjustment? Can you describe these bolts? Usage, size/pitch?

Where nut an bolt are used, use coarse. Absolutely no reason for fine in those cases.

The normal NO. 10 screw is fine. 10-32.

Large majority of your bolts will be UNC.

Gr 5 is plenty strong for anything on your truck.

For our trucks: Coarse is plenty strong and torque-able. Fine is only necessary if forced by the design or available parts.

Fine threaded bolts are stronger and require more torque than course threaded bolts of the same size and grade. This is due to the larger minor diameter (deepest part) of the thread. I agree with bartamos that “Gr 5 is plenty strong enough” for our trucks but would replace like and find. If I found a fine threaded bolt, I would replace it with the same unless I found documentation saying otherwise.

For bolts, NC are used when threaded into Ci and Al, NF is used in steel. Exceptions may exist.

Ed

The door hinge bolts are Grade 5 NF. Wish the GM engineers were around so we could ask why. ISTM, that NC would speed up the assembly line.

Lot of stress on door hinges, they thread into NF threads as part of hinge system.

Ed

I have answered the OP's question. Most seem to agree. Now we can get into a long technical discussion if you want to but will be off topic. Like for instance......the door bolts are UNF to be able to quickly tighten after adjust. Not for strength. There are lots of greater stress places on the truck and they use UNC. There is no stress on door hinge bolts/threads when it's closed. When it's open, it is cantilevered, probably weighs 60-80-100 lbs and we lean on it. All those bolts are plenty strong enough for that, UNC or UNF.

Here is the reason we don't need to get into a strength discussion regarding our trucks. Gr 5 vs Gr 8, fine vs course and all that.
For example: using a 3/8 bolt. Using Shear as it is 60% of Tensile. using Gr 5 steel.
The course thread shear is 4985 Lbs
The fine thread shear is 5860 lbs
So 6 door hinge bolts will hold 30,000 lbs. 3 bolts is 15,000 lbs. So maybe the fine threads were actually for adjustment. Maybe the pin is weaker than the bolts!

After about 4 threads engagement, the stripout is as strong as the shear.

In a bolted assembly of two plates with a bolt and nut, the threads should be "below" the shear plane(the mating plane between the plates). So that the unthreaded shank passes thru the shear plane, if possible. But again, not necessary in our applications/stresses/vibrations/usage. Needs perspective.

Fine thread hardware are harder to find sometimes.

The extra strength, which depends on the application, is not needed in our trucks unless as stated in my answer to the OP and others who have responded.

If a person wants to use Gr 8 and UNF threads it's "fine", this is just a reference for those who don't.

Stay cooped up and wash your hands boys.

Fine threading also has a very shallow pitch (helical angle). This makes vibrations less effective at loosening fasteners over time. Fine thread fasteners are ideal for environments where vibration is considered an issue. Blue loctite helps too.
Another tidbit I learned from engineers, while working for a large heavy equipment manufacturer, is that a bolt needs to be two and a half times longer than its diameter for maximum holding. Typically considered in blind holes. Beyond that doesn’t add much.

I doubt if strength was an issue in the engineer's design.
I will wager that the use of fine threads were called for to keep parts which you don't want to come loose, but are prone to.
I would bet that fine threaded fasteners were more expensive to procure, so GM must have had just cause to account for the additional cost.
1) the 2 bolts which hold the bottom of the radiator support to the front crossmember. The entire front of the truck body parts and radiator are kept in place by just these two bolts.
2) hinge bolts. Repetitive opening and closing of these heavy doors calls for a fastener less likely to back out.
3) the bolt-on crossmember which supports the torque tube ball and provides additional support to the frame in conjunction with the riveted-on engine mount crossmember. (In '54, GM eliminated the bolt-on crossmember when they redesigned a more robust engine support crossmember.)



On a side note, GM used fine threads behind the dash to hold the gauge cluster and speedometer on just to irritate me personally, given that it is nearly impossible to get to some of those. If I were ham-handed with sausage fingers, I would find another hobby just because of these fine threaded studs.
Carl

I spent roughly 6 years working in a body test lab for an OEM. We ran durability tests on components like doors, hoods, hinges, latches, etc, So, I may be able to shed a little light on this.

To start with, one has to think about why a threaded fastener like a bolt is even torqued in the first place. Seemingly a dumb question with the obvious answer being: "So that it won't loosen up and fall out." While this is obviously true, another reason is to provide a specific clamp load for the joint that the fastener is holding together.

For a given fastener torque, a fine thread fastener will provide more clamp load than a coarse thread fastener. If you think about the threads on a bolt as an inclined plane, you can picture why this is the case. This same characteristic also makes them less likely to back out.

Also, as previously stated, fine thread fasteners are somewhat stronger, as their minor diameter (thread not rolled as deeply into the fastener) is larger than their coarse thread counterparts.

Joints are often classified as "hard" and "soft". A hard joint would typically be something like a rod bolt or main bearing cap, where there was little or no compliance in the components that are being joined. In contrast, soft joints are typically those found in automotive sheet metal like hinges, strikers, reinforcements, or mounts. They often involve multiple layers of sheet metal that must be squeezed together. Applying and retaining a specific clamp load on joints like these can be a challenge, since the sheet metal itself can be compliant, or there still might be small gaps between the various layers that might shrink over time resulting in a loss of clamp load. Sometimes the fastener might have to be torqued beyond what might normally be considered adequate, in order to mitigate these effects, and keep the component from slipping.

Something like a door hinge is a classic example of a soft joint. If the original engineers specified a fine threaded fastener for that joint, it is very possible that they were concerned about the above in order to keep the hinges from moving around. Previously, it was stated that there is no stress on the hinge bolts when the door is closed. While it is true, that the hinge bolts are not under as much tensile stress when the door is closed as when it is open, those hinge joints still have to support the weight of the door as the vehicle is driving down the road. Not sure what the weight of a '46 PU door is, but it will effectively increase many times over when the vehicle hits a bump or pot hole. I could easily imagine 10 Gs (and I'm probably being conservative here) going into that area of the body so a 50 or 60 pound door now effectively weighs 500 - 600 pounds. Once again, the concern is not that the load will exceed the strength of the fasteners, it is whether or not the clamp load is sufficient to keep the hinge from moving.

That's not to say that sometimes there might be other reasons for an OEM to use a fine thread bolt. Perhaps using one meant that a common fastener set could be used in multiple locations in order to reduce inventory in the assembly plant. And, I'm also not totally ruling out the theory that some engineers do, in fact, harbor a secret grudge toward service technicians.

On the way to my office, I often used to walk by the area where full vehicles were tested. Basically, they took data from a vehicle on a test course, and played it back into a test vehicle with its suspension connected to large servo hydraulic actuators. These rigs attached to reinforced concrete blocks in the floor containing maybe 100 cubic yards of concrete, yet still, on some of the severe events, you could feel the floor of the building vibrate - there was that much force going into the test rig (and vehicle). Sometimes, you would swear that the car or truck was going to jump right off of the rig. I guess what I'm trying to say is that there is much more force and flexing going on in a car or truck body than you would think.

The two and a half times the diameter is a rule of thumb that I have heard before. Bolt stretch is critical for maintaining clamp load. Did you ever wonder what the indentations on the heads of some flywheel bolts are for? The fastener guru told us that since the bolt is too short to provide sufficient stretch along the length of the bolt, the indentations provide for additional stretch in the head area to help compensate.

Lastly, I would be cautious about implying that grade 5 bolts will be plenty strong for anything on our trucks. If, in fact, the system or component was built with grade 5, then obviously no problemo. However, if there was a grade 8 fastener put in a location by the factory, there was probably a very good reason for it. Critical interfaces like motor and suspension mounts, or the steering gearbox come to mind. I personally would never, ever, install a weaker fastener than what was originally specified.


Best Regards...

Bob, Thanks for taking to time to add to our knowledge base. I can't speak for others but I need all the help I can get.

RonR

LOL, I'll buy into that. The F**d engineers diabolically located the battery on my wife's Escape under the cowl,requiring disassembling half of the components in the engine bay to get it out.
Good info Bob. Thanks for contributing.

Unfortunately, I think that just about anyone who has worked on cars longer than 10 minutes has experienced things like that. Having spent some years as a dealership tech, prior to my time at an OEM, serviceability always remained very personal to me. I had more than my share of knock-down-drag-out-fights over serviceability related issues - and I didn't even work in that group. I would often tell them to picture themselves on the side of a busy road, or in a dark parking lot trying to get their car going. Suddenly, things like battery placement, fuse and relay center locations, or even headlight replacement can take on a whole new dimension.

I used to work with a grizzled, older technician who, coincidentally, was also named Bob. He spent most of his life working on cars, and did not come to work for that OEM until he was well into his 50's. Knowing in advance what his answer would be, we would often "manipulate" the new engineers into asking him the question: "So Bob, why did you want to come to work here?"

We would then stand back and watch their reaction as he gave his stock reply: "Well... I've worked on cars all my life... Nearly forty years... Owned my own gas station and shop for nearly twenty.... With all that... I figured that before I'm done, I just had to go out and kill me an engineer. So here I am."

The expressions on their faces were priceless. All got the joke, but clearly some got it much more slowly than others.

I still crack up when I think about that...

Best Regards...

Bob

That's Priceless!!

Another great thread!
Great topper CrowbarBob, many of us wanted sometime or another to go find us an engineer.

I also use to like to scare engineers!
Come up here and look at this crane bearing 40 foot off the turbine deck.
By the way, you have to hang way out to see it.
Don't worry, I'll hold onto your pants legs!

Never lost a desk sitting engineer and then they would let the craft fix it!
Don

Now I'm a little scared, being a Mechanical Engineer. Although I don't/didn't work in the automotive field, so maybe I'm safe. I try my best to incorporate serviceability into systems I design.

When I was in the Army, M60 tanks would come into the shop with connectors broken off the rear comm box. The cable and connector went thru a hole in the fender support, and when the tank threw a tread, the box got pushed up and broke the connector off. I sent in a suggestion to move the box back 2 inches on the fender so the flexible cable was the only thing inside the hole. The response was "We've redesigned the fender support brace with a bigger hole. We'll replace the support brace when the tanks come back to the depot for overhaul. Your suggestion is denied." It didn't matter that all it took was drilling 4 holes 2 inches to the rear of the existing holes. They had to spend a ton of money to replace the bracket, instead of incorporating a field fix for a few minutes time.

Sorry Kevin!
I was not knocking all engineers. Just those that would not listen to the grunts in the field.
I think you will understand by your last post.

Don

No worries, Don! That "scared" comment was a bit tongue in cheek.

I too have read up extensively on the reasons for fine threads versus coarse. One simple reason that I didn't see mentioned was something that I read in an article awhile back. It stated that it's easier to cross-thread a course fastener than a fine thread. Maybe to some degree that in certain instances using a fine was an easy way to help prevent damaging components during assembly.

John

John, you have it backwards.

Whether I do or not, I don't remember where I read it so I cannot present the article. But, that's the way I remember it. At our shop we repair threads (coarse and fine) several time a week.

I will say that I cringe whenever I see people use impact tools without starting the nut or bolt by hand first. Just a pet peeve of mine.

Johnb

That is a pet peeve of mine also. We ran into that with assembly line people. Mostly small hardware. They got trained to never start the screw with the driver. After a week, they were starting it with the driver. Took the driver away.

I've had to be a real butt head at Discount Tire. Explaining to the kid that one twist of his hand on a lug nut is a quarter turn. So if he does it four times it's not enough before hitting it with the impact. I finally gave up and don't watch anymore. Those are fine threads, easy to cross. With an impact, you don't know it.

When I'm working on truck bolts, I only use my impacts for removal. (except lug nuts). I remove and install small hardware (phillips) with my driver after starting threads by hand or manual screw driver.

You know you're in good company when everyone gets excited to talk about bolts!

Two things to add here:

-Course thread is recommended for tapped holes in cast iron, plastic too.

-The rule of thumb thread engagement is 1.5 (not 2.5 diameters), ie nuts.
That rule holds true for stainless screw into aluminum. 1.5D is plenty to ensure the head twists off before you strip anything.

I noticed the 2.5 diameters also. Not sure where that came from. If it's about engagement of threads, 4 treads is as strong as the bolt.

Nuts are already designed to give the 4 threads to make them as strong as bolt. You don't have to worry about that.
If it's a tapped hole in steel, it's still 4 threads or more.
Aluminum: helicoils.

1.5D engagement on a 1/4-20 is 7.5 threads. Engineers and Designers don't use rules of thumb. They use math and testing.

Regarding the clamping force of UNC vs UNF. When properly torqued, which is very iffy, a UNF only gives 13% more force. This is not a factor in old truck design. The UNF was used for other reasons. Hinge plates don't squeeze at torque levels we use.

A hard joint is what we are all used to. A soft joint is when a gasket or spring or rubber is involved, as an example.

I was at a workshop a few years ago, and one of the speakers had a couple of hours to talk to us about nuts and bolts. I didn't think he would be able to fill up the first hour. He should have had all afternoon. He kept us glued to our seats, and I am sure that most of us in there still remember it today. many of these kind of workshops you go to you don't remember a month later. This was not one of them.
Jim

Like Ott said, we are all "nuts" together.

Here is what I have found in regards to stripping one thread over another.
When you add a little muscle to a course thread to overcome a little over-spray paint on it, it feels exactly like adding a little muscle to a fine thread which was started cross-threaded.
The coarse one ends up just fine, the fine one ends up really coarse.

Has anyone mentioned that fine thread seem to be easier to undo after 70 years of rust attack? I think it is due to smaller space between male and female threads to hold water.

Phak1 and I agree, a lot of old trucks have fine threads through out. The torque can be higher AND the fine thread will not shake loose as soon. Roads prior to 1956 were not paved etc. However we all know that with or without penetrant, watch out for knuckles and fingers. For a purest like me I always have my employees replace original. I do so on my favorite pet projects I work on. Ed

To Phak1's point and you agreeing. Obviously if you have a bolt that threads into a threaded hole somewhere, you have to use the same threads. The poster himself made this point.

My point was, and to answer the posters question, was that if you find a bolt AND a nut, you DO NOT have to replace it with a fine threaded set. You CAN use UNC without any danger or problem. Also you can use grade 5 anywhere on that truck.

Then the discussion went off on the pro's a con's of fine thread. A true waste of time. Then a discussion of replacing "like with like" without specifying that bolt and nut sets are an exception. That is ridiculous. The question was answered long ago.

I agree with Phak1 also because he agreed with me .........except as noted.
If you find a fine threaded bolt and nut, do you really know it's factory? The only ones you know are factory are the fine bolts that thread into a factory cage nut or other female permanent thread.

The things you read now on the internet about coarse and fine were not well known or tested or understood in 1946. Fine thread may have just been for adjustment or short engagement. Vibration levels of aircraft, space vehicles, building wind deflection, clamp force requirements, testing methods, alloys, engineering of fasteners and strength of materials were not as well established or known. The use of fine threads in 46 was not for some fear of failure of a coarse thread in a truck on bumpy roads.

FYI, I just cleaned up the mounting bolts for my steering gear. The 3 thru-bolt/nut combos were fine thread, and the one bolt that threaded into the steering gear itself was coarse thread.
They all had the same "R" mark on their heads that I found in various other places on the truck (including the fine thread hinge bolts), so I suspect they were original.

WikiLeaks says:

R is a marking from the BSF "British Standard Fine" system. R means approx. Gr 5

Or Rochester Bolt and Nut or some other MFG's. Pre SAE hash mark bolts.

Must be Rochester Bolt and Nut, because both the fine and coarse thread bolts were marked with the "R". My truck was built in Oakland, CA, not Britain.