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Old 05-17-2017, 07:26 AM   #1 (permalink)
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coarse threads vs fine threads off-road suspensions

When using grade 8 bolts on off road vehicles which is generally better to use? Between vibrations, sudden hits, weather, etc.. Coarse is most common but they seem more readily available at any hardware store.

I found the write up below repeated over and over on different websites. It looks like most sites copied it and just use it. there wasn't much variation in search results.


Coarse Threads

Coarse threads are more durable and have greater resistance to stripping and cross-threading. The height of each thread is greater than the corresponding fine thread so there is more material between each thread making flank engagement greater.

Coarse threads are less susceptible to being nicked or damaged, so they do not have to be “handled with care” as much as fine threads. A nick to a fine thread can cause more of a problem proportionally due to the shallowness of the thread, e.g. gaging or assembly.

Coarse threaded fasteners install much faster than fine threaded fasteners. A ˝-13 bolt assembles in 65% of the time it would take to assemble a ˝-20 bolt. The ˝-20 bolt advances one inch in 20 revolutions, while the ˝-13 bolt advances one inch in only 13 revolutions.

Coarse threads are not affected by plating buildup as much as fine threads. The same amount of plating on a coarse thread would use up a greater amount of the plating allowance on a fine thread. Fine threads experience more gaging and assembly problems due to plating buildup than coarse threads, as there is less material between each thread flank.

When using CoilThread Locking Inserts, or other stainless steel threaded fasteners, coarse threads are much less likely to experience galling than fine threads. Fine threads have more rotations as we discussed previously and this coupled with the closer pitch diameter fits of fine threads increases the tendency for fine threads to experience thread galling.
Fine Threads

Fine threaded bolts are stronger than the corresponding coarse threaded bolts of the same hardness. This is in both tension and shear due to the fine threaded bolts having a slightly larger tensile stress area and minor diameter.

Fine threads have less tendency to loosen under vibration due to their having a smaller helix angle than coarse threads. Fine thread Locking Insert grip coils are more flexible than coarse thread insert corresponding size grip coils, and are less likely to take a set under vibration conditions.

Fine threads because of their finer pitch allow for finer adjustments in those applications that need this characteristic.

Fine threads can be more easily tapped into difficult to tap materials, and thin walled sections.

Fine threads require less tightening torque to develop equivalent preloads to the corresponding coarse thread bolt sizes.
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Old 05-17-2017, 08:24 AM   #2 (permalink)
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I assume you are thinking link-bolts.

If your tabs/brackets are strong enough and you properly torque your fasters then either would be sufficient.

If I were building a race car and buying all the best high end stuff, I'd run L9 fine thread bolts and L9 collar lock nuts and L9 washers...

If I were building a trail toy play rig, I'd save a grand or so and run regular ol coarse thread Gr8 bolts and nuts with some loctite.
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Old 05-17-2017, 09:03 AM   #3 (permalink)
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Originally Posted by eat.sleep.wheel View Post
I assume you are thinking link-bolts.

If your tabs/brackets are strong enough and you properly torque your fasters then either would be sufficient.

If I were building a race car and buying all the best high end stuff, I'd run L9 fine thread bolts and L9 collar lock nuts and L9 washers...

If I were building a trail toy play rig, I'd save a grand or so and run regular ol coarse thread Gr8 bolts and nuts with some loctite.
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Old 05-17-2017, 09:15 AM   #4 (permalink)
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Just from a pain in the ass standpoint, I prefer coarse threads....

The fine threads dent and damage too easily, and don't handle any dirt or sand from wheeling on the bolt when you disassemble them, and if you are slightly misaligned and try and to pound the bolt through the link mounts and the misalignment spacers and the heim, the fine thread bolts don't take that well. I've had much better success with coarse thread bolts.
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Old 05-17-2017, 09:46 AM   #5 (permalink)
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Coarse threads for the ease and less chance of crossthreading when working on the trail. And coarse threads I can find just about anywhere it seems
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Old 05-17-2017, 09:52 AM   #6 (permalink)
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Coarse thread for the above reasons.

Fine thread is better for higher torque loads, and much easier to thread lock.
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Old 05-17-2017, 03:55 PM   #7 (permalink)
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If you have a tab made of 3/16 or 1/4 and a properly sized shank bolt (most of the time 3/4 in in diameter, I personally do not find fine thread needed for that application.
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Old 05-17-2017, 05:16 PM   #8 (permalink)
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My link bolts are 3/4" grade 8 coarse thread with grade 5 nylon lock nut (all from fastenal), I tighten them as tight as I can with my 24" ratchet and they still move. Not loosen much from what I can tell but they rotate from the sharpie marks I put on them after I tighten them. Tabs are 3/16" with 4130 cnc machined step washers, correct bolt shoulder engagement.

Do I need different hardware? What should these be torqued to? My torque wrench goes to 250 ft lbs, I've tried that and they still rotate. I check torque and re-mark all suspension hardware before every race. Am I stretching something?

Good timing on this thread.

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Old 05-17-2017, 05:34 PM   #9 (permalink)
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the trade off on deeper threads with coarse thread bolts is theybend easier at the threaded section because the minor diameter is smaller than the same size fine thread bolt, obviously i prefer the shank of my bolts to pass through all mounting surfaces. i always use fine thread bolts when ever possible(on anything that sees substantial loads) because from my experience they tend to be stronger, the only time i use coarse is on something that gets removed often or the store doesnt have what i need in fine thread.
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Old 05-17-2017, 05:38 PM   #10 (permalink)
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Originally Posted by chris demartini View Post
My link bolts are 3/4" grade 8 coarse thread with grade 5 nylon lock nut (all from fastenal), I tighten them as tight as I can with my 24" ratchet and they still move. Not loosen much from what I can tell but they rotate from the sharpie marks I put on them after I tighten them. Tabs are 3/16" with 4130 cnc machined step washers, correct bolt shoulder engagement.

Do I need different hardware? What should these be torqued to? My torque wrench goes to 250 ft lbs, I've tried that and they still rotate. I check torque and re-mark all suspension hardware before every race. Am I stretching something?

Good timing on this thread.
coarse threads loosen easier because the incline is greater than a fine, change your bolts to fine thread if you have a problem with them loosening
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Old 05-17-2017, 08:42 PM   #11 (permalink)
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Originally Posted by chris demartini View Post
My link bolts are 3/4" grade 8 coarse thread with grade 5 nylon lock nut (all from fastenal), I tighten them as tight as I can with my 24" ratchet and they still move. Not loosen much from what I can tell but they rotate from the sharpie marks I put on them after I tighten them. Tabs are 3/16" with 4130 cnc machined step washers, correct bolt shoulder engagement.

Do I need different hardware? What should these be torqued to? My torque wrench goes to 250 ft lbs, I've tried that and they still rotate. I check torque and re-mark all suspension hardware before every race. Am I stretching something?

Good timing on this thread.
Why are you not using grade 8 nuts on grade 8 bolts? 250ft/lbs is probably just under what you actually need. Check your bolt specs.

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the trade off on deeper threads with coarse thread bolts is theybend easier at the threaded section because the minor diameter is smaller than the same size fine thread bolt, obviously i prefer the shank of my bolts to pass through all mounting surfaces. i always use fine thread bolts when ever possible(on anything that sees substantial loads) because from my experience they tend to be stronger, the only time i use coarse is on something that gets removed often or the store doesnt have what i need in fine thread.
I really don't think there is any substantial difference between the 2. CAT lists the same strength for fine and coarse, difference heads have different strengths. Correctly used bolts should have the shaft fully through the mounting surface so the thread should never be loaded in shear.

The only time I use fine is if it's on thinner material and I can get a few more threads.

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Just from a pain in the ass standpoint, I prefer coarse threads....

The fine threads dent and damage too easily, and don't handle any dirt or sand from wheeling on the bolt when you disassemble them, and if you are slightly misaligned and try and to pound the bolt through the link mounts and the misalignment spacers and the heim, the fine thread bolts don't take that well. I've had much better success with coarse thread bolts.
x100. And fine tends to get gummed up real quick.


Reads this, everything you want to know on bolts:
Nuts, Bolts, Fasteners and Plumbing Handbook Exerpt
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Old 05-18-2017, 12:04 AM   #12 (permalink)
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I really don't think there is any substantial difference between the 2. CAT lists the same strength for fine and coarse
CAT may not but endless other listings show fine thread is stronger than coarse when all other aspects are equal

here is one of many examples, it may not be substantial but its true


Frequently Asked Questions on Bolting Matters

Quote:
The potential benefits of fine threads are:

1. Size for size a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).

2. Fine threads have also less tendency to loosen since the thread incline is smaller and hence so is the off torque.

3. Because of the smaller pitch they allow finer adjustments in applications that need such a feature.

4. Fine threads can be more easily tapped into hard materials and thin walled tubes.

5. Fine threads require less torque to develop equivalent bolt preloads.

On the negative side:

1. Fine threads are more susceptible to galling than coarse threads.

2. They need longer thread engagements and are more prone to damage and thread fouling.

3. They are also less suitable for high speed assembly since they are more likely to seize when being tightened.

Normally a coarse thread is specified unless there is an over-riding reason to specify a fine thread, certainly for metric fasteners, fine threads are more difficult to obtain.
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Old 05-18-2017, 05:55 AM   #13 (permalink)
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My course thread don't come loose that I notice but I have sheered 2 Grade 8 Course 5/8 uppers. One of them was 30 miles into KOH. Perhaps they were loose?

The trail fix fine thread issue is where I am hung up. I have had to hammer link bolts in that are close but not close enough. This is where I have confidence in course threads.

I am building a new and hopefully very competitive race car. All new bolts means potential opportunity to make changes for the better. I will probably go fine thread on my coil carriers and bypasses. But the link bolts are up for debate still with me.

In theory once they are in torqued, locktited with nylocks, I shouldn't have to mess with them much. In reality I have had bolts break going 50mph and it did not work out well for me
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Old 05-18-2017, 11:10 AM   #14 (permalink)
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Fine thread bolts have their advantages, but I would stick to a coarse thread bolt for our application for the reasons listed above.

I'd be double checking my torque specs and ensuring that I'm using grade matched bolts/nuts, they're getting torqued properly, and they're secured with Loctite.

If you still notice loosening, I'd consider throwing some Nordlock washers on it. A bit pricey, but you can continue to use your coarse thread fasteners for the reasons stated above. They're available on Amazon if you want to price check.

https://www.youtube.com/watch?v=IKwWu2w1gGk
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Old 05-18-2017, 12:03 PM   #15 (permalink)
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My link bolts are 3/4" grade 8 coarse thread with grade 5 nylon lock nut (all from fastenal), I tighten them as tight as I can with my 24" ratchet and they still move. Not loosen much from what I can tell but they rotate from the sharpie marks I put on them after I tighten them. Tabs are 3/16" with 4130 cnc machined step washers, correct bolt shoulder engagement.

Do I need different hardware? What should these be torqued to? My torque wrench goes to 250 ft lbs, I've tried that and they still rotate. I check torque and re-mark all suspension hardware before every race. Am I stretching something?

Good timing on this thread.
Standard dry torque on 3/4 grade 8 is right around 375lb/ft, lube on the threads will lower this by 10-50% depending on what the lube is. I highly recommend the pocket reference book for all things fastener related. Gives standard torque values for just about any fastener out there as well as clamping force.
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Old 05-18-2017, 02:38 PM   #16 (permalink)
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Fine thread bolts have their advantages, but I would stick to a coarse thread bolt for our application for the reasons listed above.

I'd be double checking my torque specs and ensuring that I'm using grade matched bolts/nuts, they're getting torqued properly, and they're secured with Loctite.

If you still notice loosening, I'd consider throwing some Nordlock washers on it. A bit pricey, but you can continue to use your coarse thread fasteners for the reasons stated above. They're available on Amazon if you want to price check.

https://www.youtube.com/watch?v=IKwWu2w1gGk

Why not just double nut it...those nordlocks are spendy
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Old 05-18-2017, 08:09 PM   #17 (permalink)
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If you're using coarse threads in a shear application (such as a link joint/bracket) you're a fucking hack. It's a red flag to me that somebody has no idea of what they're doing. Fine thread fasteners are the same price as their coarse thread counterparts if purchased in bulk.

Plain nuts or nylocks also peg the meter. Stovers are the commonly available preferable nut, the price is not much higher and they work considerably better.

Cheap washers... Don't even bother. They will do more harm than good. 90% of washers I run into are cheap garbage that isn't suitable for use on disposable furniture.

Never assembly anything dry. Always use a torque wrench on a critical component and use the correct torque.

Many fastener failures would be prevented if people actually purchased fine thread fasteners, used properly sized high quality hardened washers, and torqued said fasteners correctly. A loose bolt is a soon to be broken bolt. Shit washers will not shoulder the bolt/nut correctly, and will either cause a point overload on the fastener, or will extrude and lead to the loosening of the fastener. Coarse threads with plain nuts or nylocks will not maintain torque in environments of high stress cycles. A bolt that was never torqued correctly is not even given a chance.

But, that's just me.
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Old 05-18-2017, 08:15 PM   #18 (permalink)
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you guys bashing your threads on rocks should learn how to cut down the extra length of bolt sticking out past the end of the nut and if your link holes are not lined up learn how to use a screw driver/pry bar so they do line up and you dont have to hammer the bolt through
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Old 05-18-2017, 08:25 PM   #19 (permalink)
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If you're using coarse threads in a shear application (such as a link joint/bracket) you're a fucking hack. It's a red flag to me that somebody has no idea of what they're doing. Fine thread fasteners are the same price as their coarse thread counterparts if purchased in bulk.

Plain nuts or nylocks also peg the meter. Stovers are the commonly available preferable nut, the price is not much higher and they work considerably better.

Cheap washers... Don't even bother. They will do more harm than good. 90% of washers I run into are cheap garbage that isn't suitable for use on disposable furniture.

Never assembly anything dry. Always use a torque wrench on a critical component and use the correct torque.

Many fastener failures would be prevented if people actually purchased fine thread fasteners, used properly sized high quality hardened washers, and torqued said fasteners correctly. A loose bolt is a soon to be broken bolt. Shit washers will not shoulder the bolt/nut correctly, and will either cause a point overload on the fastener, or will extrude and lead to the loosening of the fastener. Coarse threads with plain nuts or nylocks will not maintain torque in environments of high stress cycles. A bolt that was never torqued correctly is not even given a chance.

But, that's just me.
This, and those of you hammering bolts in so hard that you're fucking up the threads, are idiots. If your suspension is so bound up you can't get the bolt in you have bigger issues. Hell buy a prybar, something.
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Old 05-18-2017, 10:08 PM   #20 (permalink)
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Why not just double nut it...those nordlocks are spendy
Watch the video that was in that post.

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If you're using coarse threads in a shear application (such as a link joint/bracket) you're a fucking hack. It's a red flag to me that somebody has no idea of what they're doing.
The bolt should never see a shear load if preload is maintained (and the joint is designed properly). The friction between mating parts carries the external load, keeping the fastener in pure tension.

Fine thread fasteners indeed have a larger tensile stress area, but only in the threaded region. By rights, the threads should never be in the shear zone to begin with. In that case, both the fine and coarse thread bolts have identical shear resistance (in the event that preload is lost).

Fine thread fasteners have marginal benefits. Installing them with a click type torque wrench against an imperfect fabricated surface throws what benefits they do have out the window. I would be looking into better retention practices rather than changing thread pitch.
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Old 05-18-2017, 10:54 PM   #21 (permalink)
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By rights, the threads should never be in the shear zone to begin with.
for every 1 picture you find on this website with bolts in it that has a shank going all the way through the mounting surfaces i can find you 10 with threads in them, almost no one runs the proper length shanks in the world of custom 4x4 fab.

im constantly preaching here on this site and to my friends the importance of a proper length shank, how ever in the real world most have threads in the mounting surface and thats where fine threads are much better.

imo this thread would be of way more value if it was about the importance of the proper shank, thats the biggest problem i see out there(when it comes to bolts being bent/broke and wallowed out holes) as you stated if the shank is the proper length then the coarse vs fine thread is closer debate, how ever fine still wins

Last edited by rockyota83; 05-18-2017 at 10:59 PM.
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Old 05-18-2017, 11:08 PM   #22 (permalink)
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for every 1 picture you find on this website with bolts in it that has a shank going all the way through the mounting surfaces i can find you 10 with threads in them, almost no one runs the proper length shanks in the world of custom 4x4 fab.

im constantly preaching here on this site and to my friends the importance of a proper length shank, how ever in the real world most have threads in the mounting surface and thats where fine threads are much better.

imo this thread would be of way more value if it was about the importance of the proper shank, thats the biggest problem i see out there(when it comes to bolts being bent/broke and wallowed out holes) as you stated if the shank is the proper length then the coarse vs fine thread is closer debate, how ever fine still wins
A correctly installed bolt is never in shear, only tension. There will always be a threaded area between the loaded surfaces. The shank does not have to be super long.
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Old 05-18-2017, 11:37 PM   #23 (permalink)
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for every 1 picture you find on this website with bolts in it that has a shank going all the way through the mounting surfaces i can find you 10 with threads in them, almost no one runs the proper length shanks in the world of custom 4x4 fab.
Just as no one is going to take the necessary precautions to ensure they see the benefits of a fine thread bolt. A single burr / ding on a thread or even trapped debris has far more detrimental effects on a fine thread than a coarse thread. There's simply less clearance in a fine thread assembly. Higher risk of not hitting your desired preload when using torque as your indicator.

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im constantly preaching here on this site and to my friends the importance of a proper length shank, how ever in the real world most have threads in the mounting surface and thats where fine threads are much better.

imo this thread would be of way more value if it was about the importance of the proper shank, thats the biggest problem i see out there(when it comes to bolts being bent/broke and wallowed out holes) as you stated if the shank is the proper length then the coarse vs fine thread is closer debate, how ever fine still wins
Shank length is only relevant if you lose preload and the fastener is put into shear. I would argue that diameter is more important; not because of the increase shear resistance, but because of the larger clamp load it's capable of producing.

This is a difficult discussion because there's so many variables. Since I can't control the assembly environment, I'm going to stay in the coarse thread corner.

I also don't race, nor do I have any issues with my fasteners.
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Old 05-19-2017, 12:17 AM   #24 (permalink)
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A correctly installed bolt is never in shear, only tension. There will always be a threaded area between the loaded surfaces. The shank does not have to be super long.

oh looky more engineer shit that doesnt compute to the real world of 4x4 backyard fab the vast majority of bolts on our rigs are seeing shear forces, bray d brings up another great point of diameter. the forces seen in what we do most people either dont have a large enough diameter bolt to get enough tension to remove any shear forces or they are not tightening the bolts to the proper torque, which once again the fine thread comes into play with its greater shear strength of the threaded section.

youre right bray d we could run around in circles arguing about this subject for a long time love me my fine threads
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Old 05-19-2017, 12:19 AM   #25 (permalink)
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The bolt should never see a shear load if preload is maintained (and the joint is designed properly). The friction between mating parts carries the external load, keeping the fastener in pure tension.

Fine thread fasteners indeed have a larger tensile stress area, but only in the threaded region. By rights, the threads should never be in the shear zone to begin with. In that case, both the fine and coarse thread bolts have identical shear resistance (in the event that preload is lost).

Fine thread fasteners have marginal benefits. Installing them with a click type torque wrench against an imperfect fabricated surface throws what benefits they do have out the window. I would be looking into better retention practices rather than changing thread pitch.
Shear application =/= pure shear loading, and I did not intend to imply that.

And you're wrong on everything else. If you can't fab to the point that your washers/fasteners are seating on flush and flat surfaces with holes that are the correct diameter, just give up.
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