OP
Thanks Jerry, makes perfect sense, much appreciated.
Out of interest, does anyone know how long the bolt is for the standard issue US bumper plus hook is? Interested if UK one is shorter.
Are Grade 8 bolts overkill for the front bumper?
- Thread starter UKTJ
- Start date
Out of interest, does anyone know how long the bolt is for the standard issue US bumper plus hook is? Interested if UK one is shorter.
Remove your factory bolt, measure its length. Measure the thickness of the mounting base of your new tow hook. Add base thickness of your new tow hook to the length of your current bolt and that is the length you need for your replacement bolt. Buy the bolt that is at least as long as the that total measurement, better to be a little longer than required than shorter.
Gunnersmate
TJ Enthusiast
Bolt Grade Metric and SAE
FYI
The numbers Stamped on the Head of a metric bolt indicate what grade it is
FYI
The numbers Stamped on the Head of a metric bolt indicate what grade it is
OP
Good info, but in this case it needs to fit into an existing thread, so has to match the bolt being replaced.
OP
My thinking was that if the same bolt as for the US version with the OEM tow hoom is used on the UK version with no tow hook the additional length required will be less, as there is already a bit of unused thread.
I have just measured the length of the bolts from my bumper, the front one is 1 3/4 and the back one is 2 1/4. How does that compare to the length on a US spec TJ?
You're overthinking this. Just measure your bolt and select your replacement bolt's length based on the above described procedure.
Gunnersmate
TJ Enthusiast
your threads should metric
understanding metric bolt sizes
Example
M8-1.0 x 20
Here's what the description means…
M = Metric thread designation
8 = Nominal diameter, in millimeters
1.0 = Pitch (distance from thread to thread), in millimeters (determines whether it is Fine Thread or Coarse Thread)
20 = Length, in millimeters
The number stamped on the Head is the Grade See Chart I posted above
Export models have the same mix of SAE and metric size bolts US models have. His front bumper bolts will be 1/2x13 as they are here in the US.
OP
If I lived in the US where 1/2"-13 grade 8 bolts were easy to come by I would. But I have yet to find a single UK source. Most of the world is metric, what is common to you is rather obscure most other places.
I ended up getting the OEM Torx from USA and they cost almost as much as the OEM hooks with shipping!
The OEM are Grade 5, I also bought from a Nut& Bolt supplier in OZ 4 grade 8 hex head and 4 grade 5 Allen head, they had neither Torx nor Allen head in grade 8. but they all fitted.
The nutserts are thread through so a little bit longer will not hurt but too long and they loose their strength for purpose.
My two longer export Torx from the bumper loop bolted down the hook but were not long enough.
If you buy a cheap set of aftermarket tow hooks like Jerry posted in post # 10 they come with the hex head Grade 8,s about $20au for the entire kit from any auto shop and this is what everyone (except stupid me) in Australia uses on their TJ,s.
If you want the Torx they will have to come from USA and they will be Grade 5.
I am not sure they even make Torx in grade 8 after searching the net for days?
I got a bee in my bonnet and ended up with 12 new bolts plus the 2 I took out so 16 bolts lol.
Total for fitting OME tow hooks on an export TJ $200au almost all shipping costs lol
The OEM are Grade 5, I also bought from a Nut& Bolt supplier in OZ 4 grade 8 hex head and 4 grade 5 Allen head, they had neither Torx nor Allen head in grade 8. but they all fitted.
The nutserts are thread through so a little bit longer will not hurt but too long and they loose their strength for purpose.
My two longer export Torx from the bumper loop bolted down the hook but were not long enough.
If you buy a cheap set of aftermarket tow hooks like Jerry posted in post # 10 they come with the hex head Grade 8,s about $20au for the entire kit from any auto shop and this is what everyone (except stupid me) in Australia uses on their TJ,s.
If you want the Torx they will have to come from USA and they will be Grade 5.
I am not sure they even make Torx in grade 8 after searching the net for days?
I got a bee in my bonnet and ended up with 12 new bolts plus the 2 I took out so 16 bolts lol.
Total for fitting OME tow hooks on an export TJ $200au almost all shipping costs lol
LONGJP2
TJ Expert
Too late now, but I would bet a company (or Ebay seller) that is selling hooks, would throw in some grade 8 bolts if you explained the situation.
OP
I did get hex head bolts with my after market hooks, but they are metric so useless for fitting to the TJ bumper. Never bothered to check grade. I have spent a fair bit of time online and I am yet to find any UK supplier with 1/2-13 grade 8 bolts with any type of head.
SAE J429 Grade 5 bolts have a minimum yield strength of 120,000 PSI. SAE J429 Grade 8 are rated to 150,000 PSI. So the material of a Grade 8 is roughly 25% stronger in tension.
Of course those numbers are meaningless without more details of the application.
A TJ bumper bolt is 1/2-13 UNC, at whatever length makes sense for the application.
A Grade 5 1/2-13 has a minimum break strength of 17,000 lbs tension, a yield strength of at least 13,000 lbs tension, and a proof strength of at least 12,000 lbs tension. Meanwhile the Grade 8 is a bit over 21,000 lbs, 18,500 lbs, and 17,000 lbs, respectively.
Note that minimum break strength is the tension required to break the bolt. The bolt will begin to deform past the yield strength, but will continue to hold. In most cases, yielding is considered a type of failure, as it changes the structure in a way that often significantly alters loads.
Proof load is a load that any bolt can be tested to with no visible signs of damage or deformation. It is only a single load test, and does not take into account repeated loading.
All of this is nice, but still not very applicable to the TJ bumper. Why? Because when you start winching on a TJ bumper, you get combined tensile and shear stresses. With good bumper design and a low-mounted winch, the tensile stresses can be minimized.
But the shear stresses will always remain. A bolt has roughly 60% as much strength in shear as in tension. Based on information provided by Fastenal, the approximate shear strength of a Grade 5 1/2-13 UNC bolt is 9,000 lbs, and a Grade 8 is 11,300 lbs.
But those are shear strengths to failure, not to yielding. Now the expected yield strengths in shear are somewhere around 6,900 lbs and 9,800 lbs, respectively.
Given the way most TJs are set up, the vast majority of winching force is applied to the top two bolts. If the winch is pulling at 2" above the mounting surface, the moment about the bumper multiplies the top shear stress by a factor of 1.5, given that the reaction force against the bumper overturning is provided by the bottom bolts.
Many TJ manufacturers rate their bumpers to a 12,000 lbs winch. A stalled out 12,000 lbs winch will therefore apply roughly 18,000 lbs of shear across the top four bolts, or 4,500 lbs per bolt. So that alone will not shear a bolt, not Grade 5 nor Grade 8. Technically you still have a 2:1 factor of safety against total failure with Grade 5, and 3:1 with Grade 8. Not great, but your winch line will probably snap first.
But let's say whatever you're pulling isn't moving. So you grab a pulley, and run the cable back to one of your recovery points. An anchor point is generally placed in front of the frame rail, roughly centered. So that 12,000 lbs additional force is now distributed 50% top and bottom. The top two bolts now have an additional 3,000 lbs of shear for a total of 7,500 lbs of shear per bolt, and the bottom bolt has 3,000 lbs of net shear (6,000 lbs from the recovery point attachment negated by 3,000 lbs from the overturning force from the elevated winch).
So now your Grade 5 bolts on top start stretching, and the bumper begins to pull away on that side. They don't quite rip out, but they will probably have to be cut out with a grinder. The Grade 8 bolts are still fine.
Now let's say pulling 24,000 lbs just isn't enough, and the load slips. It transmits a 1.5x shock force to the line. Your grade 5 bolts snap, the bumper peels off the frame, and you get to watch some expensive things happen. The grade 8 bolts yielded and came insanely close to snapping, but didn't.
Now obviously that's a pretty unusual use case. But it kind of gives you an idea of what you could expect. In automotive design, engineers generally choose to use a factor of safety of about 3:1 to failure. Technically, we exceeded that when the single winch line began to pull about 12,000 lbs, even with the Grade 8 bolts.
Keep in mind that bolt failure is not the only way a bumper could fail. The material held in by the bolts could tear out, or the welded nuts could even tear out of the frame. Grade 8 has a modestly higher target torque value, and the additional clamping force will also provide some additional resistance to tear-out of the base material. In addition, the reduced stretch of the Grade 8 will also result in more even stress distributions in the surrounding material, reducing the odds of tear-out a bit more.
Flange bolts are preferred to normal hex bolts or cap screws because they distribute the load better. When paired with an appropriately graded washer of a good size, this provides the maximum resistance to tear-out.
Also note if the bolt does yield, it loses its preload and the odds of tear out of the surrounding material increase vastly.
In addition, a stretching or yielding bolt in shear changes shape, and often ends up being subjected to a combination of shear, bending, and tensile stresses, thus vastly accelerating the process of total failure.
Another factor not discussed is that the bolt preload (the existing tension due to torquing) decreases the shear strength by a significant factor. At the same time, that preload also adds strength in that it increases the friction between the two joined plates.
TL;DR: Buy Grade 8 flange bolts with quality washers when possible. But you probably won't die if you have to go with Grade 5.
Of course those numbers are meaningless without more details of the application.
A TJ bumper bolt is 1/2-13 UNC, at whatever length makes sense for the application.
A Grade 5 1/2-13 has a minimum break strength of 17,000 lbs tension, a yield strength of at least 13,000 lbs tension, and a proof strength of at least 12,000 lbs tension. Meanwhile the Grade 8 is a bit over 21,000 lbs, 18,500 lbs, and 17,000 lbs, respectively.
Note that minimum break strength is the tension required to break the bolt. The bolt will begin to deform past the yield strength, but will continue to hold. In most cases, yielding is considered a type of failure, as it changes the structure in a way that often significantly alters loads.
Proof load is a load that any bolt can be tested to with no visible signs of damage or deformation. It is only a single load test, and does not take into account repeated loading.
All of this is nice, but still not very applicable to the TJ bumper. Why? Because when you start winching on a TJ bumper, you get combined tensile and shear stresses. With good bumper design and a low-mounted winch, the tensile stresses can be minimized.
But the shear stresses will always remain. A bolt has roughly 60% as much strength in shear as in tension. Based on information provided by Fastenal, the approximate shear strength of a Grade 5 1/2-13 UNC bolt is 9,000 lbs, and a Grade 8 is 11,300 lbs.
But those are shear strengths to failure, not to yielding. Now the expected yield strengths in shear are somewhere around 6,900 lbs and 9,800 lbs, respectively.
Given the way most TJs are set up, the vast majority of winching force is applied to the top two bolts. If the winch is pulling at 2" above the mounting surface, the moment about the bumper multiplies the top shear stress by a factor of 1.5, given that the reaction force against the bumper overturning is provided by the bottom bolts.
Many TJ manufacturers rate their bumpers to a 12,000 lbs winch. A stalled out 12,000 lbs winch will therefore apply roughly 18,000 lbs of shear across the top four bolts, or 4,500 lbs per bolt. So that alone will not shear a bolt, not Grade 5 nor Grade 8. Technically you still have a 2:1 factor of safety against total failure with Grade 5, and 3:1 with Grade 8. Not great, but your winch line will probably snap first.
But let's say whatever you're pulling isn't moving. So you grab a pulley, and run the cable back to one of your recovery points. An anchor point is generally placed in front of the frame rail, roughly centered. So that 12,000 lbs additional force is now distributed 50% top and bottom. The top two bolts now have an additional 3,000 lbs of shear for a total of 7,500 lbs of shear per bolt, and the bottom bolt has 3,000 lbs of net shear (6,000 lbs from the recovery point attachment negated by 3,000 lbs from the overturning force from the elevated winch).
So now your Grade 5 bolts on top start stretching, and the bumper begins to pull away on that side. They don't quite rip out, but they will probably have to be cut out with a grinder. The Grade 8 bolts are still fine.
Now let's say pulling 24,000 lbs just isn't enough, and the load slips. It transmits a 1.5x shock force to the line. Your grade 5 bolts snap, the bumper peels off the frame, and you get to watch some expensive things happen. The grade 8 bolts yielded and came insanely close to snapping, but didn't.
Now obviously that's a pretty unusual use case. But it kind of gives you an idea of what you could expect. In automotive design, engineers generally choose to use a factor of safety of about 3:1 to failure. Technically, we exceeded that when the single winch line began to pull about 12,000 lbs, even with the Grade 8 bolts.
Keep in mind that bolt failure is not the only way a bumper could fail. The material held in by the bolts could tear out, or the welded nuts could even tear out of the frame. Grade 8 has a modestly higher target torque value, and the additional clamping force will also provide some additional resistance to tear-out of the base material. In addition, the reduced stretch of the Grade 8 will also result in more even stress distributions in the surrounding material, reducing the odds of tear-out a bit more.
Flange bolts are preferred to normal hex bolts or cap screws because they distribute the load better. When paired with an appropriately graded washer of a good size, this provides the maximum resistance to tear-out.
Also note if the bolt does yield, it loses its preload and the odds of tear out of the surrounding material increase vastly.
In addition, a stretching or yielding bolt in shear changes shape, and often ends up being subjected to a combination of shear, bending, and tensile stresses, thus vastly accelerating the process of total failure.
Another factor not discussed is that the bolt preload (the existing tension due to torquing) decreases the shear strength by a significant factor. At the same time, that preload also adds strength in that it increases the friction between the two joined plates.
TL;DR: Buy Grade 8 flange bolts with quality washers when possible. But you probably won't die if you have to go with Grade 5.
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Post a list of the sizes you need, I can check my local industrial hardware store. I can get flanged g8 too. I’ll check the prices. I could probably ship to you for much cheaper than you could get there or online.
for your needs, you should probably be looking shear specs of the bolt.
for your needs, you should probably be looking shear specs of the bolt.
If the recovery hook is rated at 10,000 lbs like most two hole deals are then the hook breaks before the bolt. I think bolt strength is always given as a function of diameter and not length, that sure is odd if a longer bolt is weaker. I wonder if those stupid bolt manufacturers just don't understand how this stuff really works.
I know it’s hard to believe but those stupid bolt manufacturers do understand how at least their products work. Even without on board engineers, they can follow known standards like SAE.
$25 for the Torx on ebay and $25 postage to Australia lol
TJ4Jim gave me his tow hook but the shipping to Australia was around $55 from memory!
They are better off coming from a company that does regular international shipping, ad hoc international shipping is expensive even for ebay sellers.
You are better off to bite the bullet and just get them from US like I did in the end! then you can get the correct Torx which look better than the Allen or hex head!
I got fed up looking and got them from Amazon USA which was cheaper than ebay USA.
If I was to have recovery hooks fitted elsewhere I would use hex head grade 8 or equivalent metric but for the OEM the grade 5 is what came out of the factory and I am sure they would have tested them ?
I now get a lot of stuff from US free shipping with Prime, first purchase covers the one years Prime fee.