12mm vs. 14mm vs. 9/16"

[GodStang]

I have gotten a lot of questions on this so I decided to get some info together and will keep adding as I get more. Most if not all 99-04 Cobras with IRS come with a 12mm 10.9 (grade 8 equivalent) in the front of their irs were it connects to the frame. A common upgrade is to get 14mm bolts and replace the 12mm. I was getting ready to upgrade to MM subframe bushings. We were in the store with the front bushing sleeve seeing how well the 14mm fit. It fit a whole lot better than the 12mm. Well as we started to leave my dad grabbed a 9/16" (14.2875mm) bolt and slid it in and it fit perfect. No slop. Well some people asked what is the difference besides less movement of the irs to help get rid of the clunk. So I ran some calculations. Hope this helps.

Tensile Strength of a Grade 8 bolt (10.9 Metric) is 150,000 lbs per square inch.

12mm: 0.1753007in^2 X 150,000 lbs/in^2 = 26295.105lbs

14mm: 0.2386044in^2 X 150,000 lbs/in^2 = 35790.66lbs

9/16in: 0.2485042in^2 X 150,000 lbs/in^2 = 37275.63lbs

So for tensile strength 14mm has 9495.555lbs more than the 12mm and the 9/16" has 1484.97lbs more than the 14mm and 10980.525lbs more than the stock 12mm.

In our application we are more interested in Shear strength then in Tensile strength. Shear strength of a Grade 8 bolt (10.9 Metric) is around 60% of the tensile strength. We are going to use 91,000 lbs per square inch since that is what a lot of people were using.

12mm: 0.1753007in^2 X 91,000 lbs/in^2 = 15952.3637lbs

14mm: 0.2386044in^2 X 91,000 lbs/in^2 = 21713.0004lbs

9/16in: 0.2485042in^2 X 91,000 lbs/in^2 = 22613.8822lbs


So for Shear strength 14mm has 5760.6367lbs more than the 12mm and the 9/16" has 900.8818lbs more than the 14mm and 6661.5185lbs more than the stock 12mm.

I know some people have mentioned NAS bolts. I have looked every where for these bolts and the best I can find is $250 a piece online so I will not be using these on my car. I feel that the 9/16" grade 8 is more than adequate in strength and in size to minimize clunk and help improve the IRS.

Now I am also doing research for a better bolt to go in the top of the IRS so that I will not have to file down head to fit 10.5 rims. Alot of people are complaining about the MM low profile head not being able to be torqued well. Right now the best I can find is $159 for 1 bolt.

Please feel free to ask questions and add info.

Here some pics of the bolts and bushings:

These are the MM bushings with the sleeves notice how two sleeves are narrower then the other two. The narrow two go up top and are 12mm.

Here is the stock 12mm neck to the new 9/16"

Here is the 9/16" and nut

Proof of grade 8

 

[Torch10th]

:shrug: I had no issues being able to torque down the MM bolts...


Great info. When I do bushings I'll go with the 9/16's grade 8 bolt.

 

[quick01snake]

so w/ a calculated shear strength of ~15K lbs for the stock bolt, why are we even worried about strength, unless you plan to replace it w/ a less than grade 8 bolt...hell at the stock size the bolt has a factor of safety probably approaching 3...i thought this upgrade was purely to remove slop, nothing to do with increasing stregth, that being just a side effect of upping the bolt size...

 

[Torch10th]

Correct. The upgrade is to remove slop and "clunk" from the IRS.

The added strength is just a benefit. The cost between a grade 8 9/16 and the equivalent 14mm bolt is probably nill, so go with the 9/16th.

 

[kxt]

I went w/ 9/16s grade 8 and no problems here. As the diameter of the bolt increases the shear strength of the bolt increases assuming they are the same grade so.... since 9/16" is > 12mm then 9/16" is stronger.

 

[quick01snake]

I went w/ 9/16s grade 8 and no problems here. As the diameter of the bolt increases the shear strength of the bolt increases assuming they are the same grade so.... since 9/16" is > 12mm then 9/16" is stronger.

that's like saying you can find phone numbers in a phone book...duh (no offense)

all i was saying is why go into all this strength analysis if stock is PLENTY strong, and obviously anything larger of the same grade is going to be much stronger...so the added strength i wouldn't consider a "benefit", as it provides no advantages over stock...you're just looking for something that is physically larger....and now i have spent more time stating my point than Godstang did w/ the calculations, so i guess i'm done...

 

[03Steve]

Awesome info GodStang! :rockon:

 

[GodStang]

Yes that is correct about strength. I have had people ask about is the 9/16" grade 8 adequate compared to the stock bolt in the area of strength that is the whole purpose of the calculations. Yes it may be common sense to some of us but to others they have no idea what the grades mean and what grade the stock is. I am also stating that the common upgrade of 14mm is .2875mm smaller than the 9/16" therefor the 9/16" fits better for the lower subframe bushings reducing slack of the 14mm.

 

[viperbluelx]

Most of the clunk comes from wore differential bushings or too much backlash in the factory gearset. I have NO clunk at all and I still have the stock 12mm bolts up front.

 

[GodStang]

That is why I am also doing the bushing upgrade and all. I am just making sure no stone is left unturn. I am not saying this is the thing. I am just giving people another option.

 

[quick01snake]

sorry...i will shut up now...LOL

 

[UCBeau]

Very interesting, thank you for posting the info

 

[1999WhiteSVT]

good call to your dad for grabbing that 9/16 bolt.

heres a dumb question. what length is needed and how many bolts total? i havnt even had a chance to get under the car yet, but when i get around to the new bushings i dont want to make too many trips to the store.
thanks

 

[kxt]

^I think its 4.5" long.

Also I think most of the clunk comes from the CV joints in the half shafts. At least for me thats the only place there is play in the driveline

 

[GodStang]

They are 4 inches long and you only need two for the front. You also need 2 lock washers to go with it. As of right now I have not found a better top bolt and it is still 12mm it has a smaller "sleeve" at the top.

 

[viperbluelx]

Also I think most of the clunk comes from the CV joints in the half shafts. At least for me thats the only place there is play in the driveline

no

 

[01cobravortech]

My 01 came with 14mm bolts for all 4 subframe mounts. I have the MM subframe poly bushings and they don't come with any bolts unless you order. Thats weird that my IRS had 14mm all around. Definitely a worth while mod in my book. Suspension has felt different since install from last summer. However, I have noticed from looking under my rear that the rear control arm bushings have deteriorated more and have had substantial rear alignment problems and movement. Will soon be doing the Delrin kit for the rear arms because I have a lot of weird skipping in the rear when one tire goes over dips or crappy roads and the other one doesn't. Maybe its time to replace the KB rear steer tie rods. Maybe I'm mistaking this for a bump steer issue with the present tie rods?

 

[RoyWoods]

:shrug: I had no issues being able to torque down the MM bolts...


Great info. When I do bushings I'll go with the 9/16's grade 8 bolt.

You must have gotten lucky. The MM ones I got were worthless.

 

[GodStang]

Added some pics to answer questions.

 

[ROADFOOLSRIDE]

Thanks for the post. I'll be putting bolts in with my bushings as well.