No look at the pic above. Its only 4" I have done multiple Cobras all with this setup and it works great!!
14mm IRS bolt question
- Thread starter 2001gunmetalgt
- Start date
No look at the pic above. Its only 4" I have done multiple Cobras all with this setup and it works great!!
The bolt is important, but it's not critical. If you had wheel hop before replacing the bolt with a larger one and the new bolt cured wheel hop, the original bolt was most likely very loose!
:thumbsup::coolman::beer:
Jpsh post this on Cheaters how toGrade 8 9/16 is the way to go. I have been running 9/16 since 2003. The 14mm still has alot of play in it but the 9/16 has almost zero play. Got mine from Lowes.
And yes its the shaft not the head and its the front bolt not the top bolt.
Here is a write up I did long ago on one of my swaps:
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
Sounds good. I haven't worked on mine at all, because I bought it in December and stored it away right when it got home. I'm gonna do a brace and switch that bolt, and Nitto's and see how the car acts and go from there.
Since you've got some experience with this...do you use any washers or just the nut and bolt?
Last edited:
Where you think I got it? I posted it there long ago so I was able to get to it quickly.
As far as washers they bolt up just like the stock ones. Meaning I have just drawn a black. I will check my car when I get home.
Users who are viewing this thread
