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4/04/2016 5:02 pm  #1

MN12 IRS in '68 Cougar

This is my first post on this site, but I have been reading quite a bit for a while now.

As the subject line says, I have an old Cougar that I am in the process of installing a T-bird IRS in.  I am using the factory cradle, cast steel differential and lower control arms.  This is not a speculative kind of thread; I have the old, leaking 8" and leaf springs out and am in the process of... trimming the cradle to fit under the car.

This brings me to my dilemma.  To make the cradle clear the rear seat area I have trimmed most of the front mounting area from the cradle.  I have had to cut most of the rear mounting area off as well.  The cradle is essentially dangling from the frame by a temporary cross member attached to the upper control arm mounts and clamped to the bump stops on the frame with large "C" clamps.  I may need some advice as to how to mount this thing.


4/05/2016 7:07 am  #2

Re: MN12 IRS in '68 Cougar

I guess a little information about the project may be useful. 

It has an efi'ed 302 and 4r70w trans.  I am anticipating 300 hp. to the wheels.  I have installed the fatman strut suspension system in front (the old version of the kit).  I am also in the process of rewiring the car.  The interior is gutted and I have had a 6-point roll cage installed.  It has been a garage ornament since 2008.  I've had a few different plans and intentions for the project in this time.  Job and wife and baby and work mean that I don't have a lot of time or money to throw into it.

I am definitely not a fabricator.  This is probably one of the more difficult projects I have taken on.  Any guidance that you all would be able to provide would be greatly appreciated.


4/17/2016 12:05 am  #3

Re: MN12 IRS in '68 Cougar

As I recall the Cougar was a slightly heavier and upmarket Mustang.
Pics of where you are with this might be helpful.

At these prices I want my gas 100 octane, leaded, and my windshield washed!

5/30/2016 12:51 am  #4

Re: MN12 IRS in '68 Cougar

I tried putting a MN12 IRS in my kit car. My first attempt was to use the X (or K) member as you had. Guess what, I found out, just as you had, that the front two arms extend too far into the passenger compartment. So, I too shortened these. Then I found that I needed to narrow the LCAs. Well that screwed up my spring perches so I had to convert to coil-overs. Ultimately I bought a set of Factory Five LCAs then was able to weld up a mounting cage as they do and probably saved 50 lbs. Its a lot of work though.


1/07/2017 12:43 pm  #5

Re: MN12 IRS in '68 Cougar

Got to love Murphy's Law...  Sorry for the delay.

I'm actually lucky the Cougar is slightly larger than the mustang.  If memory serves, it's 3 inches longer than the mustang.  All of this is in the rear frame rail section of the car.  The cradle itself tucks up between the passanger compartment and the gas tank nicely... with some creative trimming.

 I ended up needing to cut both front and rear mounting ears off the cradle and replace them with 2, 1x1x.120 wall tubing cross-members (one front and one rear).  The cross-members are currently held to the frame by 4, 3/16 plates with 3, 1/4-20 bolts, in single sheer, running through the frame, in each plate.  I will be enlarging the mounting bolts at some point as mock-up continues.

I will be using coil-overs, installed in the original spring pockets, to support the car.  I am presently working on the mounting brackets for this.  I have moved the lower mount as far out to the edge of the spring pocket as possible.  This places the upper mount outboard of the frame; between the frame and the wheel arch.  This should provide sufficient clearance for the wheels and tires.  I have also provided for reinforcement of this area by extending the rear IRS cradle mounts.  The upper mounts will also be made out of 3/16 steel.

The challenge that I have found is that the top of the coil-over mount ends up close to the frame.  To compensate (without having to cut the frame) I will need to lean the tops of the coil-overs out (toward the tire) by 5* from vertical.

I would appreciate any guidance regarding this.

Last edited by Frank]\[]<@ (1/07/2017 1:17 pm)


1/25/2017 1:53 pm  #6

Re: MN12 IRS in '68 Cougar

In looking at your install, it looks like you can actually mount the stock coil spring with a fabricated plate at the top.  Is that correct?


7/19/2017 9:21 am  #7

Re: MN12 IRS in '68 Cougar

It's amazing how much life gets in the way with these projects.

Anyway, Greg55_99, you are right; the stock coil spring would have fit very well.  When I pulled the IRS from the junk yard, I had them back-half the frame for me.  The upper mounts on the original frame sitting in my shed.  Unfortunately I've already committed to the 'coil over' thing.  I cut the coil spring locators off of the lower control arms while I was stripping and re-bushing them.  Kick myself for doing it now.  It would have made the project much easier.

So, to update.  I have sorted out the upper and lower coil-over situation.  I made it down to plan "D" before I found a viable solution.  I'll post pictures a little later.  I made the lower mounts out of 3/16 angle iron.  I hope it will be strong enough.

I was also able to drill out all of the mounting bolt holes.  There will be a total of 16, 7/16" bolts and 2, 1/2" bolts (for the coil overs) holding the cradle to the frame. 

That brings me to my question:  what is the best way to keep the frame from collapsing when I bolt the cradle in permanently?


7/19/2017 9:26 am  #8

Re: MN12 IRS in '68 Cougar

Keep going!  I am a fan of backyard innovation!



8/04/2017 6:16 am  #9

Re: MN12 IRS in '68 Cougar

Here's a shot of the rebuilt 8.8.

Front and rear mounting points on the passenger's side.

Here's a shot of the Passenger's side coil over and knuckle.

Here's what happens when you decide to go play with the car after an appendectomy -- all hopped up on pain meds.

(I don't know why most of the pictures are sideways.  Just turn your monitor)

sidewase and tiny...  perfect.


Last edited by Frank]\[]<@ (8/04/2017 6:19 am)


8/04/2017 1:56 pm  #10

Re: MN12 IRS in '68 Cougar

Can you make the pics a little bigger? 



8/04/2017 4:28 pm  #11

Re: MN12 IRS in '68 Cougar

Looks like its coming together - Careful with that wrist!  You Know "Murphy!."

Cheers - Jim

UNDERCONSTRUCTION! Highly Modified C3 Corvette
         Dual Wishbone IRS w Subframe + Custom Uprights

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