Thread: MN12 IRS install in a 1970 F100 SWB

This thread will walk through the install of a Ford Thunderbird IRS into a F100 SWB pickup. This swap can be done on virtually any truck with a 34" wide frame. The process remains the same regardless of make. The IRS was put in 1989 - 1997 Ford Thunderbirds, Mercury Cougars and Lincoln Mark VIIIs. They are readily available in multiple configurations. They came with either disc or drum brakes and the Lincoln units came with an aluminum third member housing. They can be easily purchased used for between $150 & $400. I paid $125 for the unit used in this thread.

These IRS units come completely mounted on a subframe that is mounted into the donor vehicle with 4 main mounting bolts. HD aftermarket sway bars and axle shafts are available. With the 8.8 third member almost any gear ratio you would desire is available along with locker units. If you have the fabrication skills required to do this installation you can easily fabricate an additional third member support as well.

The bolt circle is 5x4.25 and can be swapped out using bearing units from Mustang IRS units for a 5x4.5 bolt circle. The 5x4.5 circle was only offered with disc brakes to my knowledge. The 5x4.25 came with either disc or drum brakes.

I will be using a mount kit from Truck IRS. At the time of this article the mount kit cost was $395 plus shipping. The kit includes:

(2) rear mounts
(2) front mounts
(6) gussets
(4) upper shock mount ears
(2) sleeves

In addition to the mount kit you will need to purchase:

(1) 2x2x33.6875 steel tube
(2) shock absorbers
(2) standard to metric brake line adapters
(1) U Joint
(1) MN12 IRS Unit Complete

Shock absorbers:
The shocks are not being mounted as in the Thunderbird so shorter shocks are required. There are plenty of options available. I bought mine at Advance Auto. They are front shocks from a 1970-1982 Chevrolet Blazer. The Monroe shock built for Advance is part number is 59041. The Monroe part number is 33033. The Monroe shock built for Napa is part number 94005. If you would prefer something different you are looking for a shock with a compressed length of 10 1/8" & an extended length of 14 5/8". You want 1/2" eye type mounting on both ends.

Brake line adapters:
The brake lines in the Thunderbird are metric. The brake lines in the F100 are standard so an adapter is required. The adapters are Edelmann 271300 although any adapter with matching specifications will work.

U Joint:
As it turns out Ford for the most part uses 2 basic U Joints. A large and a small. The drive shaft in the pickup used the small version while the Thunderbird IRS uses the larger version. Any good parts store can find you the proper U Joint to mate your driveshaft to the Thunderbird IRS. I have seen this install done putting the old 9" yoke on the front of the 8.8. This pickup will be receiving a 460/C6 swap soon so a new driveshaft will be needed. At that time I will have it built using the Thunderbird U Joint on the rear. This will make servicing in the future easier as parts will be standard and not a blend of 2.

I am not employed by Truck IRS nor have I ever been. I have no vested interest in Truck IRS. The pickup used in this thread is my personal pickup. I like the swap and wanted to share the process with others should they desire to do something like this to their pickup.

First remove the bed from the truck. Once the bed is removed you willwant to mark the centerline of the axle on both sides of the frame. At this point remove everything that will be in your way for a rear end swap. This will usually include the exhaust and possibly fuel tanks depending on configuration of the truck. The truck used here has the fuel tank in the cab so the only thing removal required was the tailpipes from the exhaust. New tailpipes will need to be built so you can throw the old ones away.

Disconnect the brake lines from the brake hose and remove the brake hose from the original rear end. This hose was clamped off with vise grips to keep the brake fluid from leaking out. Disconnect the upper shock mounts fromt the frame. You can either take off the top of the shocks or I recommend just cutting the rivets out of the frame and taking the frame mount with the shocks. You will need to remove them later anyway. Disconnect the parking brake cables that run to each wheel. Remove the driveshaft.

At this point the only thing holding the old rear end in place is the 4 spring mounts. Again you can unbolt these or just cut the mounting rivets and take the mounts with the springs. You will remove them later if you do not take them with the rear suspension. You should now be looking at a bare frame from the back of the cab to the tail of the frame. The rear end you removed should look something like this.





It is now time to start the actual install. Measure back (toward the tail of the truck) from the axle centerline marks you made on the frame 14.5" and mark the frame. Now check from the tail of the frame that both new marks are the same distance from the end of the frame. If they are not double check your 14.5" dimension. If it is correct then split the difference from the tail and move the mark so that each side is identical. Example: You have a .5" difference between the sides with the left side being shorter than the right. Split the difference (.5" / 2 = .25") and move your mark on the left side .25" towards the front of the truck and the right side .25" towards the rear of the truck. The marks should now be identical from the tail of the frame.

You are most likely working with an older truck. The bushings in teh original spring eyes may be completely worn away allowing the original rear end to move front to rear. This could cause the difference in your dimensions from the tail. For a Ford SWB pickup the dimensin from the tail of the frame should be 22.3125". At this point level the truck. Now from the marks made on the frame that are behind the centerline marks mark a vertical line up 1.75". A note on this dimension, you may need more or less depending on your frame configuration. You are creating a horizontal cut into the frame rail on each side that is level. It will also be as long as the rear mount as to provide a complete surface for the weld required. 1.75" is the dimension used on the pickup in this install.

Now that you have your vertical line figured and in place mark a horizontal line from that point forward (towards the front of the truck) the length of the rear mount. With your outline in place cut along the vertical and horizontal lines removing this section of frame. This is where we will be installing our rear mounts. With the rear mounts bolted to the rear mounts of the subframe align the complete unit into the frame squared up and tack the rear mounts to your frame.

Double check your location on each side and if all is well take the 2x2 square tubing and place it on top of the mounts, centered on the rear mount bolt holes, between the frame rails. With it clamped at each end to the mount and level weld the new crossmember (the 2x2) to the top of the rear mount and to the inside of each frame rail. At this time you can weld the 3 gussets on each side in place and finalize the welding of the rear mount to the frame rail. You should at this time look like this.

With the rear mounts in place you will notice that the mounting holes for the subframe are slotted allowing movement of the subframe to the front or the rear. Measuring forward from your rear vertical cut line 14.5" you now have the centerline to align the IRS to.

Using a floor jack level the subframe. Then align it on the centerline marks on your frame. You are now positioned to tack the front mounts in place. Loosely mount the front mounts to each of the front mount points on the subframe. You will notice these holes are slotted allowing the mounts to be moved in and out. With the front mounts against the frame tack them in place.

Verify you are level and if all is good go ahead and weld the front mounts to the frame.

Passenger side front mount in place.


Another view of the passenger side front mount.


A view from the rear of the left side front mount.


Another view of the left side front mount.


A closeup of the left front mount.


A closeup of the passenger side front mount.

You will notice in the previous pics the spring mounts are still in place on the frame. These are slated for removal.

Passenger side with both spring perches removed.


Cutting off the left front spring perch.


It is now time to install the coil springs. We started with removing 1/2 coil from each spring for ride height. Up to 2 coils can be removed. I would recommend going slowly when cutting coils. We disconnected the rear mounts to drop the subframe down and installed the coils that way. You can just as easily disconnect the spindle and install the springs in that manner. Now a few pics with the coils in place.

Putting the coil springs in. Left side view.


Here is a shot from behind with both springs in place.

The wheels are on this morning for a look. Still need to do the shocks, brakes, driveshaft & a few odds and ends. In these shots the hoist is completely on the ground. The truck is sitting on it's suspension.

Left side from the front.


Right side from the rear.


Right side from the front.


From behind.

At this time it is time to do the tedious small things that make it a complete install. As stated in the beginning I aquired a blend U Joint to put the Thunderbird yoke adapter onto the end of my driveshaft.



Now we have some pics of getting the brake lines ran. The original hose was replaced although it did not need to be. A tee could have been put in line at the point of the old hose and solid lines ran to the new hoses for each wheel. I bought new hoses for each wheel and it was determined the best mounting point was on the front mount.



The new upper hose is mounted to an existing hole in the carriage that we tapped to accept a 5/16" bolt.


Now just some pics of the lines installed.




All that is left is bleeding the brakes out! We now have the driveshaft back in along with the brakes operational. We still need to mount the shocks but you are really close to the test drive.

Time to install the shocks. The 2 sleeves that were in your mount kit are for the lower mounts. The holes in the control arms are metric. They need to be bored out to .625". The sleeve can then be placed in the new hole allowing for usage .5" mounting hardware.

The passenger shock getting lined up for the upper mounts to be put in.

From the front view.


Same side from the rear view.


Passenger side upper mount.


Drivers side upper mount


Bed on, from the rear, still on the hoist.




Under the truck, left side, from the front.


Under the truck, right side, from the front.

Now the final product. A note concerning this particular truck. The front suspension is from a 2005 Crown Victoria Police Interceptor with 1.5" drop springs.





Got some pics to give an idea of scale. I am 5'11" tall.

I wondered what all was involved in this swap. I had a cherry Mark VIII that got totaled when some dumb A$$ ran a stop sign and hit me in an F-250. I stripped the car for everything but the drivetrain and sold it to a guy that was planning to put everything in a ford truck also, don't remember what year. Then, I got a replacement 403,xxx mile Mark VIII and dropped valves and 3 cylinders. Quad cam mod motors aren't usually cheap but I got one out of another Mark VIII with on 6k miles. Now I'm paranoid to drive that car. (I always have bad luck when I drive Lincolns.)

Originally Posted by Dog Dish Deluxe

I wondered what all was involved in this swap. I had a cherry Mark VIII that got totaled when some dumb A$$ ran a stop sign and hit me in an F-250. I stripped the car for everything but the drivetrain and sold it to a guy that was planning to put everything in a ford truck also, don't remember what year. Then, I got a replacement 403,xxx mile Mark VIII and dropped valves and 3 cylinders. Quad cam mod motors aren't usually cheap but I got one out of another Mark VIII with on 6k miles. Now I'm paranoid to drive that car. (I always have bad luck when I drive Lincolns.)

I am not sure exactly what your question is. I wrote this article as a step by step covering the install under my F100. It is basically the same for any frame that has 34" wide rails.