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This thread will walk through the installation of a Ford Thunderbird IRS under a Ford F100.

I will be posting pictures as the install takes place. It is scheduled for this week sometime. IRS is Independant Rear Suspension. The difference between an independant suspension and a solid axle is as follows. The independant suspension allows one or both or the tires to operate independantly of the other. If you hit a bump or an uneven section of road only the tire involved will be affected. With a solid axle if you hit a h*** with the right tire the left tire is also affected. As the right tire goes upward the left tire travels downward or possibly upward with the right tire. An independant suspension allows for the passenger compartment to maintain stability giving better control over the vehicle. Better handling in curves and overall a better ride.

Solid Rear Axle

Independant Rear Axle

Hub to hub on the Thunderbird IRS is 63.25". This swap is wider hub to hub than old truck widths. The only issue involved is wheel offset. If someone is wanting deep dish style wheels modifications would be required to accomodate such. Modifying the IRS would not be that difficult. If somebody was going to go that far I would be thinking custom tubular control arms. Shorter axle shafts would be required but nothing different from narrowing a solid axle. In my case I am opting for the newer style wheels as far as offset but will be having a set custom made to try to get as much deep dish look as I can get.

An advantage to the wider hub to hub is handling. It moves the pivot point outward making the vehicle less top heavy. The IRS is either coil sprung or bagged in typical configuration. The install does not interfere with wheel well clearance in any way. Without narrowing you can not run wider than stock wheel well width. Mainly because the end of the wheel well is pretty much right at the frame rail.

Animated images are owned by Truck IRS and are used here with permission.

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The start of the install is to remove the bed from the truck. Once the bed is removed anything preventing removal of the old rear end or install of the IRS needs to come out. In this case the mufflers were far enough forward so they could stay but the tailpipes had to go. This truck is getting a 460/C6 soon so the exhaust will stay as you see it in the pics until the engine swap. As luck would have it there are really only two U Joints used by Ford unless into a heavier truck. The truck's driveshaft had a small size and the Thunderbird had a large. My local auto parts store was able to find a u joint that would allow putting the TBird end on the truck driveshaft. This is a temporary as when the engine and trans swap takes place a new driveshaft will be built. I am intentionally not posting the u joint number as your driveshafts may have different u joints that need merged. Any good parts store should be able to locate the u joint you need.

I purchased new brake hoses and adapters to convert the 3/6" line over to metric so it could be screwed into the TBird hoses. The adapters are Edelmann 271300. The shocks are not being mounted as in the TBird so shorter shocks are needed. There are plenty of options available. I bought mine at Advance Auto. They are front shocks from a 1970-1982 Chevrolet Blazer. The Advance Part number is 59041. The Monroe part number is 33033. Napa also has Monroe shocks with the 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 eye type mounting on both ends.

I purchased a mount kit from Truck IRS. At the time of this entry the kit cost was $395. plus shipping. Ok here we go. Once the rear end and all other required items are removed we measure in 22 5/16" (SWB) 26 5/16" (LWB) from the end of the frame rail on each side and mark the frame. A note on the LWB dimension. This is not a verified number as this install was done on a SWB truck. I do not currently have access to a LWB to check. My recommendation would be to mark the axle centerline on the frame. Measure back toward the tail of the truck 14 1/2". We will be making a cut vertically from this point 1 3/4". Then with the frame level we mark the frame for a horizontal cut forward the length of the rear mounts. A note for those not installing into a F100. You need the rear mount level with the truck level. The reason for the cut into the rail is to provide a flat surface to weld the mount in with a full length bead. If you can accomplish this without cutting into your rail 1 3/4" you could just as easily cut more or less as need be. So far so good. Now comes the fun part, time to weld the rear mounts in place. After considerable checking for squareness the rear mounts are tacked into place. You will notice the h***s for the rear mount are slotted so you have a fair amount of room to shift the carriage around after the mounts are welded in place. In this install the spring perches and upper shock mounts are still on the frame. Just in case we got scared and wanted to put the 9" back in. They will be coming out here shortly. The mount kit comes with 6 gussets, 3 for each side. It also comes with the upper shock mount ears. Here is what the rear mount should look like at this stage.

At this point and visible in the picture the 2x2 piece of .125 wall tubing is weled into place. The length of this piece is 33 11/16" long. It is positioned centered with the mounting bolts & level. Weld to the top of the rear mounts as well as the frame rail. Here are a few pics of the install so far.

Cutting out the upper shock mount on the passenger side.

The upper shock mount (original) on the passenger side prior to removal.

A shot of the left side from the back of the cab.

A view from above before the 2x2 crossmember was welded in. The line with vice grips on it is the original brake hose.

A view from the rear of the driver's side.

One advantage to using the Thunderbird IRS: everything is mounted to the carriage. You have 4 main mounting bolts, 2 brake lines, 2 shocks and the driveshaft. Once they are disconnected the entire unit is out of the truck. HD axle shafts are available for this rear end rated for up to 650hp. Larger diameter sway bars are available aftermarket. Aftermarket drop springs are also available. Being an 8.8 you can get gears in practically any ratio you desire including a locker. I will be cutting coils out of my springs to get the correct ride height. If I have a problem with the springs after cutting I will look into other options. That is it for now.

The front mounts are in along with the 2x2 crossmember & coil springs. At this time 1/2 coil has been removed. Not much more to report at this time.

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.

View from above of the 2x2 in place.

View from above front.

Cutting off the left front spring perch.

Passenger side with both spring perches removed.

Putting the coil springs in. Left side view.

Here is a shot from behind.

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.

I want to say something concerning the time it is taking to do the install. My truck is in a shop and my install is not a priority job. Under normal conditions it would have been finished way before now. I had to put my truck in a shop because of a heart attack I had requiring a defibulator implant. One of my new rules for life is I am no longer allowed to be around welding. I planned on doing this install until the heart attack happened. The work is being performed by Greg Wilson at Moonlight Automotive in Greenwood, IN. There are two people I trust welding on my frame and he is one of them. To keep costs down my truck gets worked on as time permits.

Ok enough of that off to more pics! First we have the driveshaft installed. It uses that blend u joint spoke of above. I was somewhat concerned about length differences but it looks as though the 9" and the 8.8 are the same or at least very close to the same length from the center of the axle to the front of the third member. I have seen pics of putting the 9" yoke on the front of the 8.8. I do not like this as the 8.8 has a plate on the front the driveshaft bolts to. Since I will be having a driveshaft built in the future I wanted the TBird rear left as a TBird. When finished it will take a u joint for a 1995 TBird. Nothing special or difficult to find. So this install can be done without altering your current driveshaft if that be your desire. Just have a real parts guy find you the correct u joint.

Now we have some pics of getting the brake lines ran. The original hose was replaced although it did not need to be. A T 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 plan is finishing her up Monday if time permits. Oh just for the die hard Ford people out there while I was at Moonlight yesterday taking pics a guy walked in and asked me if I was the owner. Cutting the story short he absolutely loves the swap. Then he went on to tell me he had a lowered truck. I asked and he said an 86 Chevrolet. I asked how he lowered it and he said springs and shackle kit. "Rides like crap and bottoms out on the bump stops out back". I thought it kinda cool a Chevrolet person was doing a thumbs up to a Ford truck!

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

Now just some pics of the lines installed.

The truck is on the ground and for the most part finished. The left bearing is bad and caused the tire to rub. Had to leave it and have the bearing replaced. Couldn't drive it very far at all the way it is. I did get to take it for a short run and it rides excellent!

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

Same side from the rear view.

From the rear.

I did not make it back to get picks of the shocks installed before the bed went on but do have a couple pics of the upper mounts. Passenger side.

Drivers side.

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.

Outside. Not the best pics but I will take more tomorrow.

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







Looking good! Easy swap huh? Bummer about the bearing, that's why I'm going through every thing before I put it together. There's a reason stuff you find in a junk yard is there. It's always been my luck, I buy the "reason" someone junked the car.
A friend of mine has put one under an old Econoline PU, actually, he put the suspension that I'd originally bought for my truck, using an AeroStar front end. He had a machine shop drill the hubs out to match the front wheel pattern. Cost him $50, including the new studs. I priced out rebuilding my IRS, seems like it's in the $500-600 range, new bushings, bearings, seals, axles, etc.

The bearing ended up being ok. For some reason someone had started taking the axle nut off. Torqued the nut and all is good. IMHO for what was done it is an easy swap. I got the IRS unit at less than half of "normal" asking price around here so I have room for things like bearings and the such.  Eventually she will get a rear disc swap and bearings swapped out for Mustang units getting me a matching bolt pattern front and rear.

Bruce T. Lytle said:

Looking good! Easy swap huh? Bummer about the bearing, that's why I'm going through every thing before I put it together. There's a reason stuff you find in a junk yard is there. It's always been my luck, I buy the "reason" someone junked the car.


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