IRS Differential Installation Problem

[Rhino]

We are having a major installation problem aligning the mounting bolts on the IRS differential. There were no problems getting it up into the chassis. We used an engine hoist and straps to install it. The manual suggests getting help from a friend to lift it in from the bottom. Unless your friends are all NFL lineman, I'suggest the engine hoist. Our problem came in getting the front and rear bolts to line up laterally.

At first, we loosely bolted in the rear. When we attempted to put the front bolts in, no amount of pushing, prying or cursing could move the front over far enough to let the bolts slide into the chassis' bushing/sleeve assembly. We tried again by first installing the 5/8" front bolts. The rear mounts would not line up from side to side. We hoped we could get enough side play in the front bushings to allow us to thread in the 1/2" rear mounting bolts. No luck.

We then sighted through the rear bushing/sleeve assembly. The lateral misalignment was about .070" We could take up about half of that by rigging up a long pry bar. There's not enough slack in the mounts to correct that much side to side misalignment. The only place to solve the problem seems to be in the front.

We have considered two possible solutions. One would be to open up the holes in the front mounts on the differential. I don't like drilling out those mounting tabs any larger. They have already been drilled out to accept the 5/8"front mount bolts. Slotting the front holes could be acceptable. However, I no longer have access to a mill. Besides, I have no idea how I'd mount the differential on a mill. Grinding slots could take hours, days, weeks... Our other idea is to use 1/2" grade eight bolts instead of the bigger 5/8" bolts. The differential originally had 1/2" bolts up front. That might not give us enough side clearance. It would also leave the 1/2" bolt room to rattle around in the 5/8" sleeve.

There doesn't seem to be a good solution. We would greatly appreciate knowing if any of you have had this problem. What is the best solution you found? Thanks for the help.

Terry

[michael everson]

I told myself the next time I ran into this i was going to just bolt up the lower bolts, then get a drill the same size as the upper steel bushing and just drill the threads out of the aluminum cover. then by new longer bolts and nuts.
Mike

[KCaniglia]

Isn't the front support one piece? Was it made wrong? What if you drill out threads on rear of diff to give you some movement. I believe orginal front bolts were 14mm.

OBTW the aluminum housing is 24lbs lighter.

[JohnK]

My IRS also had some misalignment when we attempted to install it. I didn't measure so I can't quantify the amount of misalignment, but here's how we installed it. First off, it is relatively easy for 3 people to handle it, and being able to move it around by hand was helpful. I started by pushing the metal inner sleeves of the rear bushings almost all the way out of the outer poly bushings. We then put the IRS in position and got the rear bolts just barely started (1-2 threads engaged) with the metal inner sleeves still sticking out. This allows a lot more wiggle in the rear bushings than if the metal inner sleeves are seated in the outer bushings. With the rear bolts barely engaged, you can now wiggle the IRS around enough to get the front bolts pushed through. Then go back and tighten the rear bolts (they'll pull the metal inner sleeves in with them as you tighten them). EdwardB, in his 20th anniversary roadster build thread, discusses making some "guide pins" for the front bolts out of 5/8" bolts here. I made a set of these also, but ended up not needing them. I think that, between the wiggle room you'll get by pushing the metal sleeves out of the rear bushings, along with possibly making a set of guide pins, you should be able to get it positioned properly.

[Windsor]

When you "installed the rear bolts first" .... and later "installed the front bolts first", did you leave the first set of bolts loose or did you tighten them before proceeding to the other side?

(ETA) Well, JohnK pretty much covered the point I was trying to make.

That's what I get for leaving the browser tab open for a couple of hours before responding.

[Rhino]

John K
Pushing the inner sleeves out sounds good. The amount of offset looks pretty severe, but the extra wiggle room should help. It doesn't look like the sleeve can be pushed out very far before the bolt starts to push it back in. We'll give it a try tomorrow. Every little bit should help.

[Rhino]

We are going to drill only as a last resort. The back of the housing is threaded aluminum. I talked to a nearby shop who said they know builders who have widened the front hole into a slot. I see no sign of a manufacturing defect, but without specs, it would be hard to tell.

[Rhino]

sleeve.jpg
I tried to attach a photo of a bushing to show the amount of offset.

[Rhino]

It's difficult to see the offset. It's a small crescent at the lower right. Not sure if I can figure how to post a larger photo.

[edwardb]

This is the post aJohnK mentioned. From almost 10 years ago (hard to believe...) and used when Factory Five first introduced the newly designed IRS on the 20th Anniversary Roadsters. I made the pins by grinding down bolts. Something similar can also be purchased. With the rear bolts started but loose, the pins will line things up and take up the misalignment you're describing. I've done it on several builds and has worked every time. https://thefactoryfiveforum.com/show...l=1#post209902

[Namrups]

This is the post aJohnK mentioned. From almost 10 years ago (hard to believe...) and used when Factory Five first introduced the newly designed IRS on the 20th Anniversary Roadsters. I made the pins by grinding down bolts. Something similar can also be purchased. With the rear bolts started but loose, the pins will line things up and take up the misalignment you're describing. I've done it on several builds and has worked every time. https://thefactoryfiveforum.com/show...l=1#post209902

I also tried Pauls (EdwardB's) approach and it worked without issues.