IRS Center Section (again)

[karlos]

Hi all - new member here. I'd like to start by saying that I looked at the FAQ and researched old threads for the info I'm after. There are a of course lot of threads having to do with the FFR IRS, but none seemed terribly current. My apologies if this is already a dead horse.

I have an MK4 kit on order which I'm still tweaking. Although I know the IRS was the right decision for me and I'm not likely to change it, I'm finding (so far) that the availability of the necessary donor parts is fairly limited and the cost is corresponding high. Seems that the FFR IRS design is based on donor cars that are getting long in the tooth, leaving the builder with limited options. FFR probably could have been a little more upfront about this. Anyway, the used donor parts I've been able to find are fairly thrashed and I know I'll be left with a center section rebuild on any used part I happen to find. Given the specialized tools and expertise that are required, it's probably best that I not try to take this on myself. Looks like I can mostly source all new parts, but again I'm gonna need to send it out to have it all assembled. So maybe I should just cut to the chase and look for a new/rebuilt center section, and this is where my question comes in. Where can I source a new or rebuilt with new parts center section? I've looked at the Forte website and while this looks like a good option, it's not exactly cheap at $1745. I've also checked the Breeze website, but I don't see any complete center sections. Is there another source or option I haven't found yet? Thanks in advance for any help.

Regards,

-Karl
Denver, CO

[Bob Cowan]

AFAIK, Forte's is still the best bet. But the time you buy the used center section and then have it rebuilt, you're going to spend about that much anyway. Might as well have one shipped to your doorstep.

[thardin]

I think the parts are still out there. Take a look on some of the forums for the vehicles that would provide donor parts (T-bird or Lincoln). There was a guy named DECY on the forums several years ago that seemed to source this stuff from junkyards, strip the gears and centers and ship them out. As for Axles, I would highly recommend an aftermarket set like Raxles, etc.

Been checking ebay? Type "lincoln aluminum differential" and at least 5 of them come up for reasonable prices.

[thardin]

If you are not prepared to do the work yourself (install locker, gears, set-up, etc), I agree, probably best to go with something from Forte. I have had great luck with them.

[Jacob McCrea]

I have an iron center section with a Detroit TrueTrac limited slip for sale over in the classifieds section. Everything is new except the case, the gears (3.27 factory Ford) and the pinion flange. I am selling it for not much more than my out-of pocket costs, and all relevant specs (pattern, backlash, runout) are documented in photos. I do, however, still need to bead blast the cover, as other responsibilities keep taking priority over that.

[karlos]

Thanks for all the good information, guys. All roads seem to lead back to Forte. The Lincoln Mark VIII center section on Ebay is intriguing (thanks to thardin for the lead). It uses the correct aluminum differential housing, but I don't believe these came with a factory-installed Traction-lok diff, so I would need to change it. When all is said and done I doubt I'd end up at much less than the Forte unit, but I would have a lot more leg work. Doesn't seem worth the extra effort. So I suppose I'll give Forte a call on Monday and see if they'll knock a few dollars off for a package deal including knuckles, hubs, and a center section. Oh, and I'll check out the iron offering in the classifieds too, thanks. Anyone know the difference in weight compared to the aluminum housing?

[bansheekev]

Being in the west coast, shipping of large heavy items like this isn't trivial. I locally sourced a 1998 Lincoln MKVIII aluminum center section from the local pick n pull for $50. Took about 90 minutes to get it out. There were 4 Lincolns to choose from when I was there. It was a 2.73 open differential but I really didn't care as I was going to do an all new internals build anyway. Purchased a new Eaton TrueTrac, new Ford Racing gear set, new bearings, seals, pinion flange, etc and took everything to the local shop to be hot tanked then assembled. Total cost including shop labor was $975.

You just need to know the correct part numbers to purchase. I've got them all of you are interested...

I also sourced the outer suspension uprights and hubs from the same Lincoln for another $50. New Ford Racing hubs and bearing and poly bushings from VPM all sent to Stu Spencer to be rebuilt. Came in significantly under new ones here as well...
Kevin

[CraigS]

Don't get hung up on the aluminum. Yes it's lighter. But one friend said he went iron for extra weight for extra traction off the line. That is not incorrect IMHO. It would take an F1 driver to feel the difference between aluminum and iron and, I may be incorrect, but I think the aluminum case requires a spreader tool to install the center section where the iron case doesn't. If you are really worried, you can do an iron case and move the battery up front. Done.

[karlos]

Thanks; you've got me reconsidering. Feel free to double check my part numbers (all sourced from Summit Racing).

Traction-Lok Differential FMS-M-4204-F318C: $256
3.73 Ring & Pinion FMS-M-4209-88373: $160
Installation Kit FMS-M-4210-B2: $100
Stub Shaft Bearings & Seals FMS-M-4413-A: $30
Pinion Flange FMS-M-4851-B: $34
Used Housing (Ebay): $149

Total = $ 729

This leaves me with more than a $1000 budget on labor to do the rebuild. Gonna call around to a few machine shops tomorrow...

[thardin]

Make sure you let them know the housing will likely be empty (which you will want for shipping purposes). The set-up requires a different approach if you are setting up from a baseline of an existing set of gears, versus installing from a base case. In order to set it up from a bare case, you really need some specialized tools to do it efficiently.

Also, make sure that whoever sends you the bare case stamps the main caps to ensure they do not get mixed up (side and orientation).

Its worth looking into an aftermarket IRS cover that allows you to drain fluid and also allows you to preload the caps (adds strength).

You might want to do the research and use SVO / Cobra internals to get the additional strength. When installing the unit, you WILL wish you had gone with Aluminum, if you have not.

Also, yes, the aluminum "requires" a case spreader while the iron case does not. But any professional doing an install would likely use one as part of their normal routine. Its the best way to hold the unit for the rebuild anyway and ensures you get the shims in easily, rather than pounding delicate shims into a tight space. Unless you are replacing aftermarket shim stacks with single stock shims (unlikely) you should use a spreader in either case.

[Alig2]

Thanks; you've got me reconsidering. Feel free to double check my part numbers (all sourced from Summit Racing).

Traction-Lok Differential FMS-M-4204-F318C: $256
3.73 Ring & Pinion FMS-M-4209-88373: $160
Installation Kit FMS-M-4210-B2: $100
Stub Shaft Bearings & Seals FMS-M-4413-A: $30
Pinion Flange FMS-M-4851-B: $34
Used Housing (Ebay): $149

Total = $ 729

This leaves me with more than a $1000 budget on labor to do the rebuild. Gonna call around to a few machine shops tomorrow...

I'm also going through the process of sourcing an irs.
Big red flag for you, the axles from FFR are 28 spline, the Traction-Lock from Ford is 31 spline.
The 28 spline is no longer available new. The choces are: get a used one and rebuild it, have your rebuild guy supply one, change axles to 31 spline, or change the carrier side gears to 28 spline. A further complication is the side gears are different for irs vs solid axle, there is an additional groove to catch the axle c-clip and retain the axle. Old gears from the irs can be swapped into a non-irs trac-lock, the groove can be machined in the existing gears, or new gears can be purchsed. All in all, an irs is not very plug-n-play, which is why many just purchase a rebuilt unit. I'm going the parts swap/machining route, since I'm too stubborn and cheap.

[thardin]

Big red flag for you, the axles from FFR are 28 spline, the Traction-Lock from Ford is 31 spline.
The 28 spline is no longer available new. The choces are: get a used one and rebuild it, have your rebuild guy supply one, change axles to 31 spline, or change the carrier side gears to 28 spline.

Great POINT !

That is what I was referring to when indicating "cobra internals". You were much more specific, thank you.

[karlos]

Wow, you're right - I completely missed the fact that the Ford differential is a 31 spline. Probably explains the 31 in the part number. Duh.

Eaton has a few different 28 spline 8.8" offerings, but they run considerably more than the Ford. Just when I thought I had it figured out...

[Alig2]

It seems about time for FFR to come up with something a little easier to implement, the t-bird/mark vii solution made sense 10 years ago., today you have be more creative or pay the piper.

[Bob Cowan]

What do your axles look like? The standard T-Bird tri-lobe axle ends are no longer available. If you're buying new ones from one of the aftermarket companies, then they probably look something like this
http://i1296.photobucket.com/albums/ag13/BobsExocet/Rear%20Assembly/IMG_7552_zpsndlmhflv.jpg (http://s1296.photobucket.com/user/BobsExocet/media/Rear%20Assembly/IMG_7552_zpsndlmhflv.jpg.html)

If so, then changing the stubs from 28 to 31 spline is pretty simple.

[rich grsc]

I wouldn't be running 3:73 gears, not on a car this light. The 31 spline differential will work with IRS, the 28 spline will not.

[bansheekev]

Here is what I used on the inside:

Differential, Eaton Detroit Truetrac, 28 spline, 8.8" Ford (Eaton) - 912A562
3.55 Ring & Pinion Set (Ford Racing) - M-4209-88355
2003-2004 COBRA PINION FLANGE (Ford Racing) - M-4851-B
Bearing/Seal, 2 Stub Shaft Pilot Bearings, 8.8" Ford (Ford Racing) - M-4413-A
8.8" Ring Gear And Pinion Install Set (Ford Racing) - M-4210-B2

Kevin


Thanks; you've got me reconsidering. Feel free to double check my part numbers (all sourced from Summit Racing).

Traction-Lok Differential FMS-M-4204-F318C: $256
3.73 Ring & Pinion FMS-M-4209-88373: $160
Installation Kit FMS-M-4210-B2: $100
Stub Shaft Bearings & Seals FMS-M-4413-A: $30
Pinion Flange FMS-M-4851-B: $34
Used Housing (Ebay): $149

Total = $ 729

This leaves me with more than a $1000 budget on labor to do the rebuild. Gonna call around to a few machine shops tomorrow...

[karlos]

Kinda settled on the 3.73 rear based on the 0.64 5th gear in the TKO 600 I plan to use. But there is a variant with a 0.82 5th gear. Maybe this is a better choice along with a 3.55 or 3.27 rear? Will be running a Coyote as the power source, so I don't think the low-end torque will be overwhelming.

I'm not following the comment about the 31 vs. 28 spline differential. Did you inadvertently reverse the numbers, maybe? Pretty sure the FFR-supplied half-shafts are 28 spline, right?

Thanks to everyone for keeping the newb outta trouble!

[bansheekev]

I'm running a Coyote, TKO-600 with the .64 5th gear, and 3.55's. I am very very happy with this combo. 1st gear is very usable and cruising on the freeway at 70MPH it's revving 2100 or so RPM. You can carry on a conversation with a passenger on the freeway no problem. With the .64 5th and going to a 3.73 1st gear would get shorter and the RPM on the freeway would increase. Tremec has a speed/RPM calculator on their website I believe...

Keep in mind that is primarily a street driven car. If you're going to road race there are other considerations...

Kevin

[karlos]

Sounds good. Thanks for your help - much appreciated. Don't want to go too tall (I live in Colorado; some big hills here!), but the 3.55 / 0.64 combo sounds like a good compromise.

Really appreciate the help, guys. I think you've all saved me from doing something stupid at least a couple times already.

[rich grsc]

Kinda settled on the 3.73 rear based on the 0.64 5th gear in the TKO 600 I plan to use. But there is a variant with a 0.82 5th gear. Maybe this is a better choice along with a 3.55 or 3.27 rear? Will be running a Coyote as the power source, so I don't think the low-end torque will be overwhelming.

I'm not following the comment about the 31 vs. 28 spline differential. Did you inadvertently reverse the numbers, maybe? Pretty sure the FFR-supplied half-shafts are 28 spline, right?

Thanks to everyone for keeping the newb outta trouble!
The part # you quoted is a 31 spine differential, but as you know the shaft from FFR is 28, Ford lists 2 differentials for sale in their parts catalogue, the M-4204-F318C (31 spline), and the M-4204-F288 (28 spline). The M-4204-F288 will not work with IRS shafts, the other will. My catalogue is a couple years old, they may not carry the 28 spline any longer? Just be sure the differential is compatible with IRS shafts.

[karlos]

Here's where I ended up. Very happy with the result.

Complete center section scavenged from a '94 Lincoln MKVIII: $149
All new internals (Eaton Tru-Trac, 3.55 ring & pinion, bearings, seals, etc.): $800
Ford Racing cover (girdle): $250
Local shop fee to install parts: $350

Total = $1549


Center Section As Received
42339

After Rebuild
42340

42341

42342

42343

[MPTech]

I went a similar route, bought the empty aluminum pumpkin from a forum member, then bought all new bearings, 3.55 gears, clutch packs, etc to have it professionally built. Very happy with the results so far. Plenty or torque with my T5 and still gets decent gas mileage on the highway.

BTW, someone else may want to confirm, but I think the Ford Racing cover is going to interfere with your installation.

[Martin]

I had to remove the racing cover and put it on once I had the main part in position. Might just be me being an idiot though.

[karlos]

That's my plan as well - install the cover after the center section is located in the chassis. It looks like the Ford racing cover is about 1/16" thicker than the OEM cover at the rear attach point. Anybody have trouble getting the cover to bolt up to the mounting tabs on the FFR chassis?

[2bking]

I remember having to cut the mounting tab about 1/8 to get it to fit in the chassis. I did it on a mill to keep both sides parallel. The side closest to the centerline of the car was in the correct location so I trimmed the material off the outer side. I installed mine with the cover attached but it was very difficult.

[karlos]

Just dropped off the cover at my local machine shop. The OEM cover lug measures 1.45 thick; the Ford Racing cover is 1.50. Not off by much, but it's more than I'm willing to bend the chassis mounting tabs. So heads-up for anyone planning to use the M-4033-G3 cover: the rear mounting lug is a tad thick for the chassis and will probably need some attention.

One other gotcha I just became aware of...the bolt circle on the FMS-M-4851-B Cobra pinion flange is incompatible with the FFR driveshaft. The driveshaft uses a 1330 series U-joint along with a flange yoke that has smaller bolt circle. I could replace the pinion flange but don't really want to take the chance of messing with the bearing preload, etc. Luckily, Neapco makes a 1330 Series Ford flange yoke to fit 8.8 inch rear ends with the large bolt pattern. So a relatively simple swap of the flange yoke on the driveshaft will fix the problem. About $37 available here: http://www.dennysdriveshaft.com/p374_1330_series_flange_yoke_fits_8.8_inch_rear_en ds_large_bolt_p.html

[DJDJDRAM]

Does an S550 work, it’s all the seller has?

[BRRT]

This is a very old post that references the previous IRS setup. If you are looking at a new kit from FFR then nothing above will apply.
The current kit uses a 2015-2023 Mustang or newer center section, knuckles, hubs and brakes.

Dig into some of the newer build threads. Several guys have a lot of detail in putting together their IRS. Some bought new parts from FFR (or other vendors) and some bought dropout assemblies from totalled cars.