View Full Version : Differential Rebuild

01-27-2010, 12:59 AM
I am getting prepared to order parts to rebuild the rear differential. I would keep the rear gears and just swap out the differential but there are other issues that need to be addressed such as the pinion seal is leaking.

Brief summary of vehicle: Engine, and drive line are completely stock. Car has 50k miles. 58 passes on the drag strip (no hard launches). Primary use is for street, and may occasionaly run at the drag strip.

Below is a parts list for the rebuild:

FRPP 3.73 ring and pinion.
Ford 8.8 IRS side bearings and seals M-4413-A.
Ford gear install kit, shims, bearings M-4210-B.
Cobra Pinion Flange.
Detroit Truetrac 913A561 Ford 8.8, 31 spline.
Differential Busing kit MM1RSB-45
And to close it all up,
FORE Precisionworks Billet IRS cover (still waiting for availability since on backorder).

02-26-2010, 06:21 PM
I have ordered some of the parts. The Fore Precisionworks IRS cover is due to arrive soon. Too bad they discontinued it but I got my order in before the deadline.

02-27-2010, 10:40 AM
Any particular reason you went with the Fore Precisionworks?

03-01-2010, 09:50 PM
For starters, I wanted the strengh of the Billet aluminum part. I would have gone the LPW route if the Fore part was unavailable. Secondly, the car will now have the option to add a differential cooler in the future.

Next on my list is a more aggressive clutch when the ford rebuilt unit begins to slip (too much drag racing did the original in quickly, ford replaced it when I had the TOB issues). New clutch will be Spec Stage 3+.

I will also be making changes sooner or later on the supercharger. Still trying to convince myself to either change the pulley and tune it or just go for broke on a Whipple.

The LPW is a nice part and at a good cost, but I wanted to have some options down the road.

I have yet to break a stock part in the rear end but there is considerable amount of oil leaking from the differential cover (even with additional sealant). I figured while the cost to rebuild the trac-loc differential is mainly tied up in removal of the IRS why not upgrade the differential completely along with a mild change on gears.

Still have to order the diff bushings from maxim motorsports. Just ordered the true-trac diff, and stainless steel 1/2npt and 3/8npt plugs through Jegs. The gears, bearing/shim kits I ordered from JoeHeckRacing.

03-01-2010, 10:29 PM
Sounds like you have some fun plans!! I just had my diff rebuilt locally...seems it needs a tad more break-in for some of the seals, as there is a grinding while turning sharply, but slowly...

03-02-2010, 01:30 PM
Sounds like you have some fun plans!! I just had my diff rebuilt locally...seems it needs a tad more break-in for some of the seals, as there is a grinding while turning sharply, but slowly...
What differential are you using? The grinding while turning does not sound too good. If you are using a Torsen or Detroit(Eaton) True-trac they will make some additional gear noise.

As far as the rebuild, I hope to have it done before April (weather permitting). I have been reading up on the road course racing and it sounds like fun. The drag strip is okay just to see how quick the car is in stock form where I was able to dial in a 12.7 average. I may go to the strip again after the minor mods to see if there is any improvement on ET's. I would rather take out more time to run on a road course for which I have yet to do.

03-02-2010, 03:49 PM
YOU haven't done the roadcourse thing!!!?? Well, better start a savings account for that because you will be HOOKED!!

03-02-2010, 04:05 PM
Oh crap, I already spent it. :rolleyes:

So far, my cost of parts has reached $1700.00. I could have saved some money on the LPW cover and just rebuilt my traction-loc. That would have taken roughly $500 of the top. At least the shop I will be using will only cost $300 to rebuild the rear. I would do the work myself, but I have no experience swapping gears or dropping the IRS. I also did not have to get a hand-held SCT X3 tuner either but why not. At least I can correct the speedo depending on which set of tires I plan to run. With the tuner I could also drop the pulley size down to a 3.1 without a problem but will hold off on a pully for now.

03-02-2010, 08:52 PM
Why not save a little money on the bushing kit and get it from Bruce AC427Cobra?
It is superior to the MM one and less money. WIN -WIN!

You are going to love the true trac! And the fun factor bang for the buck of doing an open track weekend is through the roof!
Check out the you tube link in my sig! Gary filmed it! WAY FUN!!
.. :thumbsup:

03-03-2010, 04:54 AM
I already ordered the MM bushing kit. Not sure if there would be a restocking fee for returns. The major cost factors on this project are the differential and the differential cover. I wonder if I could turn a profit on the diff cover now that they are discontinued. :D I think I will keep it and go forward with the project.

I will have to take pictures of all the parts (once they arrive) and maybe a before and after picture. Not sure if the shop will alow me to take pictures of the process.

03-03-2010, 04:59 AM
The video was great. Looks like fun. Of course now I want to take the car out and the weather is not good, rain and wet snow.

03-04-2010, 12:16 PM
Glad see the cost of the rebuild hasn't deterred you away from the IRS. Let's face it, its expensive to do, but well worth it.:D

Being where you are in PA, you've got so many great track options within a reasonable driving distance (Pocono, Watkins Glen, Mid Ohio, New Jersey Motorsports Park, VIR, etc.):bounce:

Get that 04' Cobra out to an Open Track Event and see what that IRS can do for you. You'll be hooked, just like the rest of us!:thumbsup:

03-04-2010, 12:47 PM
Check out the you tube link in my sig! Gary filmed it! WAY FUN!!
.. :thumbsup:
Ha...I was just watching that video yesterday!

03-04-2010, 02:42 PM
I have been frequently visiting full tilt boogie website for information. I think there will be some other upgrades to do later on. My focus will be securing the differential. Then I can work on the remainder of the IRS bushings some other time when funding is not an issue. Too bad I am not working now since I have been laid off back in October. Now that it is getting a bit warmer out I believe I can do some of this work myself (except for setting up the differential components since I lack a press and proper tools to install the ring and pinion gears). My single bay garage is a bit small and no place to put the IRS if I drop it out except outside.

My location may be great for those who are hooked on open course racing. I am expecting that to change soon. Due to lack of available jobs that match my qualifications (Electrical Design Engineer) in PA, I have been looking elsewhere to continue with my career objectives. I was disappointed about not getting the job in TN, but there is Iowa or Indiana (which would be moving to NC). NC is one of my favorite states. Not sure where I will end up by next month. I will have all the parts but may pospone the rebuild if I land a job a few hours away from my current location.

I am prepared to say goodbye to my local drag strip but will have to do that with some rubber and a few time slips.

03-04-2010, 03:01 PM
I would rather spend the money to rebuild the IRS and make it more compliant to what it was designed to do. If I was really fixated on drag racing and nothing else, a solid and built 9" rear end would be the only way to go which would cost far more than upgrading the IRS. Every time I drive the Cobra, I am pleased with ride quality and drivability the car has. I have driven a few mustang GT's that my friends have had or still have. It does not compare to the IRS Cobra.

When I picked up the vent tube at the ford dealer, I went into the show room just to look. They had a Roush 427 on the floor. That was nice, but $32k over invoice on the GT for a supercharger and stickers just did not suffice. The build sheet they had listed all of the stickers and such and little on the performance components installed. If I ever trade in my 04, it will have to wait until 2014 if and when Ford brings back the IRS. I would love it if SVT did another cobra with the 5.0L engine and supercharger with an IRS. Even if they did, it would be hard to let the 04 go and just buy it outright without a trade in. Just what to do with 4 vehicles?

I installed a custom SCT X3 tune last night and took the car out. I was amazed on the difference in acceleration with a stock engine and gears. I am still commited in changing the gears to 3.73 which is close to the 3.55.

03-04-2010, 08:21 PM
Here is what I got so far. Waiting on the differential, gear install kit, differential bushings, and the 1/2 npt and 3/8 npt plugs.

03-05-2010, 01:40 AM
I guess it is a good idea to check all doors. Found a Fedex box by the side of the house while taking out the trash this evening.

Note: Parts came from Jegs. Eaton true-track differential, Gear change kit M-4210-B, and the plugs for the Fore Diff cover. I could have used standard plumbing parts but the square post type galvanized steel or brass parts were sub-par as for quality. I wanted parts that could be removed when needed to change/add gear oil without having to use a crecent wrench or box wrench. The square posts generally round off. The plugs use a hex drive socket and are stainless steel.

All I need now are the bushings and the project can begin (once I set a time with the shop).

03-05-2010, 09:31 AM
So that's what that stuff looks like!

03-05-2010, 06:52 PM
Considering the weight of all the parts (not including the differential carrier) makes me glad that I will not be working on my car laying on my back to discover the differential assembly is heavier than I thought it to be.