Tech question, could use some advice.

Ok, well this is a long story, but I'll try to just skip along.

Wanted to change the Diff. fluid in the car, was advised to use black RTV to seal the diff cover when re-installing. I was told that there was no need for a gasket, but now I'm not so sure.

Drained all of the fluid, cleaned the hell out of the diff cover and the rear housing. Applied a decent seam of Permatex Ultra black RTV and tq'ed the bolts down to 32'#. Let it sit for a decent bit of time. Filled the diff. Amsoil Full synthetic 75w90 + their slip lock additive. Went and drove the car in figure 8's to re-distribute the fluid to the axles, etc...

Two days later, I notice that there is fluid on the bottom of the diff.. Now I'm scared, thinking that it's the pinion seal.. Well, thank god.. It wasn't. It is leaking out of the bottom of the howsing between the Diff/RTV/Cover.. All that I can figure is that it is the lighter viscosity of the full synthetic oil.

Now, I'm a perfectionist and extremely paranoid, so I'm going to take it all back apart again. I have a couple of questions.

1. Should I use a gasket? This is a direct quote from my Haynes Manual. "Apply a thin layer of RTV sealant to the cover flange then press a new gasket into position on the cover. Make sure the bolt holes aligned properly." LRS sells a gasket for the 8.8 <---link
2. I'm pretty sure I'm going to switch to Lucas 80w90 Heavy duty + Ford racing friction modifier. Any input?
3. Now, How much fluid to use? I've heard that 2 qts + the modifier is the correct amount, but when I filled it last time it took about 2 1/4 qts + the modifier? Should I just fill it to 2qts and be done with it?

Why does this have to be so complicated?

1. Gasket - YES!!

2. With the 3.73's I would use the heavy weight, but a full synthetic should be fine.

3. Fluid amount would depend on weight being used. We always use the pinky method. (Place pinky finger in the plug hole, feel fluid it's good).

Fast Eddie wrote:1. Gasket - YES!!

2. With the 3.73's I would use the heavy weight, but a full synthetic should be fine.

3. Fluid amount would depend on weight being used. We always use the pinky method. (Place pinky finger in the plug hole, feel fluid it's good).

Thank you Eddie, now one more question on the gasket. Do you agree with the RTV + Gasket install that Haynes recommends?

WNRacing wrote:
Thank you Eddie, now one more question on the gasket. Do you agree with the RTV + Gasket install that Haynes recommends?

Yes, this prevents 95% of all leaks.

Fast Eddie wrote:

WNRacing wrote:
Thank you Eddie, now one more question on the gasket. Do you agree with the RTV + Gasket install that Haynes recommends?

Yes, this prevents 95% of all leaks.

I assume based on the Haynes, apply the RTV only on the cover side, correct? Then just mate the gasket to the rear housing with no RTV?

Sorry if I'm asking too many questions, I already did this once 3 weeks ago.. I'd rather not do it again any time soon! Gonna be almost $40.00 worth of oil wasted! :*bonk(:

WNRacing wrote:

Fast Eddie wrote:

WNRacing wrote:
Thank you Eddie, now one more question on the gasket. Do you agree with the RTV + Gasket install that Haynes recommends?

Yes, this prevents 95% of all leaks.

I assume based on the Haynes, apply the RTV only on the cover side, correct? Then just mate the gasket to the rear housing with no RTV?

Correct!!

i didnt use a gasket on mine and it doesn't leak.

Never use RTV on a rubber gasket.

BlueStreak02gt wrote:i didnt use a gasket on mine and it doesn't leak.

I have no idea man, it's pretty hit and miss apparently.. Some have no issues and some have annoying drip issues like me. It's not like it's leaing a ton of fluid, I'd say at most it has leaked 1/8 of a qt, but it's enough to make me paranoid, especially driving from Gallatin to C'ville and driving the piss out of it on the track.

I just ordered the LubeLocker Gasket from Blue Collar Performance, LRS flat shipping rates suck major butt, I wasn't about to pay $15.00 to ship a stinkin' gasket.

Hopefully I'll be get it in by Thurs./Friday so I can get it installed and let the RTV cure for a full 24+ hours before I refill it. That should give me plenty of time to get my Quick release quadrent & firewall adjuster in.. Then, keeping my fingers crossed that my Hoosiers will get here by Friday so they'll be ready for Sunday! :*drive*: :*naughty*:

WNRacing wrote:

BlueStreak02gt wrote:i didnt use a gasket on mine and it doesn't leak.

I have no idea man, it's pretty hit and miss apparently.. Some have no issues and some have annoying drip issues like me. It's not like it's leaing a ton of fluid, I'd say at most it has leaked 1/8 of a qt, but it's enough to make me paranoid, especially driving from Gallatin to C'ville and driving the piss out of it on the track.

I just ordered the LubeLocker Gasket from Blue Collar Performance, LRS flat shipping rates suck major butt, I wasn't about to pay $15.00 to ship a stinkin' gasket.

Hopefully I'll be get it in by Thurs./Friday so I can get it installed and let the RTV cure for a full 24+ hours before I refill it. That should give me plenty of time to get my Quick release quadrent & firewall adjuster in.. Then, keeping my fingers crossed that my Hoosiers will get here by Friday so they'll be ready for Sunday! :*drive*: :*naughty*:

you got put a ton on the differential rim and on the cover rim...and i mean really thick, you can't just put a thin film on and expect it to do anything, or squirt it in squiggly lines...you wanna cover the entire lip of both the diff and the cover, and the torque 'em down tight, and wipe off the excess, and you won't have any leaks.

So Eddie says RTV + Gasket.. Sxynerd says only gasket.. Anyone else care to confuse me some more?!


PS: I used a pretty excessive amount of RTV.. Almost the 3/4 tube, still leaks.

WNRacing wrote:So Eddie says RTV + Gasket.. Sxynerd says only gasket.. Anyone else care to confuse me some more?!


PS: I used a pretty excessive amount of RTV.. Almost the 3/4 tube, still leaks.

wn, there's no right or wrong answer man. it comes down to what you feel comfortable with, and what you think is the best plan of action. everyone has preferences, superstitions, and likes and dislikes in how they do things with their stang. do what's best for your needs.

WNRacing wrote:So Eddie says RTV + Gasket.. Sxynerd says only gasket.. Anyone else care to confuse me some more?!


PS: I used a pretty excessive amount of RTV.. Almost the 3/4 tube, still leaks.

Not that I haven't done it , because I have but RTV is like a solvent on rubber. It will deteriorate a rubber gasket. The only exception i really make is small dabs of sealant in those key places like timing cover, head and cam cover joint.

If you want a gasket to last, give it the respect it deserves and let it do exactly what it was designed to do. If the gasket doesn't say to use rtv then don't. Don't use Cork on the diff and only use rubber/urethane gaskets once. (obviously we've all reused gaskets before and it can be done but it's not suggested) Oil makes them swell when it comes into contact and creates a deformed gasket and wont fit back together correctly.


If it were me, I would clean all the oil from the surfaces with acetone making sure no oil residue is left. Use RTV only, apply a generous amount to the diff cover and a small thin layer to the diff. (still making sure no oil leaked onto the matting surface) wait 5 minutes, then place the cover on and hand tighten all of the bolts lightly by hand allowing the RTV to slightly cure and bond for another 5-10 minutes, then tighten with a two stage tq sequence. It should take about 30 minutes for all of this to happen, which also happens to be the curing time for RTV Black.

IT IS VERY IMPORTANT THAT NO OIL OR FLUID TOUCHES THE RTV BEFORE IT"S CURED OR YOU"LL GET WHAT YOU ALREADY HAD HAPPEN.


This is the way we normally did it for average temps, the way we did it in 120* Desert and in -20* in Greenland on our military trucks. It's the way I do all vehicles and I've never had a single issue.

Just make sure oil or fluid doesn't touch the RTV before it's cured.

I never use a gasket and it never leaks. Just put a good bead around the cover and bolt it up.