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6 Cylinduh, Really Bruh?
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Discussion Starter #1
So this weekend's project was flushing and refilling my diff after about 1000 miles of driving with FRPP 4.10 gears. I had some left over 75w-140 RP laying around with a bottle of friction modifier so I said why not. Plus when the shop did it they only put in 2 qts. I read 3 quarts is needed after adding a GT500 finned cover so I wanted to see the results for myself.

The GT500 cover is sealed to the housing with a Lube Locker gasket so taking it off was a breeze. I first got the car level and removed the bottom drain plug and let the fluid drain out. While it was draining I see streaks of metallic. I don't know if this is left over from where they set the gears and that's the paint or it was very fine particles of metal. Next I took the cover off so I could fully scoop everything on the bottom of the pumpkin. I put on a rubber glove and got to work. No chipped or knicks or anything was seen on the teeth of the gear btw. It was almost like sludge under there. The magnet on the drain cover was also CAKED in what I could best describe as graphite. The fill plug on the cover and the OEM fill plug were both gunked up too but not nearly as bad as the drain plug. Mind you - 1000 miles. After getting everything cleaned up I refilled everything 1/4" below the OEM fill plug - it took 2.5 quarts (maybe a tiny bit less) and a 4oz bottle of friction modifier. The car is now quiet. The faint moaning I noticed (which is why I went back to my stock DS because the aluminum was so noisy, but stock still made noise) is completely gone. No more gear whine, moan, or drone.

So in an effort of pure curiosity I actually now question - are you supposed to change the fluid and do a complete dump of the rear axle after you drive with new gears for a while? Maybe the half quart made the difference?

Side Note - at the time of gear install, the entire rear end was redone (rebuilt trac loc, axle bearings and seals replaced). I reused the Lube Locker and no leaks. I kept the stock DS on and rotated it 90 degrees - no more shaking above 55MPH (bonus win). Instead of running vents on the cover and axle I plugged the axle and only vent the cover now.

I almost want some type of filter back there now to filter the fluid after what I saw. And the silence... the silence is bliss.
 

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You shouldn't have too. What kind of fluid did the shop use? It was probably low if they only used 2 qts. I've always filled almost to the drain hole where I can feel the fluid with my finger and it was right around 2.75 qts with an aftermarket Frpp diff cover. You don't need additional friction modifier with the RP fluid although many people do dump it in. Too much friction modifier the clutches will slip excessively in the diff.
 

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Makes me want to change mine again...
 

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6 Cylinduh, Really Bruh?
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Discussion Starter #4
You shouldn't have too. What kind of fluid did the shop use? It was probably low if they only used 2 qts. I've always filled almost to the drain hole where I can feel the fluid with my finger and it was right around 2.75 qts with an aftermarket Frpp diff cover. You don't need additional friction modifier with the RP fluid although many people do dump it in. Too much friction modifier the clutches will slip excessively in the diff.
The shop used the same stuff, royal purple 75w140 with a bottle of modifier. I gave them 3 bottles and they have me back 1 unopened. I found it suspicious but they are reputable in the mustang community here so I took their word being my first go at gears and all.

After break in then additional 500 miles my curiosity peaked prompting my personal at home change. Like I said about a quarter inch below the hole and it was golden at right under 2.5 bottles. The amount of gunk that came out... that's what shocked me

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6 Cylinduh, Really Bruh?
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Discussion Starter #6
I would say yes to your question. New Gears.....1000 miles......change fluid/full flush...... :) :) :)
Looks like lesson learned for the future. I mean if I look at it and think about it... it makes sense. New gears. Installed by a shop. Metal running together like that at now a higher rate. Bound to be something maybe not perfect in the machining of gears or install to cause metals shavings to shed as the metal wears together. I mean I cleaned it out really good in there. I may change the fluid again in another 1000 miles, or a year, for good measure as that is how much a year I drive the car.

Thanks guys! Lesson learned for me on this one.

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Looks like lesson learned for the future. I mean if I look at it and think about it... it makes sense. New gears. Installed by a shop. Metal running together like that at now a higher rate. Bound to be something maybe not perfect in the machining of gears or install to cause metals shavings to shed as the metal wears together. I mean I cleaned it out really good in there. I may change the fluid again in another 1000 miles, or a year, for good measure as that is how much a year I drive the car.

Thanks guys! Lesson learned for me on this one.

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So with that said, I'll ask this, with a new car, after 1000 miles should you change the gear oil in the rear end or have they already been broken in?
 

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6 Cylinduh, Really Bruh?
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Discussion Starter #8
So with that said, I'll ask this, with a new car, after 1000 miles should you change the gear oil in the rear end or have they already been broken in?
That's a good question. I think mine may have been due to play over year mixed with a mechanic doing the install? I mean I know you're supposed to change the rear diff oil in certain intervals. I've read guys say to change the gear oil after the gears wear in. Some guys say if the install wad spot on you don't have to. I would think the factory machines do it so there is no room for error where shops human error plays a factor. I'm only speculating and reporting on what happened to me

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That's a good question. I think mine may have been due to play over year mixed with a mechanic doing the install? I mean I know you're supposed to change the rear diff oil in certain intervals. I've read guys say to change the gear oil after the gears wear in. Some guys say if the install wad spot on you don't have to. I would think the factory machines do it so there is no room for error where shops human error plays a factor. I'm only speculating and reporting on what happened to me

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Thanks! Just wondering for future reference. I think it would make sense though after 1000 miles on a new car to change the gear oil.
 

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6 Cylinduh, Really Bruh?
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Discussion Starter #10
Thanks! Just wondering for future reference. I think it would make sense though after 1000 miles on a new car to change the gear oil.
You do make a valid point which has been brought up a lot.

I bought a brand new f150 recently. Twin turbo v6 and the dealer said I can let it rip on day one. No break in needed. That shocked me being a twin turbo application nevermind it being brand new in general.

This is a good question and I'll probably look more into this. I feel like what I'll find is that tolerances are so tight at the factory via machine built mass produced vehicles that things like break in is not needed.

On the flip side my girlfriend bought a brand new Mercedes amg which are hand built. She has to break the car in not exceeding 3k rpms for, I believe, 5000 miles.

I will tell you what I found shaving wise in the pumpkin and stuck to the magnets had me searching for some type of filtration system which they don't have because the rear end is splash lubed not pressure like oil or power steering. Watched an entire series from Banks that explains how the rear lubes itself via the gear turning. It also explained why covers are shaped the way they are.

One thing I did do recently that helped quiet the whine on my power steering pump was add a true cooler in place of the stock hard lines Ford put in. It's amazing the temp difference on the line in versus line out. I took that que from guys that autocross their cars. I redid the entire power steering system so I wanted it to last. Neat mod but I digress.

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6 Cylinduh, Really Bruh?
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5,218 Posts
Discussion Starter #12
True Cooler? Can you tell me more about it?
I went with this model as another guy with a supercharged mustang used it

Derale 13223 Frame Rail Fluid Cooler https://www.amazon.com/dp/B004XONO98/ref=cm_sw_r_cp_apa_i_OEHlCbNVGHKSC

I then used these to convert to Barbed
Earl's 700167 Super Stock Straight -6 Female to 3/8" Barb https://www.amazon.com/dp/B000CN7EB8/ref=cm_sw_r_cp_apa_i_oIHlCbQWJM97S

I then got engine and transmission fluid line. Not fuel line! It must be designated for use with transmission fluid as that's what our pumps use.

Derale 13016 Transmission/Engine Oil Hose https://www.amazon.com/dp/B004XONNTE/ref=cm_sw_r_cp_apa_i_BPHlCb202YREF

I then removed the hard line that came stock and plumbed this in and mounted with L brackets from home depot.

https://www.homedepot.com/p/Everbil...MIw-rM7ufR3wIVhLjACh2n-QgZEAQYBSABEgJ03fD_BwE

I painted the L brackets with high temp black caliper paint I had laying around. Now the fluid is truly getting cooled and not just running through a hard line.

I'll have to take new pics with my cobra bumper on. My old pics with the GT bumper on are on my old phone back up on my pc at home.

I believe I read somewhere GTs came with a hard line because models such as the Cobra had a true cooler there from the factory.

I don't auto cross but you can physically feel a difference when gripping each line after driving. Additionally it's on the low pressure side so minimal chance of leaking.

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I went with this model as another guy with a supercharged mustang used it

Derale 13223 Frame Rail Fluid Cooler https://www.amazon.com/dp/B004XONO98/ref=cm_sw_r_cp_apa_i_OEHlCbNVGHKSC

I then used these to convert to Barbed
Earl's 700167 Super Stock Straight -6 Female to 3/8" Barb https://www.amazon.com/dp/B000CN7EB8/ref=cm_sw_r_cp_apa_i_oIHlCbQWJM97S

I then got engine and transmission fluid line. Not fuel line! It must be designated for use with transmission fluid as that's what our pumps use.

Derale 13016 Transmission/Engine Oil Hose https://www.amazon.com/dp/B004XONNTE/ref=cm_sw_r_cp_apa_i_BPHlCb202YREF

I then removed the hard line that came stock and plumbed this in and mounted with L brackets from home depot.

https://www.homedepot.com/p/Everbil...MIw-rM7ufR3wIVhLjACh2n-QgZEAQYBSABEgJ03fD_BwE

I painted the L brackets with high temp black caliper paint I had laying around. Now the fluid is truly getting cooled and not just running through a hard line.

I'll have to take new pics with my cobra bumper on. My old pics with the GT bumper on are on my old phone back up on my pc at home.

I believe I read somewhere GTs came with a hard line because models such as the Cobra had a true cooler there from the factory.

I don't auto cross but you can physically feel a difference when gripping each line after driving. Additionally it's on the low pressure side so minimal chance of leaking.

Sent from my SM-G920T1 using Tapatalk
Thanks!!! I'll have to look into this more. I appreciate you taking the time for the write up and information sharing!!!!
 

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6 Cylinduh, Really Bruh?
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5,218 Posts
Discussion Starter #14
Thanks!!! I'll have to look into this more. I appreciate you taking the time for the write up and information sharing!!!!
No problem! For what it's worth it isn't something I'd do on your own unless you auto cross but since I was replacing the rack, pump, and all the lines I figured why not do it to hopefully extend the life of everything. Minimal costs and time to do

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6 Cylinduh, Really Bruh?
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Discussion Starter #16
I am not sure I would attempt that either.
Perhaps attempt was the wrong word. I should have said do. I wouldn't do it unless I autocrossed or already had the system apart. It's not like our pumps blow up under normal use. Since the system was apart I went ahead and did it. If everything were normal, and since it's just a street cruiser, I wouldn't have done it otherwise.

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Rent Asunder!
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RP already contains a good bit of FM. I don't think you'd want to add a bottle of FM to it. Be mindful not to let the FM levels get too high as FMs contain sulfur, and too high of sulfur levels in the oil can make it corrosive, resulting in spalling on the gears. You generally don't want sulfur to exceed 2.5%. RP 75w-140 is already around 1.8-2.0%, same as Amsoil Severe Gear.
 

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True Cooler? Can you tell me more about it?
I went with this model as another guy with a supercharged mustang used it

Derale 13223 Frame Rail Fluid Cooler https://www.amazon.com/dp/B004XONO98/ref=cm_sw_r_cp_apa_i_OEHlCbNVGHKSC

I then used these to convert to Barbed
Earl's 700167 Super Stock Straight -6 Female to 3/8" Barb https://www.amazon.com/dp/B000CN7EB8/ref=cm_sw_r_cp_apa_i_oIHlCbQWJM97S

I then got engine and transmission fluid line. Not fuel line! It must be designated for use with transmission fluid as that's what our pumps use.

Derale 13016 Transmission/Engine Oil Hose https://www.amazon.com/dp/B004XONNTE/ref=cm_sw_r_cp_apa_i_BPHlCb202YREF

I then removed the hard line that came stock and plumbed this in and mounted with L brackets from home depot.

https://www.homedepot.com/p/Everbil...MIw-rM7ufR3wIVhLjACh2n-QgZEAQYBSABEgJ03fD_BwE

I painted the L brackets with high temp black caliper paint I had laying around. Now the fluid is truly getting cooled and not just running through a hard line.

I'll have to take new pics with my cobra bumper on. My old pics with the GT bumper on are on my old phone back up on my pc at home.

I believe I read somewhere GTs came with a hard line because models such as the Cobra had a true cooler there from the factory.

I don't auto cross but you can physically feel a difference when gripping each line after driving. Additionally it's on the low pressure side so minimal chance of leaking.

Sent from my SM-G920T1 using Tapatalk
Where is your cooler mounted?
 
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