Mitchell overdrive

Lawrie

Lawrie

Well-known member
Im just doing some work on a friend's model A, it has a Mitchell O/D installed.
It has about maybe 30K miles on it, it shifts nice is quite ,and oil is clean.
as Im doing some work on the rear end replacing an axle shaft, O thought maybe I should look at the O/Drive.
The car will do a couple of long trips this year, maybe around 15K miles, so dont want it to give any trouble on these.

Lawrie
 
The Mitchell overdrive has bronze or brass synchro rings. Some kind of yellow metal. They recommend 85W140 GL5 gear oil, which is the kind that you find in the auto parts stores. The GL5 oil is hard on yellow metal. It actually does too good of a job protecting the metal with the anti wear ingredients. This causes little flakes of the yellow metal to flake off. A better choice is the GL4 oil which has less of the anti wear stuff and is recommended for transmissions with the yellow metal.

I did not find any 85W140 GL4 oil at any of my local auto parts stores. But I did find some single weight oil, 90 GL4. I recommend changing the oil to this. I was not able to find any 85W140 oil with the GL4 classification online.

Update: Just re read the label. The gear oil below is NOT recommended for synchronized transmissions.

Valvoline has a 85W140 oil that has both GL5 and GL4 designation. They say it is good for non-synchronized transmissions. See https://www.oreillyauto.com/detail/...y-Lz_gUGVhLQltytg9J-BT-AutI5yxwTi1mMuKiFJJKlh

The newer Mitchell overdrives have a grease point somewhere. Mine is too old to have this so I know nothing about it. Consult with Mitchell or read the instructions. I don't think it needs to be greased very often.

Also, make sure the vent tube is secured at its high point under the car.
 
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Spent a lot of time working on older Jeeps and many of them have manual transmissions with yellow metals. You can find GL4 at NAPA auto parts. Hard to find at the other big box stores, though.
 
nkaminar said:
The Mitchell overdrive has bronze or brass synchro rings. Some kind of yellow metal. They recommend 85W140 GL5 gear oil, which is the kind that you find in the auto parts stores. The GL5 oil is hard on yellow metal. It actually does too good of a job protecting the metal with the anti wear ingredients. This causes little flakes of the yellow metal to flake off. A better choice is the GL4 oil which has less of the anti wear stuff and is recommended for transmissions with the yellow metal.

….

The newer Mitchell overdrives have a grease point somewhere. Mine is too old to have this so I know nothing about it. Consult with Mitchell or read the instructions. I don't think it needs to be greased very often.

Also, make sure the vent tube is secured at its high point under the car.
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While I understand nk’s concern, Mitchell specifically recommends GL-5. I have a hard time contradicting a manufacturer’s recommendation. You can always call Steve and discuss with him. I use 85-140 GL-5 as recommended.

The newer overdrives have lube points on the input and drive coupling splines, inside of the respective housings. If you have these lube points you will see covers (short hex headed bolts) in the housings. Remove these covers and rotate the shafts until you see the lube points. One of them has a grease fitting on the coupling, and one has a set screw that you need to remove and screw in an extended length grease fitting (take it out and replace the set screw after you lube it). I don’t remember off the top of my head which is which but it’s obvious once you get the caps removed.
 
I use Master Pro SAE 140 GL-4 Gear Oil from O'Reily's. I was on the fence because Mitchell recommends SAE 85-140 GL-5 but the oil manufacturers say not to use GL-5 with yellow metals :confused: I use the 140 GL-4 in my Mitchell Synchro Transmission, Mitchell O/D and my rear end.

In the literature that came with the unit, Mitchell recommend NOT using synthetic gear oil.

https://www.oreillyauto.com/detail/...onal-gear-oil/mp03/80040?q=Master+Pro+SAE+140

55071136633_c684bed71b.jpg


This from the interweb:
"While the high concentration of sulfur-phosphorus EP additives makes GL-5 indispensable for differentials, this same chemical composition presents a significant risk in certain other components, particularly manual transmissions. Many manual transmissions, especially in older or classic vehicles, utilize synchronizers constructed from so-called “yellow metals,” such as brass or bronze. These soft metals are susceptible to corrosion from the active sulfur compounds present in the high-performance GL-5 additive package."
 
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nkaminar said:
.
The newer Mitchell overdrives have a grease point somewhere. Mine is too old to have this so I know nothing about it. Consult with Mitchell or read the instructions. I don't think it needs to be greased very often.
Click to expand...

The grease fittings are on the drive shaft couplings inside the Torque Tube, both front and rear. You need to remove a plug in the torque Tubes to gain access.
 
The first time I changed the oil in my overdrive I got some yellow flakes. That prompted me to change to GL4.
 
nkaminar said:
The Mitchell overdrive has bronze or brass synchro rings. Some kind of yellow metal. They recommend 85W140 GL5 gear oil, which is the kind that you find in the auto parts stores. The GL5 oil is hard on yellow metal. It actually does too good of a job protecting the metal with the anti wear ingredients. This causes little flakes of the yellow metal to flake off. A better choice is the GL4 oil which has less of the anti wear stuff and is recommended for transmissions with the yellow metal.

I did not find any 85W140 GL4 oil at any of my local auto parts stores. But I did find some single weight oil, 90 GL4. I recommend changing the oil to this. I was not able to find any 85W140 oil with the GL4 classification online.

Valvoline has a 85W140 oil that has both GL5 and GL4 designation. They say it is good for synchronized transmissions. See https://www.oreillyauto.com/detail/...y-Lz_gUGVhLQltytg9J-BT-AutI5yxwTi1mMuKiFJJKlh
Click to expand...

I just noticed the label on the Valvoline oil referenced above says it is not recommended for synchronized transmissions. That is what I used in my Mitchell transmission and over drive. I will drain it and look for yellow flakes.
 

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Post #5 "corrosion from the active sulfur compounds present" - when I see that word corrosion I think long term acids. One thing for sure, the manufacturers never put forth a lubricant to their customers without the product/formula going to the chemical engineers.

Jay makes a valuable point in post #4. However, this new information makes me receptive to considering a different formula.

Heck we get into the lubricant thing on here constantly, and everyone thinks they know all about it. Turns out they don't.
Guidelines for selecting a motor oil:
1) select a class of lubricant (paraphine, ash, synthetic, synthetic blend)
2) select a formula
3) select a viscosity
4) select a brand you trust (may need ot do homework - I have actually called product engineering at an oil company before. Have you?)

Be careful
 
Lawrie said:
I meant to say, should I look inside at the condition of the overdrive unit,
Lawrie
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No. You stumbled onto this, and it is working. Pick your formula, and change the lubricant. Take it for a spin, and recheck the fluid level. You have what you have.
 
The oil Used in the unit is fine, and what was recommended by Mitchell as per the paperwork
What I was enquiring about was ,should I open up the unit to check its condition, as its done a lot of miles and will do a lot more in the year.
Lawrie
 
Lawrie said:
I meant to say, should I look inside at the condition of the overdrive unit,
Lawrie
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I don't believe there are any user-servicable components inside. I asked Steve Mitchell once about whether a customer could switch between the 26% and the 34% (I think that's the taller one) and he told me that it can be done, but should be done at the factory.

BTW, if you need to get in touch with Steve, don't email him, call him. He doesn't really use email but will cheerfully swap stories and advice on the phone.
 
Gene F said:
Heck we get into the lubricant thing on here constantly, and everyone thinks they know all about it. Turns out they don't.
Click to expand...

Haha, You are so right! But because I know I don't know everything, and the oil manufacturers themselves recommend not using GL-5 Gear Oil in transmissions with yellow metals, I chose to use GL-4 Gear Oil.

I was just relating to what I use, not telling anybody to what to use.

As for the Mitchell O/D, it is a pretty robust unit.

55071325154_3be0730657.jpg
 
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Thank you, thats the info I was after, and looking at the pics it is a substantial unit, I will leave it assembled and just change the oil.( that is refill it )
Lawrie
 
Lawrie, You know how hard the O/D works in my tourer. About 70,000 miles have gone under it now and still going strong. (I use GL5), not that they have all been done with trailer in tow but at 30K, I don't think you need to worry about it.
 
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I have never seen the guts of one of those. Thanks. Given that photo, it appear either formula are fine.
 
There are no fittings in the driveshaft couplers. You'll need one of these to grease them.
 

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That comes with the new OD unit.Screw it in,grease it,unscrew it,and replace the plug in the housing.I have two units,and I wash them,put a caplug over the threaded end,put it in a baggie,and put it under the seat until the next time.
 
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