Rear End and Transmission Fluid Change

[Bill B.]

I have a 65 Coupe, 327/300, 4 speed and 3:36 gears. I changed the transmission oil and rear end oil in 2001 and have put about 7,000 miles on the car since. Is there a recommendation for changing the fluids based on calendar time as opposed to mileage? I searched the archives and was unable to find a answer. Thanks in advance for any advice.

Bill B.

[Mark E.]

Re: Rear End and Transmission Fluid Change

Wow! 7,000 miles in 14 years! You're the kind of previous owner I like to buy from...

Have you looked at the recommended service interval in the owner's manual? I THINK (I don't have mine in front of me) that these fluids have change intervals based on time as well as mileage.

These fluids are not susceptible to combustion gasses like the engine oil is, so they probably don't degrade as much over time. With that said, they do absorb moisture which contributes to corrosion and degrades the effectiveness of the additives.

In any case, after 14 years, just change the fluids...

... and don't forget your coolant, brake, and power steering fluids.

[Mark E.]

Re: Rear End and Transmission Fluid Change

An additional thought-

You didn't mention this, but of all the fluids in your car, brake fluid degrades the most quickly over time. Brake fluid is hygroscopic so it can cause rust in the system even if the car is never driven.

I flush the brake fluid in all of my cars at least every two years. I believe this explains why I have had few problems with leaks over the years.

[Bill B.]

Re: Rear End and Transmission Fluid Change

Mark Edmondson....Lol, I'll keep you in mind if I decide to sell. Most of the last 14 years were spent in Saudi Arabia and the UAE in field service so the car didn't get driven much but it will now. I regularly change the engine oil, coolant and have changed the brake fluid a couple of times. I looked in the service manual but didn't see anything about the rear end or transmission but I'll double check. Thank you for the reply.

Bill B.

[Ed S.]

Re: Rear End and Transmission Fluid Change

The reason there is nothing in the service manual about trans & diff fluid change intervals is because the fluid outlasts the hardware.

[Mark E.]

Re: Rear End and Transmission Fluid Change

The reason there is nothing in the service manual about trans & diff fluid change intervals is because the fluid outlasts the hardware.

In seriousness, if you drove a mid-60s daily driver Chevy 20k-30k miles per year, then yes, maybe the gear oil would out last the hardware.

But if you have a hardly-ever-driven collector car, time decay of fluids is a real concern.

[Duke W.]

Re: Rear End and Transmission Fluid Change

GM did not recommend manual transmission and axle oil changes back in the day. I recommend changing axle oil on Positrction axles every 30K miles to flush out clutch wear debris, and it's not a bad idea to change gearbox oil at 60K. Time is not an issue.

On a car with low mileage accumulation brake fluid should be changed every 2-4/5 years. Warm, dry climates can go toward the high end. Otherwise corrosion will take a toll and you will have problems in the 10-15 year time frame. Mileage is not an issue.

Change anti-freeze (Zerex G-05) about every five years or 30K miles, whichever comes first.

Based upon your mileage accumulation, you don't need to change the transmission or axle oil unless you get bored and need something to do. Neither takes much time or is expensive and will certainly do no harm if done properly.

Duke

[Bill B.]

Re: Rear End and Transmission Fluid Change

Thank you Duke and everyone for the replies. I'm bored so I think I'll change the lube just to be safe. I change the coolant every 3 years and I'll do the brake fluid now as it's been 4 or 5 years.

[Mark E.]

Re: Rear End and Transmission Fluid Change

Duke- What is a "Beyond Control Poster"?

Sounds impressive.

[Don H.]

Re: Rear End and Transmission Fluid Change

Look at side of the upper box of his post, (and yours) opposite from his name. There is shown his (and your) join date and No. of posts. He's been here 3 months longer than you and has 12,000 + posts. You have 50. That is a beyond control poster. Son, you have a ways to go if you hope to catch up.

[Mark E.]

Re: Rear End and Transmission Fluid Change

I've wondered about the various titles. Does a list of these exist, maybe with the criteria for each?

I'm not asking for promotion; just curious.

[Duke W.]

Re: Rear End and Transmission Fluid Change

Yes, I'm sure many think I talk too much, especially after a few of my favorite craft beers.

BTW, the original spec for the manual trans and axle oil is Mil-L-2105D. That milspec was cancelled years ago and replaced by the commercial spec, SAE 80W-90 GL-5, which is commonly available in auto parts stores. Brand makes no difference... just make sure it has the API "donut" with that information.

From memory the trans capacity is 2.5 pints and 3.7 for the axle. If you have Positraction you will need one 4-oz. bottle of limited slip diff. additive, which you can buy much cheaper at NAPA or most parts stores than a GM dealer. I don't have a part number handy, but you should be able to find it at napaonline.com.

Some use GL-4 in the trans, which may be okay, but I still recommend GL-5. GL-5 is an absolute must for the hypoid gear axle!

The axle does not have a drain, so you will have to siphon or pump it out. Make sure the axle oil is hot and drain it ASAP after a drive so all the Postiration clutch wear particles are in suspension and will be expelled with the fluid. Your Muncie may not have a drain plug either. At some point they were eliminated, but I'm not sure of the year. A quick peak underneath should reveal the answer.

I think there is a list of "top posters" on the TDB, but I haven't looked at it for years. I don't think anyone will ever catch old Joe L. until he goes to that big lobster smorgasbord in the sky.

Duke

[Donald A.]

Re: Rear End and Transmission Fluid Change

For trans I use Sta-Lube GL-4 from NAPA. Great article in the last issue of CR on this.

[Frank D.]

Re: Rear End and Transmission Fluid Change

As do I - lots of stuff about corrosion of 'yellow' metals with GL-5 which some refute in certain blends.
Don't even want to take the chance - I used Amsoil GL-4

[Duke W.]

Re: Rear End and Transmission Fluid Change

The factory fill for C1 to C3 manual transmissions was GL-5 or its predecessor Mil-L-2105D, and I have yet to hear a report that any suffered synchronizer corrosion damage. Has anyone else? Worn synchronizers are common, but corrosion?

It's another internet myth. The GL-5 EP additive can cause corrosion damage of "yellow metal" in laboratory tests, but it doesn't happen until oil temperature is well above the normal operating temperature. That's why it's never actually observed in old transmissions.

Duke