Replacing Drivetrain Oils - Help!

[Mighty Burger]

Hey guys, I'm new here and new to the automotive field, please be gentle with me

 

I recently got a 1983 Datsun 720. It's a 5 speed manual, 4-wheel drive and has a 2.4L engine, odometer reads 160k miles. Small bed, standard cab. For the age it's running pretty great, but there's a couple things I want to look into fixing.

 

I have lots of trouble shifting from first to second, especially if it hasn't warmed up. It also makes a ton of noise when I put it into four-wheel drive - I know extra noise is normal, but it doesn't sound like it should. I got to thinking and I'm pretty sure none of the previous owners touched the oil in the transmission, transfer case or the two differentials, so I want to change out the oil in all of them (the engine oil was recently changed). I have a couple questions about that.

 

What type of oil should I use? I've looked around a ton and haven't found a conclusive answer that instills confidence.. A couple people have mentioned 75w90 gear oil. So for my transmission and transfer case I would use GL4 75w90 gear oil, and for both differentials I'd use GL5 75w90 gear oil right?? If I can't find that type of gear oil would I be safe using a close equivalent motor oil?  And how much oil would I need to buy? (For reference I live in Idaho, we get mildly warm summers around 80s and cold winters below freezing but rarely dipping below 0F)

 

Is there anything I need to be really careful of when doing this? Do you guys have any advice? I've only done a couple minor things so far, like running a new battery cable from the positive terminal to the starter, so I don't have lots of experience but I'd love to learn.

 

I've already asked enough questions but another thing I've noticed, there's a slight rattling noise in the engine, almost like a diesel engine. I think I need to do a valve adjustment. Is that something an amateur like me could handle?

 

Thanks for the help, I'm having lots of fun learning about this sort of stuff.

[Crytek89]

@datzenmike does a good job explaining what you need in this thread -> 

 

[Mighty Burger]

Thank you @Crytek89. Looks like I should be getting 80w90 oil instead of 75w90. I'll see if I can find any. I called around and only one shop in town carried GL4 75w90, they only had Amsoil brand and it would cost me $75 to get a gallon of it.. ouch!!! I'll look around again and see if they have the 80w90. If I cannot find it anywhere, would the 75w90 be acceptable?

 

Also, an update. I took my truck to my high school electronics teacher. We've known eachother for a while, he used to be an airplane mechanic! And he also had a Datsun 720 at one point that he worked on a ton. He helped me check all of the oils, and my suspicion was correct. The transmission oil was thick, dark a little murky, looked degraded. The transfer case oil looked good but was a little low. The front differential was almost bone dry, the little bit of oil that was in there was an awful thick creamy goop - I'll be avoiding 4 wheel drive until I get this taken care of. And the rear differential was horrible. The oil looked creamy, like coffee creamer, nasty and polluted, looks like water got in it. After a little looking around we found a vent hose that goes to the rear diff that was broken off, so every time water got through to that point it just went into the differential. I have absolutely no clue whether I could ever find a replacement part that would fit.

I should probably change out these oils pretty quickly here..

[620slodat]

You were wondering about transmission oil. I plan to use "AC Delco Synchromesh Transmission Fluid - Friction Modified" GM USA part #10953509 (very slowly building a diesel Datsun  620 pickup). This number is from 2013 so it could have changed by now. Supposed to be a synthetic that will make your transmission shift much better, and should also work for cold weather. This is something I found somewhere else, then found this info is also good for Datsun/Nissan transmissions as it is GL-4 rated. Also, Datsunmike has used it and can give first hand experience with it.

 

Do you think that you might possibly at any time go through water up deeper than your stock vent on your rear axle? If so, look into putting a longer vent hose clear up high into your motor compartment and put some kind of filter on it. The same thing could be done to other vents for the transfer case, front differential, etc. and tied into the same vent hose used by your rear differential.

 

Don

[datzenmike]

8 hours ago, Mighty Burger said:

. A couple people have mentioned 75w90 gear oil. So for my transmission and transfer case I would use GL4 75w90 gear oil, and for both differentials I'd use GL5 75w90 gear oil right?? If I can't find that type of gear oil would I be safe using a close equivalent motor oil?  And how much oil would I need to buy? (For reference I live in Idaho, we get mildly warm summers around 80s and cold winters below freezing but rarely dipping below 0F)

 

 

 

 

Transmission..... GL-4 only. 75w90 is fine. Two liters or just over 2 US quarts should do. Fill till you can't put more in. Don't try to force more in. Drain bung is on the bottom of the front case and has a magnet on it, so have a good look at it for bearing pieces and chunks. A fine black paste is normal. Clean it for next time you inspect it.

 

Trans Case .... Same as transmission. 1.5 quart.

 

Steering box... Same as transmission for oil. One pint

 

Differential.... GL-5 75w90 is fine. 1.3 liters or almost 3 pint. 

 

 

The T-100 is known for being noisy when it's been abused.

 

You can substitute synthetic oils but they will seep past old seals. For the transmission I'm using GM Delco Friction Modified Synchromesh Transmission Oil. I suspect there is only one maker and other brands add a colorant and put their own name on it. It's synthetic and thinner viscosity and I replaced the rear transmission seal and it's been ok. Shifts are faster (with the same effort) and it now downshifts where before it would be very stiff and end with a small grind.  With 75w90 the first few shifts were stiff and it only shifts well when warmed up but with the synthetic the shifts are all good even down below freezing.

[bottomwatcher]

I just replaced my vent pipe on my axle a few days ago. All it is a brake line any autoparts stores carry it. 10mm threads. Just thread it in, cut off the extra and tuck the end into the hole on top of the spring perch. Less than $8 from autozone.

[datzenmike]

Where are you in the US? what state?

[Mighty Burger]

I'm from Idaho.

 

I'm a little concerned about that seal leak. It seems like a difficult task, and I don't think I have the skills to replace that seal. I don't want anything to start leaking if I put synthetic 75w90 in it.. But the only oil I can find of this type (GL4) is synthetic, and it would be nice to be able to shift smoothly when it's freezing outside.

[datzenmike]

It may not leak with synthetic but it has with my T-100, differential and transmissions. The seal at the back of the transmission is $5 and easy as taking the driveshaft off.

[Mighty Burger]

Well, I just went and bought four quarts GL4 and three quarts GL5, 75w90. Gear oil's expensive...

[datzenmike]

20 hours ago, datzenmike said:

 

 

Differential.... GL-5 75w90 is fine. 1.3 liters or almost 3 pint. 

 

 

 

 

[Mighty Burger]

I'm changing the oil in both the front and the rear differential. With a little looking around I found this: https://imgur.com/a/xx4xIw0 (liters on the right), which seems to indicate I would need 2.25 liters or around 2.4 quarts to fill both differentials. Please correct me if I'm missing something. If I have any leftover oil I could just return it, I made sure to keep the receipt. Thank you for being patient with me.

[datzenmike]

2.2 pint front and 2.75 pints for the rear. Roughly 5 pint or 2.5 quarts. Bang on. Forgot it was 4x4. Change every 30K and check level at least every year. Visually check oftener for wetness or leaks.

[Mighty Burger]

Thank you. I'll cross my fingers and hope it won't leak, if it does, at least the replacement seal doesn't cost much at all. Hopefully this oil change will help to make the truck run as long as possible.

[datzenmike]

It's the synthetic oil that may seep. It's very thin and slippery. Ordinary GL is fine.

[Mighty Burger]

Well, darn. I purchased synthetic oil, it was the only type of GL4 any store in my rural town had. I might consider returning it and finding some sort of conventional gear oil online. I did look around, and I wasn't able to find any. In fact I was barely able to find any GL4 at all, but maybe my google-fu needs some work..

I searched a couple videos, and taking the driveshaft off to replace the seal seems like quite a task especially for me. I would guess the fact it's 4wd and the fact the previous owners lifted it would make it even more tedious. How much would a mechanic charge for this job? Maybe I just need to man up and be more confident in my abilities..

If it starts leaking, would taking out the synthetic stuff and putting in the regular oil help? If not, would it help to use some sort of additive like the stuff from Lucas to stop the leak?

 

That was quite the ramble. I'm just trying to figure out what the best thing to do is.

Sorry to keep bugging you. I really appreciate the help.

[datzenmike]

It may not leak but every one I've done started leaking after switching to synthetic. One differential, two seals in the transfer case and two different transmissions.

 

 

Definitely do it yourself, mechanics are crooks. 12mm or 14mm??? wrench, a screwdriver and a heavy hammer with some weight block of wood. The driveshaft bolts to the transfer case, unbolt (4 bolts) and pull it out of the transmission. Pry old seal out with screwdriver. Tap new seal in lightly with hammer and piece of 2x4. Use suitable diameter socket to fully seat the seal into the end of the transmission. If you raise the truck level on a hoist there should be no problem with oil spilling out. Grease the outside of the spline before assembling so it does not start up dry.

 

 

[Mighty Burger]

Awesome, thank you. Honestly this was much more helpful than that video I saw, makes a lot more sense now. Pictures helped. I hope it doesn't get to that point, but I should be able to handle just undoing four bolts, moving some things around, replacing the seal and putting everything back together. The video I saw was a whole intricate process, tons of bolts and different bits being taken off, including a tire (???) etc. but it seems a lot easier than that. Hardest part would be reaching up there with all of the 4wd components. I assume if the transfer case or either differential develop a leak it wouldn't be hard to fix those either with a similar process?

[Mighty Burger]

Update!

 

I've replaced all of the oils. The stuff that was in my truck was rancid. It's sounding happier now and the gears shift beautifully. Good stuff! There are a couple issues though.

 

I mentioned how the front differential was almost empty. I now know why - the moment I put oil in, it started leaking badly. The leak comes from the point where the shaft coming from the transfer case meets the front differential. My guess is there's a seal in there that's shot. I have no clue where I could find a replacement seal for it. I'll stay off 4wd for now.

 

Also. You guys didn't warn me about the smell!!! Probably the worst smell I've come across. That's what happens when you have burnt oil rotting away for 36 years. Anyways I got some on my shirt and I put it in the washing machine with other clothes - big mistake!! Now everything smells rancid. I've ran the batch through a few cycles and it's done nothing. I've looked around and people are saying to soak the clothes in coke overnight. It sounds like one of those stupid life hacks that just wastes everybody's time but people say they've had success with it. The clothes are sitting in the stuff right now, I'll see how they do tomorrow. I'm skeptical, probably gonna have to end up buying a new closet

[datzenmike]

It's from the sulfur anti scuff ingredients.  

 

The seal is the pinion seal. Most auto parts shops can find you one.

 

 

[Mighty Burger]

Thank you guys for helping me get new oil in everything. Also, the coke trick worked, ended up having to soak it overnight twice but the smell did go away.

 

I'm coming back to ask about the breather tube bolt for the rear differential so the stuff in there doesn't get water in it and turn milky again. I'm having a lot more trouble than I thought I'd have searching for that little tube bolt. Local auto parts store doesn't have anything that fits which is really odd. Is there anything I could get online that would fit?

 

Also, after talking with the previous owners they said something's bent that causes the seal in the front diff to fail very quickly. Seems like I might just have to not use four-wheel drive, maybe except for getting unstuck from snow, I don't think it's something I could fix without spending a bunch of money.

[datzenmike]

Should be metric.

 

 

Try on line.

[Mighty Burger]

I've been searching for the past hour on Amazon, Ebay and Napa and I can't find anything. I hate asking to be spoonfed but for the life of me I can't find what I'm looking for. I've found some with 10mm diameter threads (assuming that is the correct size due to someone else's comment) and all of them have an absolutely massive barbed tube connection.

[datzenmike]

As long as the threads fit and you are replacing it anyway, just use any hose that will fit the barbs.

 

Take the fitting into a machine shop or fastener store and have them measure it and give you the proper thread pitch. Use plumber's tape to be sure it doesn't leak.

[bottomwatcher]

Like I said I just replaced mine several weeks ago. 10x1mm fitting brake hard line . If the remnant of the fitting is in the axle tube pull it and take it to ANY autoparts store they have it in stock. $8 buys a long enough tube to make two!