No, longer than that.
I leafed through the maintenance log I keep for my ’76 Trans-Am (I have one for all my vehicles; it’s the only way to keep track of things short of having someone else keep track of them for you) and found the entry: April 7, 1999. Fourteen – almost fifteen – years ago that I last changed the differential fluid (gear lube). That’s too long – even for an antique muscle car that’s not driven regularly. In part, because over that span of time, even a very minor leak (and what 40-year-old car doesn’t leak?) could gradually result in a too-low level that you might not notice until a mechanical failure brings it to your attention. And even if the level is right, contaminants accumulate and the bottom line is it’s just not smart policy to neglect this job, given how easy (and inexpensive) it is vs. how expensive (and difficult) it is to deal with a major rear axle issue.
My car is a Pontiac (and a Trans-Am) but the procedure is pretty much the same for most RWD cars. If you’re working on a ’70-81 Firebird or Camaro, it will be exactly the same because all these cars (with the exception of a few high-performance 1970 models) have the same 10-bolt rear axle. This axle was also used in same-era Chevy Novas (and Pontiac Venturas and Buick Apollos) as well as Malibus and pretty much every other RWD/V-8 car GM made during those years.
Mine takes 4.5 pints of gear lube – plus a bottle of limited slip additive, which you’ll need if your car has a limited slip axle. Most Z28s and Trans-Ams from the ’70s came standard with a limited slip (also known as “posi-traction”) rear end. But it could be ordered with almost any Camaro or Firebird – as well as Novas (and so on). Look for a little metal tag that reads “limited slip only” secured to the rear axle cover via one of the bolts that holds the cover to the axle housing. You can buy the little bottle of additive at any GM dealer; many car parts stores also stock it. The gear lube (be sure to get the right weight; check the manual for your particular year and application) comes in plastic quart bottles. If your car is like mine, you’ll need three. I use synthetic, but regular’s ok to. I prefer synthetic because it flows better at lower temperatures and gives a higher level of protection at higher temperatures/under severe loads/abusive conditions. My Trans-Am, with a street-strip 455 making probably close to 500 lbs.-ft of torque up front and pretty aggressive 3.90 gears in the pumpkin, needs the extra protection. Your car may not.
In addition to the fresh gear lube (and limited slip additive, if applicable) you’ll need the following to do this job:
* Half-inch socket/wrench.
* Gasket scraper.
* 3/8 inch extender bar.
* Catch pan.
* About a foot of old fuel hose, 7/16 in diameter.
* Jack/jack stands.
First, warm the car up – ideally by driving it for about 15 minutes. Then raise and support the back end high enough off the ground to give you room to work around the axle. Loosen but do not remove the 10 bolts holding the pumpkin cover in place. Slide a catch pan underneath and then pry the cover loose. Let all the old lube drain completely. Remove the bolts – being careful to keep track of them – and slide the pumpkin cover away from the axle housing. The ring and pinion are now visible. I use my hand to scoop out any remaining old lube in the bottom of the case. Use a razor blade or scraper to peel off any old gasket material – being careful not to let any fall into the axle housing. Clean it out if some does fall in there.
Use the 3/8 inch extender bar to loose the fill plug, which is typically located on the upper passenger side of the axle housing. Be careful not to lose this plug! Also be careful not to damage it while trying to extract it. If the plug is tight, spray it with some PB-40 or similar penetrating fluid, let it sit then try again. Use leverage (breaker bar, etc.) to ease it out.
Wipe out/clean the pumpkin cover and bolts. If yours has a magnet, be sure to carefully clean and re-install it. Place the fresh gasket on the cover, using one or two of the bolts to hold it in place. Align the cover, being sure the gasket is also lined up correctly, and reinstall the cover. Tighten the bolts in a cross-hatch pattern to avoid warping the cover and to assure a good seal.
Now comes the fun part!
There are several ways to get the fresh fluid into the housing. Some use hand pumps, some use turkey basters. I like to use the plastic bottles the gear lube comes in – with a length (about 10 inches) of 7/16 fuel hose secure to the nipple on top of the bottle. Using this set-up, you can insert the flexible hose into the fill plug while getting the bottle into position. Now, squeeze! You ought to be able to empty most of the bottle’s contents this way. Do the same with the second bottle. This will get you close to 4 pints (there will still be some left in both bottles). Use the third bottle to get another half-plus pint in to fill to capacity. You’ll know you’re there when the lube level in the case is right at the level of the fill hole and there is some slight seepage. Double check to be sure the level’s right, then re-install the plug.
Hopefully it won’t be another almost fifteen years before I get to this again!
Throw it in the Woods?
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