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Installing A Rear Axle Drain Plug And Changing Rear Axle Oil

Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
I decided to start a new thread so C3 owners can see how easy it is to install a 1/4" NPT drain plug in their C3 rear axles AND to show them the easiest way to put their new gear lube in.

1. You'll need a 3/16" drill bit, a 7/16" drill bit, a 1" drill bit (Harbor Freight), a 1/4" NPT tap, a 1/4" Allen head pipe plug, and a 1/4" Allen socket.

2. Using the 3/16" drill bit first drill a hole thru the exact center of the center strut rod bracket and continue thru the bottom of your rear axle housing. It'll start draining so have a coffee can under it ahead of time. After it has completely drained measure and note the fluid level in the coffee can then discard the oil (or keep it if you're just installing the drain plug).

3. Using the large 1" drill bit drill a hole thru the bottom of the center strut rod bracket ONLY. Then de-burr the top and bottom of the 1" hole.

4. Using the 7/16" drill bit drill thru the bottom of the rear axle housing.

5. Using the 1/4" NPT tap cut threads into the bottom of the rear axle housing JUST deep enough for the 1/4" NPT Allen head pipe plug to screw about 3/4 to 7/8 of the way in.

6. Screw the 1/4" NPT Allen head pipe plug in tight.

7. Shove a 3 foot length of old 5/8" garden hose into the rear axle's fill hole then shove a transmission funnel into the hose.

8. Using a hot plate heat the new (or used) rear axle lube and friction modifier until it's around 200 degrees then slowly pour the mixture into the transmission funnel. With the lube heated to 200 degrees it'll run right in.

9. Reinstall the fill plug (sorry, no picture) and you're done. With the 1/4" NPT drain plug installed it'll be REAL easy to change your rear axle lube the next time.
 

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  • 1971 Corvette Rear Axle Drain Plug Installation Tools 009.jpg
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  • 1971 Corvette Rear Axle Installing 85W140 Oil 007.jpg
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  • 1971 Corvette Rear Axle Heating 85W140 Oil 006.jpg
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Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
Blacker Than Black

When I changed my 85W140 rear axle lube this afternoon I was astonished over how black it was after being used for about 3 years. It was as black as lampblack and that is about the blackest thing there is. But at least I got it changed and my new Sta-Lube friction modifier added. As before I added 8 ounces which is two ounces more than the specifications call for. And to avoid making a big mess I poured in the new lube thru a 3 foot piece of 5/8" I.D. garden hose. When it comes to adding new rear axle lube a piece of 5/8" garden hose and a transmission funnel is the only way to fly as it has a big I.D. but is small enough to allow excess to run out.
 

GTR1999

Well-known member
Joined
Sep 4, 2003
Messages
1,146
Location
New Haven CT
Corvette
1969,1972,1975
I prefer to set the hole 1.5" from the pad and not through the bracket. Vendors have been selling a kit that does the same thing into the bottom of the rear cover. I never liked that method either.
 

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