Adding a rear end drain plug

Most older car rear ends dont have drain plugs. The manufacturers figured that the oil lasts for the "life" of the car, and if you really want to change the oil, you can just take the rear cover off and deal with the mess. But these days, if you own a classic car, you probably expect it to last forever and you intend to change the rear end oil every 5 years or so. And you dont want it to be a big messy operation.



So how do you add a drain plug? The first thing that I wanted to do was drill a hole through the bottom of the cast iron housing, and tap it for a big pipe plug, similar to the filer plug. But then I got to thinking about how difficult that metal might be to drill and tap, and I dont even own a pipe tap bigger than 1/4 inch, and I am not sure the casing is thick enough where I would need to drill, etc. The very bottom of the housing is not a good place for a plug anyway, because it would tend to get hit by stuff. If you look at modern rear ends, they either offset the drain plug away from the very bottom, or they put a big heavy guard around it for protection

Then I came up with another idea...

How about just adding the plug to the bottom of the back cover? You might not be able to get the last 1/2 cup of oil out, but you will get the other 2-1/2 quarts. And it is a low risk operation. If I totally screw this up, I can just get a new cover.

The cover itself is too thin to safely hold a thread, so it needs additional metal added. I used a 2 inch circle that I cut from a piece of 1/4 inch steel with a hole saw. That is an effort in itself, but with a quality hole saw and a drill press, and much patience, I got all the way thru. The plug shown here is a standard 1/2-20 oil drain plug used on most American made engines. Normally you would use a larger plug for 90W oil, but there are two advantages to this type of plug: it is easily obtained with a magnetic end (to collect metal fragments), and I already have a 1/2-20 tap. The oil might drain kind of slow, but it will drain.

One thing that I consider important is to tap the threads perfectly perpendicular to the seal surface. Otherwise it will be putting more pressure on one side of the seal than the other, and might leak. This is easily done on a drill press, by using the drill press to guide the tap. You still turn the tap by hand, but use the drill press as a guide to keep the tap straight. Another recommendation: don't fully tap the hole. This piece will need to be welded to the cover, and doing so could cause minor damage to the threads, so plan on finishing the threads after the welding. You still want to start the threads now, while it is easy to get the tap started straight, but stop after the tap is far enough in to align itself well. You can then come back later and finish the threads without needing the guide.

I cut the bottom edge off and beveled one side so that I can get as close as possible to the bottom of the cover. I milled both sides of this piece, just because I could, but that is overkill. Hand filing would work just fine. You need the outside to be flat enough for a good seal, and you need the inside to be clean enough for a good weld.

A few things to consider when placing the hole location:

1. Miss the ring gear! I dont know how much clearance there is between the ring gear and the cover, and there is no reason to find out. The non-ring gear side has lots of room.
2. Dont put it too close to a cover bolt, or you will likely have interference issues when you put everything back together.
3. Be sure which end is up! Most covers have a definite top and bottom. If you put the plug on the wrong end, it will ruin you day.

You can't tell from this picture, but I used a hammer to flatten the metal around my plug location as much as I could, so that the threaded piece, which of course is flat, would fit fairly tight against the cover, which is quite curvey. The bevel in the threaded piece faces down and fits into corner along the bolt flange. Set it in place, mark your hole, and drill.

One mistake I made here was to make the hole too small. I drilled just slightly larger than 1/2 inch, but dispite my best efforts in aligning the pieces, I hit the edge of this hole when I tried to finish tapping the threads. I had to cut the hole a little bigger with a die grinder to let the tap go all the way thru. Make your hole at least 5/8.



Use an old bolt and some washers to hold the threaded piece in place. This bolt may be ruined by the heat, so if you have some old junk bolts around, this is a great place to use one. The blue ink mark on the cover is my way of aligning the threaded piece over the hole. It didnt work that well, hopefully you have a better way.



I brazed this piece on, but I think MIG welding would have worked just as well. The MIG might actually cause less heat distortion. You can see how far out the paint burned from the brazing. MIG wouldn't have done that. Make sure you go completely all the way around, with no holes or gaps that might leak.

Clean it up and finish tapping the threads. You do have flux removal to deal with when brazing, or weld splatter when arc welding. Choose your poison.



This is how it looks from the inside. You really can't get much closer to the bottom. This is a Mr.Gasket #15 magnetic drain plug sticking through.



A new drain plug will come with a plastic washer gasket (bright white in this picture). This works great for engine oil. but for a rear end gasket that might not be removed for 10 years, I prefer the old copper gaskets: they will never crack.

I haven't put any oil in this yet, but is sure looks like it is going to work.