Rear Axle Oil Check or Refill

Have you checked your rear axle oil level recently? Maybe you have been putting it off because you do not have the special tool to remove the drain or filler plugs.

In these articles from Tech Sessions II, Mike Ash explains how an inexpensive tool can be made, and then discovers there are more than one size of plug on MGA and MGB rear axles.

Drain and Filler Plug Trivia (Volume 33, No 2 – November/December 2007)

I decided to replace the oil in the Coupe’s rear axle before putting it back on the road. I do not have the special square-end tool required to remove the differential unit drain and filler plugs, although they are available from the suppliers. Usually, I can just jam a half-inch socket drive into the hole sufficiently to remove and tighten the plugs. However, in this case, the drain plug was just too tight. I considered ordering the correct tool but, instead, I went to the local hardware store and found a set of three socket drive adapters on the table of one-price, made in China, cheap tools. I then ground down the end of the ⅜” and ½” drive adapter to fit the plug. The square hole in the drain plug is about 15/32” across, so not much has to be ground off two adjacent surfaces of the ½”drive to fit the plug (Figure 1).

You can just see in the picture that the marks on the edge of the plug show that, in the absence of a suitable tool, someone (not me) at some time has used vise grips or channel locks to remove the plug!

Rear Axle Drain Plug (Version 33, No 3 – January/February 2008)

Since my last article, I have found that there was a bit more to the trivia on rear axle drain plugs. Last time, I discussed modifying a half-inch socket drive adapter to make a wrench that will fit the square hole in the rear axle drain plug. This came about when I checked the oil level in my MGA. A couple of weeks later, I had occasion to check the oil level in the rear axle of my MG TF, which is fitted with a Z-type Magnette rear axle, and found that my newly made wrench was too large. Subsequently, I had occasion to remove the filler plug from my MGB, and found the wrench to be just right. By this time, I was beginning to realize that maybe there were two different size square holes in MG rear axle drain and filler plugs! I did a quick and dirty (in some cases very dirty) check of my spare axles, and came to the conclusion that, in general, MGB tube-type rear axles appear to use a drain and filler plug with a square hole that is slightly larger than the similar hole in the plugs fitted to the earlier banjo style axle of the MGA, Magnette, and early MGB. Figure 2 shows what I have determined to be a typical MGA style plug on the left and an MGB plug on the right.

Although they may not look it in the picture, the diameters are the same, but the square hole in the MGB plug is slightly larger, with the corners closer to the edge; which I would think would make for a weaker plug. A 3/8″ socket drive is a much closer fit for the MGA plug, but maybe a little small if any real torque is required to remove a stubborn plug. I measured the square hole in the MGA plug to be about 7/16″ across, so the half-inch socket adapter needs to be ground down a bit more than I suggested last time. Since the plugs are the same diameter and have the same thread, I concluded that at some time a later plug had been installed in my MGA.

Note from Internet Coordinator

I have had a dedicated drain plug tool called a “Sump Plug Key” for many years. I say “dedicated”, but in fact this tool is designed to work on many different types of axle, engine sump and gearbox drain plugs. It has 9 different wrenches, and has been borrowed many times by fellow Club members.

I lovingly refer to this as the “dog bone” tool. It is about 6″ long. The MGA plug in the rear axles takes the square tool.

This tool is still available from Draper Tools in the UK. Go to the website, then click on the Oil/Fluid Draining Tools catalog, page 144. Stock No. 07179, Drain Plug Key. It costs about $12 plus postage.

Note on the Rear Axle Oil

The rear axle takes about 1.6 litres (3.25 US pints) of Hypoid 90 oil. It is full when oil comes up to the bottom edge of the filler hole. The oil should be changed approximately every 6,000 miles, and the level checked if there is evidence of a weep or a leak from either the drain plug, the banjo flange gasket, the pinion oil seal, or the seals at either end of the axle tube.

While checking the oil level, also take a look at the axle vent mounted on the top right side of the tube. Remove it to clean, if necessary. A blocked vent will cause pressure build up in the axle, and could force oil past the seals.