Rear Axle Octagonal Hub Nut

Details

Written by Malcolm Eades

The nut that secures the wheel bearing in the rear axle hub is unusual because it is octagonal rather than hexagonal. This may seem like a piece of MG vanity but in fact there is a practical reason. Firstly the nut has to be thin in order to fit within the limited space of the hub. Secondly, because it is thin, it has been designed to have as much surface area as can be fitted into the circular recess of the hub and by giving it eight flats instead of six, a bit more metal gives it strength.

It poses a problem for the amateur because of its non-standard shape and the difficulty of getting adequate grip because it is tucked inside the hub. It needs a special socket but we just don't have octagonal sockets in our tool boxes. The ones specially made for the purpose are expensive, and even then some after-market products are of dubious quality. It all adds to the total cost of what might otherwise be routine maintenance. Nonetheless, if you are heavily into MGs it is probably worth the investment because it fits other cars like the MGA and MGB. MG specialists generally have them for sale but check the bulletin boards for reports on quality. Or you might try the bulletin board for a kind owner willing to lend you the socket for the weekend.

A second aspect of the amateur's problem is the torque setting needed to tighten the nuts effectively. The Workshop Manual is strangely silent on this topic and does not even refer to the need for a special tool, although Item AJA.5058 is listed later in the Special Tools Section Q. Suffice it to say that this nut must be VERY TIGHT and Barney Gaylord a.k.a. "The MGA Guru", recommends 180lb ft. See here. His excellent article also explains why such force is necessary and why it is unlikely that you will strip the thread, however hard you tighten. The average home torque wrench doesn't handle this, so off you go to the tool hire shop! It is better to use leverage to achieve the torque, by adding an extension like a length of steel tube to the wrench handle. Do not resort to what is affectionately known as the "BFH". (The B stands for big and the H for hammer).

If you discover that your own hub nuts are scarred with chisel marks on the flats, the good news is that they are probably not very tight because a ham-fisted previous owner has resorted to tightening them with a hammer and chisel. The bad news is that you will need new nuts, because the damage will almost certainly stop you from getting a decent grip when you try to tighten them. If they have been like this for long there may be more bad news if the bearings have worked themselves loose in their housings. It is possible to deal with a worn housing using a system called "Speedisleeve" which lines the housing with a thin metal sleeve. Details here.

If you are contemplating any kind of hard driving, like competition work, it is essential to do this job properly because if a nut works loose and the bearings start to move, undue forces are inflicted on the half-shaft and eventually it will break.