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The Big Nut on a YB – Rear Axle

Alas this article is really only for the YB owner (and any MG TD, TF, Wolseley 4/44, Morris MO Cowley and Oxford, Riley Pathfinders and Wolseley 6/90 who stray this way….they all use the Nuffield Hypoid rear axle!) Very quietly I must admit that the YA/YT rear axle is a better design for servicing as its brake drums can be removed easily. The YB’s has its hubs and drum as one casting, often requiring a puller to remove them. But the YBs rear axle is much stronger and last much longer, but its a sod to work on.


The YB Nuffield Rear Axle


The ‘Big Nut’ I am talking about is very, very important. If it is not tightened sufficiently, then the splined hub will move on the half-shaft splines, ruining both the hub and the shaft. I have checked both the YB and 4/44 workshop manual, but no torque settings are given. The nut we are talking about is item number 29 in the exploded diagram. There is a photo of the nut fitted to my YB and another photo of the half-shaft removed, to show the splines and big thread. The size of the thread indicates that about 90 to 100 ft/lbs is the correct torque loading, and if the hole in the shaft’s end does not line up with the slots in the big nut, turn the nut further until they do line up. The nut is locked with a big split-pin. To get such a high effort into tightening that nut, the wheel needs to be fitted, chocked both fore and aft on the ground. Trying to use the hand-brake the drum will just slip. The Big Nut locks the hub up against a tapered collar, and I put graphite grease on the taper, the splines and the threads.


Half shaft splined end which BIG NUT attaches to.

The BIG Nut.


As always, it is not that simple. Up until YB no. 0286 the axle used BSF threads and afterwards UNF threads. Because of the age of our cars, there is no way of knowing what axle or even half-shafts you now have. It is quite possible to have a BSF threaded shaft one side, and a UNF threaded shaft the other today. Shafts from the TD/TF and 4/44 will also fit as they all have the same ‘track’. ( The TD changed from BSF to UNF at TD No. 12285. To check your Big Nut threads, the BSF ones will have 11 threads per inch; the UNF ones 14 threads per inch (tpi). You may well find that your YBs Big Nuts are in poor condition as days of old, someone used Stylsons (lazy mans wrench) to remove/refit them, tearing the corners off the hexagon.


The original MG part number for the BSF nuts is 101791, the MG Octagon CC sell them under part number SAX120. The original MG part number for the later UNF nut is 102061, MG Octagon CC SAX121.


UNF thread Identification

Why did they change the threads? The whole rear axle along with the wheel studs changed from BSF to UNF threads because Morris had been taken over by BMC; really an Austin take over. Austin were more modern that Morris and used the American Unified Thread system, called ANC and ANF, (American National Coarse and Fine.) We used it but called it UNF and UNC, Unified National Fine and Coarse. There is a diagram to help you identify UNF threaded items.


Neil Cairns.