Mark IV stub axles

Does anyone have knowledge or experience of stub axle failures on these cars? The 2.5/3.5 are considerably heftier than the 1.5 but they all have an ultimate life, and like human life we don’t know when that is.

I am an MG tragic too, and they have a history of fatigue failures starting many, many decades ago. It is a major safety failure, as you lose the wheel and drum assembly and of course the brakes. I have direct experience of this failure from back in the late fifties as a very young teenager. I had a Christmas holiday job as a helper (under age?) to an ice cream delivery supplier, and on our way back to the depot, I felt my passenger side drop a bit then noticed a wheel slowly overtaking us along the footpath. We were lucky in that the road was flat and straight and we were travelling at a gentle 40 kph or so. As a matter of detail, ‘escaped’ wheels usually go faster, as when the weight comes off, the rolling radius reverts to full size, so the speed increases because the r.p.m. remains the same.

But, traditionally, MG drivers often pressed their cars into severe competition, increasing the stresses dramatically. At least one supplier is making new ones for the early models, which would seem a good investment for the next 80 years of the car’s life. An alternative is a modification where they bore the centre of the knuckle out and shrink-fit a new machined stepped stub in place from the back. Similar, in principle to the later Jag i.f.s. stub axles. I can’t see that there is enough ‘meat’ in the Mk IV knuckle to do this.

I am considering getting some crack testing done (sometime), but are there new knuckles available?

So your idea is to cut off the broken stub axle and bore out the knuckle (2) to put in a new stub axle.
The XKs and Mark saloons have a tapered stub axle that goes in from outboard with a big nut on the back, but it looks like there isn’t room with the loop on the end of the beam axle for a big nut on this knuckle, and you definitely would not want to try threading an axle in, it would be a stress concentration at the worst possible place, so the idea is to make a stepped bore and stepped diameter axle and press or shrink it in from the back.
It would be a tricky bore to do, and you would want to make the machinist aware of these steering angles, or run the calculation yourself for the correct angle.
So it sounds feasible to me, and if you have a knuckle with a broken axle but no other cracks or damage lying about, it might be worth a try.

I’ve not heard of any stub axle failures in SS Jaguars or MkIVs. It would probably be better to obtain an original.

I know what you described with the MG. I was in a friend’s MG SA when the front near side came off due to the restorer fitting the hubs on the wrong sides. The wheel also damaged the wing as it departed.


I’ve only heard of one stub breaking, but that almost resulted in the car rolling …(over)
I have heard of at least two where the stub bent resulting in the drum rubbing on the backing plate.
But it is definitely not possible to put a new spindle in the stub. There is not enough metal to modify it to like a MKVII or XK.
They mainly crack at the radius of the spindle into the vertical section.
I have a large box of stubs that have been crack tested … and failed. I was fortunate years ago to be at Beau.ieu and saw a box of NOS stub axles ar Worcester’s stall, and bought the all and put them on all my cars
The stub that broke was probably partially the result on over tightening the wheel bearings.
The cleverest solution I’ve seen was based on the stub axles of MgS.
They hav3 ball nottaper bearings and get the end float set by having a tube between the inner and outer of very precise length.
Soi. InThis case they turned up a tapered tube, tapered because of the difference in size of the bearings, and this pressed at each end on the inner part of the bearing.
What this means is that the load is transferred to the inner bearing directly and not not by a long lever… the spindle.
This was on an SS100 that races a bit and seems to have worked.
We did ask about making new ones, but they need to be forged and that meant a minimum order of 500 units
But aside from this ,if doing a front end, you should firstly have the beam checked for alignment and king pin eye size and true.
Most don’t and later when assembled there is nothing that can be done to set the camber or caster if different one side to the other.

I agree Ed. There is not enough metal thickness in the body of the knuckle to permit boring out - it would create considerable weakness and eventually a serious fracture. I have attached a copy of a sketch, for reference, of the solution by the MG fraternity. It is a copy from Mike Sherrell’s publication, ‘TCs Forever’. (He is from Perth, WA, and recognised as the world’s expert on investigation, reverse engineering, and solving of faults that plague TCs.) I’m sure he won’t mind this being shared. Strength is maintained by the fact that the bearings are ball, not taper, with a compression bush between. Obviously there is no ability to adjust for bearing play, and when this becomes excessive, the bearings are just simply replaced.

In relative terms of car size, there is more front-to-back depth available in the knuckle to accommodate this modification and I would never suggest this be done on a IV. I agree too, that the only realistic solution is to convert to ball bearings with a tapered sleeve to match. This might be a good solution for another reason - the taper bearings have been out of production for a very long time last I enquired, and kits are only available from stockists of N.O.S. parts, at a golden price. It reduces the bending forces on the stub at its critical plane by a significant amount.

The standard factory set up for the MGs is with ball bearings and a spacer bush, and the lot tightened together against the back shoulder of the stub axle. For those interested, check the NTG spares site for a picture of the tapered spacer as used on T-Type MGs. It is made precisely to match the distance between the internal bearing installation shoulders inside the hub. Conversely, many MG owners are now converting to tapered! I don’t think they realise they are increasing the bending stress on a 70+ year stub axle.

The fact here is that this is a critical component right at the top of the list of serious ‘outcomes versus the what-if failures’ and it is wise to be aware of the risk, albeit small.