Halfshafts require good ultimate strength – which will depend on the both type of steel used and the design.
The material strength is a similar issue to bolts: Mild steel bolts are like putty, all the ones in our cars are high tensile of varying grades – some stronger than others depending on the application. The ultimate strength of a bolt is reflected in what torque it should be tightened to so if we look at 3/8 UNF bolts in MGA/B/Midget as an example, the more normal high tensile bolts torque to about 30 lb-ft, head studs 45-50 lb-ft, and crown wheel bolts to 60 lb ft. Halfshaft steels have a similar range.
Midgets in particular had weak halfshafts – early cars used the EN8 steel inherited from the Austin A30 origins, later cars EN17. Uprated ones on the market use EN24, stronger again. We use an even stronger one steel, 2767. in our special halfshafts.
A common misconception is that harder halfshafts are needed. While a certain degree of hardness is required of course to withstand the load applied to the spline without it wearing away, hardness isn’t the issue – it is the core strength of the material that’s important. The stronger steels also tend to be harder, but aren’t hardened as such – they can still be cut and machined etc.
The other part of the strength equation is the design. The performance of a halfshaft of a given material is influenced by two primary considerations – the root or core diameter (that’s the diameter of the base of the splines) and the concentration of stress – if a given amount of stress is concentrated into a smaller area then the shaft is more heavily loaded and more likely to fail. (What footwear is best for deep soft snow – skis or just boots?)
The standard halfshafts of the early MGA and all Midgets are the worst possible design:
The large square, coarse spline form reduces core diameter and also focuses stress into the centre of the halfshaft, while having the spline cut into it as it is causes a further stress raiser at the end of the spline. Waisting the halfshaft down to the root diameter is a good way of optimising the strength of what comes standard. This is described on my page Uprating Midget Rear Axles. It’s not so much that removing metal makes it stronger, it’s that it spreads the stress along a greater length of halfshaft thus reducing the concentration of stress substantially.
Other design features which reduce stress concentration substantially are finer splines, v-shaped splines, and also the splines being formed by rolling, as per the later MGA and MGB halfshafts.
A further advantage of the fine spline form is that the core diameter is increased about 20%, giving directly a similar increase in strength.
If you own a Midget or early MGA, waisting the halfshafts is a no brainer – it is cheap and easy to do, and substantially prolongs the life of the halfshaft. Midgets in particular are very prone to breaking halfshafts. If you do break one and want to replace it with a second hand one, use one from a 1275 rather than an earlier Midget or A30 etc (unless you want to do the job all over again).
If you have significantly modified one of these models, consider getting uprated halfshafts Yes, you could be gentle in the lower gears but to me that partly defeats the purpose of the extra power. And having spent countless hours and a not insignificant sum of money, the extra costs of better halfshafts isn’t a lot extra in comparison and well worth the reliability gains.