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MG Midget and Sprite Technical - What after market halfshafts?

Just broken a halfshaft and now is the time to fit aftermarket uprated Shafts. A bit of research shows they are not all equal in strength and durability. Need some recommendations and of your experiences.

Thanks Roy
R Mcknight

Depends what you've broken but I believe there were 4 generation of half shafts on Sprites - ending in BTA 806 which were made in EN17 and the strongest of the standard variety. A H Spares price for a new one is about £130 + vat. For a little more money Peter May sells a competition half shaft in EN24 for £137 + vat.I've had these in the Frog for the past 15 years and they've never been out.If you buy secondhand try to fit them in their original sides(left to right)as breakages are more likely with reverse double notching.
F Pollock

how did you break it? I've not managed to snap one yet, thought they do start to go twisty after about 5 years.

Peter may do EN24 and Frontline do EN40B. Not sure which is best.
Rob Armstrong

I had a similar problem, MGOC spares at a similar price to A H spares as above. I decided to replace the unbroken one as well, MGBhive sell them at a good deal less, even more so if you buy from them on ebay.

Note that the head of the shaft is 1mm thinner than the originals, so you may need to make up shims to push the drums back out to the original position to avoid contact with the back plates.
Dominic Excell

How did it break .. Pulling away a bit sharpish and too much wheelspin on a greasy manhole cover, then sudden grip on tarmac and BANG!

Yes Rob EN24 and EN40 What is best? I have heard that one twists a little before it snaps and the other is more brittle and snaps like a carrot!
R Mcknight

if you don't fancy investing over 300 quid right now, I've a s/hand aftermarket one that I'm happy to pass on for peanuts.
David Smith

I believe the thinking is that EN40B is a little too brittle, and that's why Peter May uses EN24. Kim Dear @ Magic midget offers EN40B and "Competition" - he may be able to offer more insight (and he's a great retailer with terrific service).
Philip Dodd

Rob, with K power your time will come sooner or later ... unless you are a saint like Ant!
Paul Walbran

I think we are confusing the issue...

Old used good quality factory orginals (((vs))) aftermarket custom designed for hi performance

There is a 3rd option.. a completely new rear end from a differant car ...

My plan is to shorten a mazda rx7 1st generation rear end... its got positrack, disk brakes, rated to 300hp and has a # of various aftermarket gear ratios and still all fits into a13 inch wheel... granted the PCD has to be altered

At least thats may 2014 winter project plan

Prop


Prop and the Blackhole Midget

So, what's your 2013 winter project Prop?
b higginson

I broke one two weeks ago just pulling out of my drive. Cold engine so nothing harsh, just a normal start. I had a spare so only a few hours work and the mandatory blood offering and I was on my way.

But, I bought these half shafts about 35 years ago and they were made in (then) West Germany so you would expect some life but I'm guessing I got about 40,000 miles out of them. So what is the problem with these half shafts? Bad design? Poor choice of material? Misalignment in the axle housing? I have a 1965 Ford Econoline with 448,000 miles on the original half shafts. If Ford could get it right, why couldn't BMC?

As to after market ones, I've been considering the competition ones that Moss offers here in the states but I haven't researched them yet. Or anyone else's.
Martin Washington

Bad design is the primary culprit. The old style of coarse, square splines focus stress into the centre, resulting in high stress cncentration, whereas finer V-shaped splines spread the stress closer to the periphery resulting in a much lower stress concentration. This would be tolerable if the splines hadn't been simply cut into the halfshaft as they are, resulting in a higher stress riser where the splines end than if they had been left protruding - again another feature of modern halfshaft design.

BMC adopted the fine, raised splines for the B series in the early '60's but the A series wasn't so lucky - all it got was an improved material.

What puzzles me is why some of the aftermarket units, having gone to the trouble of using an uprated material, still persist with the original primitive design. I can see the point of retaining the spline section, which makes for easy compatibility without having to go for special side gears in the diff, but at the very least the halfshaft could be made with the splines standing proud. However for best results it's best to go for halfshafts with the modern spline form. The gears to match aren't that expensive, and it then becomes fit and forget.

As for the material, EN40B is a nitridable steel, but to go by the spec which is listed on-line (for example, west Yorkshire Steel) its ultimate tensile properties are in the same range as EN24 and it's yield point is a little higher. This means EN24 will start to deform a bit earlier than EN40B. Whether it's better to bend before breaking or just break at a higher load is a moot point. Would you trust a twisted halfshaft? We use EN30B or 2767 ourselves, both similar steels which have a usefully higher higher ultimate tensile strength than either EN40B or EN24 and is a material used here for 500BHP V8 halfshafts not much larger than the Midgets.
But in practice, if the design is right then any of these materials would do.

Paul Walbran

I wonder if just another bearing in the center of the half shaft would have any effect

If the 1/2 shaft is bending twisting I cant imagine the effect that has in the center of the half shaft thats un-supported... you know there has to be some kind of lateral flexing

I dont have any real thoughts beyound that...just a "Hmmm".... kind of moment


Prop
Prop and the Blackhole Midget

How you gonna get a bearing in there Prop? Lot of cutting just to alter the axle.
Lawrence Slater

****How you gonna get a bearing in there Prop? Lot of cutting just to alter the axle****

Not my problem....HAHAHA, got a mazda rx 7 rear end

I was speaking more theoretical then practical....but it was an interesting comment.

prop
Prop and the Blackhole Midget

I remember reading here some time ago about having the half shafts cut down starting at the outboard end of the splines to the minor diameter of the spline and tapering up to the existing original diameter thus removing the stress points at the end of the splines. Does anyone have any experience with this? And if this works, why don't the aftermarket makers do this (other than higher cost).
Martin Washington

the aftermarket makers are not designers and don't want the responsibiliy of producing their own design. While they copy the original they are blameless for problems.
David Smith

David, that's a very curious bit of logic given that it's a design 101 thing to avoid stress raisers of the type the original design has. By the same logic you would also use the original material. Better logic is to avoid the problems inherent in the original design. Which is certainly what we have done with ours.

It's understandable to an extent why the design was used by the manufacturers (it was cheap and easy) but I find it less understandable that makers of uprated shafts only do half a job by improbing the meterial but not the design when how to improve the latter is such a basic thing.
Paul Walbran

logic doesn't often apply in business does it? look at how much cheap tat is sold....
David Smith

Dennis Welch seem to be offering new halfshafts with the appropriate treatment http://www.bighealey.co.uk/category/catalogues/sprite-midget/axle
David Billington

This thread was discussed between 23/08/2013 and 25/08/2013

MG Midget and Sprite Technical index

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