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MG MGA - Wheel Nuts

For a longtime i have suspected that the MGA wheel nuts provided by Moss and others are too small. After discussing with Bob West about 18 months ago (he thought that the A nuts are probably Mini nuts that got mixed up years ago, and no-one has ever realised), I finally ordered some TR4 nuts from Moss last week. I fished a wheel out of the cellar and have taken some pictures of the nuts in comparison in the same hole.

The MGA nuts are smaller, and the hex shoulders out on the edge of the concave hole in the wheel. This causes stress fractures from the point of contact - I have had a few wheels fail like this, which is what started me thinking. As the pictures show the tapered surface of the TR nut is much deeper, but in its geometry it is the same as the A nut. It's deeper taper prevents the hex touching the wheel.

From the back side of the wheel, again showing the nuts in the same hole used for the front side shots, the new nut is also sitting nicely in the hole.

The lowest picture shows the two side by side.

What do others think?

dominic clancy

I would say that you are on to something here! Using a nut that doesn't fully contact all of the taper on the wheel just doesn't make sense. This is a safety issue and Moss should address it immediatly. Thanks for pointing this out Dominic.
Ed Bell

I have a load of nuts that I believe are original and they are all of the small variety as pointed out by Dominic. I too thought this was a poor idea and I had been thinking for sometime of finding a bit of hex bar and turning my own set.

Did Bob West believe that the original nots were larger?
Robert (Bob) Midget Turbo

Can't speak to Moss nuts per se, but the originals will do the same.

The problem is not the nuts - the wheels are worn out. Repeated tightening, and especially over tightening (NO air wrench!), swages the holes out. If you can find some wheels with little use, like a spare on a car that has been dead for many years, you will see that the holes are much smaller and shallower. The OE hex will overhang the tapered hole by quite a bit.

"Fatigue failures" can be a result of this swaging, or the nuts being bottomed, which effectively makes a loose wheel, no matter that the nut is tight. Always examine nuts to be certain they are not bottoming on the hub or stud shoulder.
If there is contact, grind off some of the pointy end of the nut as an emergency measure, but new wheels are in order.

Bigger nuts can be used, but be careful; following from my past writings:
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The nut must tighten down on the wheel before it bottoms on the hub or thread end - this can be a problem on worn wheels or wheels designed for larger studs, or with closed-end nuts. Use of taper nuts on ball-faced wheels can do the same. Over tightened wheels will eventually suffer from this, as the nut works it's way through the wheel - failure by loose! This can be checked by screwing the nut all the way on the stud without wheel, and measuring the projection of the stud through the nut; then install the wheel, tighten to torque spec, and repeat the measurement. The nut should be at least one full thread out from where it was sans wheel. Sometimes taking a bit off the small end of the taper can fix this, but watch for bottoming on the thread end or shoulders.

Red oxide stains emanating from tight fixings are evidence of lack of lubrication and small constant movement; the end consequence is galling or fatigue failure cracks or both. This is called fretting corrosion. It is common in dry splined connections. For a real fright, go to a truck stop and look at a bunch of trucks with Budd wheels - the ones with 8 or 10 studs. The ones with red oxide stains (or black on alloy wheels) coming out from the studs are in line for wheel or stud failure from fatigue cracking!

Most wheels using tapered or spherical nuts are “coined” around the boltholes. This provides a stress limiting/relief cushion, spreading loads at the fastener. The fastener distorts the wheel somewhat, making things less “touchy”. Cheap aftermarket wheels frequently are made of flat plate and have a tendency to work loose or crack due to local high stress under the fastener. The coining can be partly simulated by a machined recess on the back, directly under the bolt, though it does not provide the material improvement of coining. If you have flat back wheels, attend carefully to torque specs and inspect frequently.
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On a new wheel, the mentioned coining is such that the metal around the stud hole will not contact the hub face until the nut is tightened to spec. Possibly not even then, but I have no new wheels to check.

FRM
FR Millmore

Hi Fletcher

Thanks for the explanation. I made the check that the nuts are not bottoming out, ad can confirm that they do not.

I always have used a torque wrench for wheel nuts, so it's not down to over tightening.

dominic clancy

Dominic-
I have seen the nuts driven into the wheels on cars less than a year old, from air wrenches, at about 4 on-off cycles. And I have no good feel for how many "correct" tightenings might cause the damage. But, all the wheels are now near 50 years old, many on-off cycles. So, unless you have owned your car since new, or since new wheels, you have little idea. Dealers were a prime culprit!
Somewhere I once saw some wheel nuts which had a shoulder above the taper. It did not touch the wheel, so it does nothing until the wheel breaks, at which point it might keep it from falling off.

FRM
FR Millmore

This thread was discussed on 19/09/2011

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