The MGA With An Attitude

This page details the need for high torque to secure the REAR HUB NUT on the MGA (and early MGB) banjo type rear axle.

At 09:04 PM 6/11/02 -0700, Ray Ammeter wrote:
>On your website you mentioned 140 ft lbs is the proper torque setting for this nut.

You took me a little by surprise with that one, and I had to go read my own web page. For some years I have been using, as a rule of thumb and as a minimum, 150 ft lbs. I actually don't remember where the 140 number came from on my web page, but knowing myself as well as I do, the fact that it has the metric equivalent torque number in parentheses means that I read it somewhere in print, otherwise I wouldn't have translated it into the metric equivalent (probably just copied it from the original source).

>In my aftermarket manual it states the torque setting to be 180 ft lbs for the hub bearing nut. My manual is "MGA,MGB 1955-68 Autobook, by Kenneth Ball, by Autopress LTD". My original work shop manual does not have the torque setting for this nut.

Right, it doesn't. And there are a lot of other torque settings that are not in the factory manual.

>My question is: where did you find the torque setting for this nut.

Sorry to disappoint you, but I don't know. But at this time I would like to amend that torque spec with the word "MINIMUM".

>I am curious if there are other after market manuals that state 140 or even 180 for this nut.

I can't answer that either. As a career mechanical engineer and machine design specialist, I can tell you why it should be A LOT OF TORQUE. The inner race of the bearing is not a tight press fit on the axle housing, but is in fact a "Locational Clearance Fit". This means it can be anywhere from zero clearance to some slight positive clearance, but is never an interference fit. If a bearing that size was a press fit you would need a sledge hammer to install it and a herculean puller to remove it, so it isn't. Given the manufacturing tolerances of both parts for a LC fit, it will nearly always have a small clearance. When the car accelerates the axle shaft and bearing are pushed forward against the axle housing. When the car decelerate the axle shaft and bearing are pushed backward against the axle housing. If the inner bearing race is allowed to move on the axle housing it will eventually wear down the OD of the axle housing, and the fit for the bearing on the housing will become looser. When it gets quite loose to the point where it wobbles about just a little bit on the housing, then some nasty things happen to the functionality of the assembly. The brake shoes can grab and/or clunk with each revolution of the wheel. The wheel can bounce or resonate significantly if the wheel is just slightly out of balance, and if it does the noise will get louder as the wear increases. But the first problem will be that the hub seal will not hold oil for very long before it begins to leak profusely and fills the brake drum with gear oil. If you then install a new seal it will be leaking again in short order.

So the idea is to have the big nut tight enough that the inner bearing race cannot be moved about on the axle housing with the forces of acceleration and braking. This is why it requires a lot of torque. The standard torque spec for a 1-1/2" fine threaded machine screw torqued to its yield point is 65,400 in-lb, or 5450 ft-lb (just to start with things in the proper perspective). This assumes a thread engagement length about equal to the diameter of the threaded fastener. Since this particular nut is only about 1/4" thick you can divide by 6 to find the maximum torque allowable of about 908 ft-lb (to preclude stripping the threads). So when dealing with a fastener of this diameter, a few hundred ft-lb of torque would be small potatoes, and there is no way that any normal human being will ever be able to apply enough torque to strip the thread. And that is why they didn't bother to list the torque spec in the factory shop manual.

Now I can tell you from experience that 100 ft-lb of torque on this nut is not much, and is in fact very marginal for performing the required clamping function. 150 ft-lbs is not a lot more, and is only about twice what you would apply to a 1/2" wheel lug nut. Using the big blue thin wall tube socket I can usually achieve between 150 and 175 ft-lb of torque before the socket threatens to slip off of the nut and damage someone's body in the process (anyone in the vicinity), so that's about where I stop and call it sufficient. If the nut was a little thicker and I had a stronger socket I would have no qualms about giving it 200 to 300 ft-lb of torque. Bottom line is that you need to crank that puppy up about as tight as you can get it, and then give it a little extra kick. Hence my reference to "arms like Poppey the Sailor Man". If your reference manual says 180, I wouldn't argue with that. But you can tell the guy who thinks that maybe 50 ft-lbs is enough that he needs this explanation of the function of the nut.

I have seen many of these things that were disassembled and reassembled by the DPM with a BFH and a hand held punch or chisel because they didn't have the proper wrench. In every case (without exception) where I find chisel marks in the nut, it will require less than 30 ft-lb of torque to remove it, and sometimes it can be unscrewed with fingers only. Also in virtually every such case the hub and bearing assembly can be pulled off the housing with hand force only, and often with light finger force. This is an immediate indication that the bearing race is too loose on the housing, moving about and creating the wear pattern that should be avoided. When one is not prepared to actually repair the worn housing (which is of course always) the only fix is to reset the bearing with sufficient torque on this nut to be sure it will never move again, even though you know it's a sloppy fit. So go ahead and wail on the thing with the biggest wrench you can find, just don't leave it too loose.

On 2/20/2014, Colin Brown in Brighton, England wrote:
My BMC workshop manual for the Farina range, book dated 1969, gives 180 ft lb / 24 kg m. As you say, it needs a LOT of torque!

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