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MG MGA - Disc Rotor Exchange

As expected I got into the 'might aswell spiral' and I decided to change the brake disc rotors while I am overhauling the brake system.
I completed the read brakes and now I am going to do the calipers and check the master cylinder.

Back to the rotors;

I have taken off the brake caliper, and the 4nuts on the rotor and the bolts that go through. Now what?

Looking as the side behind it seem there is a lot of rust, so it could be rusted onto the hub, but a couple of knocks with a mallet didn't seem to brake it loose.

I guess I am missing something....
Do I need to take apart the wheel hub? I hope not!
I didn't want to get into undoing castelated nuts on bearings at this stage...

I was hoping that changing the rotors was simple thing... any help appreciated.

The apprentice MGA mechanic.
G Ramos

Unlike most modern cars, the rotors are a "press fit" onto the hub. I have an arbor press I use for this task. If you don't have access to a press you may have to find a shop who will do it. Perhaps one of the other guys will have a "trick" you can use.
Good luck,
G T Foster

Gerry, are you sure? I don't recall having to press my rotors on.
Art Pearse

All the front hubs I've encountered have been this way, although I suppose it's possible some are tighter than others. But, hey, I been wrong before..just not often..:)
G T Foster


An old trick I learned in vocational school many years ago for stuck drums was to heat them up with a torch (I just use a plumbers propane torch) and they usually pop loose once they expand a tad. I am pretty sure I have used the same trick for rotors.


Richard Taylor

Gonzalo, the front rotors (discs) are bolted to the front hubs. I'm suprised that you've been able to remove the 4 bolts without removing the hubs. You normally need to hold the nuts to prevent them spinning.

You need to remove the hubs and place the hub and rotor on a workbench and the rotor should come off the hub with a firm blow from a hammer. I've never had to resort to a press or heat; although that doesn't mean you won't have to if they are really rusted solid.

This means that you need to remove the castlated nut, which is not too hard. The hardest part is pulling out the cotter pin and then you may need a hub puller to remove the hub.

Good Luck

Andy Preston

If you want to remove the calipers there is no need to remove the rotor, but eventually you will if the rotors require to be rectified (recommended).
In what stage you are?
R Garcia

I wanted to change the rotors because they have some grooves in them... and I thought that since I was rebuilding the rear it would be good to change the rotors and pads to ensure I get the best braking now that the brake fluid has been drained.

I am kind of discouraged, it seems to be a big job and I am not sure where I will find a guys with a press... nevermind a hub puller. Remember I am doing this in my parking lot. ; )
Is a hub puller like a 'gear puller' with the claws and a screw? I think I can rent one of those from advance autoparts.

I was able to remove the bolts which were kind of a press fit in the holes.
Maybe the solution is to just overhaul the caliper and leave the rotor as is, and not change it; but it I loose the benefit of having new rotors with new brake pads...

I wanted to do it because I had the impression when I drove it that when I braked it was braking more from the back and less in the front. But that may only be my impression.

I will do some research to see where I get.
G Ramos

If the grooves are not too severe, they will not matter, as long as the majority of the disc is flat, not glazed and runs true. How were the brakes before you took out the pads?
Art Pearse

The brakes seem to brake ok, and the discs seem flat and true. Maybe i shouldn't change them then...

In that case I need to see how I could return rotors I just bought from Moss.

G Ramos

I forgot that you car has wire wheels. These are different from the disc wheel. The rotors on disc wheels need no bolts. The wheel studs are used instead.
There is a method that can be used to pull wire wheel hubs without needing a puller. Remove the wheel, cotter pin and nut. Place spacer (even a socket)againt the end of the axle stub. It needs to be long enough that it sticks out of the hub (maybe 1/4 to 3/8 inch or so). Wind your spinner back on the hub so the inside of the spinner is snug to the spacer. Continue to tighten the spinner. This will pull the hub from the axle stub. The tricky part is keeping the spacer in position when installing the spinner, but this does work.
G T Foster

Thanks Gerry, seems like a neat trick. Yes I do have wire wheels.
I guess I could used like a spark plug socket as a spacer or something of the sort.

The my biggest challenge for me will be to take out the cotter pin and and nut, and then re-tighten it to the right torque for the bearings to work properly. ;S

More over I am not sure I will be able to push it back in again after I have changed the rotor... Big hammer approach? Will it be easy to separate the rotor from the hub?
G Ramos


I just replaced my rotors yesterday. (I also repacked the bearings while I was at it.) Once the 4 bolts securing the rotor to the hub are undone, a couple of taps and it should come apart fairly easily. When putting the new rotor on, attach the bolts and snug them in order of opposites (ie: if you pick one bolt as number 1, do number 3 next, then 2 and then 4.) Tighten them all up incrementally until you have them all close to tight. That way, the rotor will slide into place evenly.

Once everything is ready to go back on to the spindle, a few taps with a rubber mallet should push them on enough to get the castle nut started. There are no torque specs for the 1500 or 1600, so as Barney Gaylord told me - just wail on it to get it really really tight. If the castle nut is not alligned with the cotter pin hole, try to go part of a turn tighter as opposed to looser. (Remember, left side is LH thread).

Good luck. I'm sure you'll invent a couple of new swear words while you're at it.

Cheers. Glen
Glen Perchie

Got it Glenn! Thanks guys, now I have inspiration to start the job, with a slight confidence that I will be able to complete it.

Mind you, I did tap the rotor with a malet and it didn't move, so I will have to go harder....

BTW I guess the castellated nut is a special size. Do you know what exact size it is so I can buy a socket for it?

For repacking of grease, can I just use some standard lithium grease (multipurpose)? They probably can do with some new grease cos it is all pretty dirty and gritty in there.
Do you suggest cleaning all the old grease off first?

I just realised there are some instructions in the workshop manual for this kind of operation but it references special tools. IT is a good guide though.
In fact there are some torque specs. But I will follow your's and Barneys advice, assuming that the right shims are in there to avoid the bearings from binding.
G Ramos

I used a 1/2 drive deep 1 1/8" socket. If you are planning to repack the bearings, its a little tricky getting the outside bearing out. You need to feel for the gap between the spacer and bearing and gently tap the bearing out. (preferably with a brass drift). Once the bearing starts to move, the spacer will move around more easily, allowing better access to the bearing with the drift. Be careful, it's easy to wreck a bearing if the drift slips off. Wash the bearing and hub in solvent to get all the crap out of it. I use wheel bearing grease. It's a bit thicker than general purpose grease.

To get the rotor off, you can probably whack the rotor if you're planning to replace it anyway. I made a rotor puller from an old knock-off nut for my car (wire wheels). I'll try to upload a picture.


Glen Perchie

Gonzalo, if you need to hit the rotor with a hammer use a 2 X 4 piece of wood to avoid the metal to metal impact.

There is a special grease for disc brakes rotors, I donīt know what is the difference!
R Garcia

Gonzalo, there's a few other things to bear in mind concerning the disc(rotor) 'runout' ie the amount the disc is out of true when it rotates. The manual gives 0.006 inch which is almost nothing and it helps if you take a razer blade and ensure that the mating surfaces are absolutely clean. Once assembled use a dial gauge to see how close the disc is to true. Don't be surprised if your not within tolerance and the only advice I've heard is to reassembly again which is a pain. The reason I think that there is so little tolerance is that the pads hardly pull back at all after braking and if there is not a good fit with the disc the pads will rub part of the way around on the disc.
J H Cole

R Garcia
"There is a special grease for disc brakes rotors, I donīt know what is the difference!"
The temperature rating of the grease WAS the difference. All new greases are adequate. 50+ years ago when most cars had drum brakes it was discovered that those newfangled disc brakes needed high temp grease in their bearings.
R J Brown

Ok, I will try to set something up with a dial gauge to measure how true it is. That may actually be the most difficult part to get right...
G Ramos

R J Brown,

R Garcia
"There is a special grease for disc brakes rotors, I donīt know what is the difference!"
The temperature rating of the grease WAS the difference. All new greases are adequate. 50+ years ago when most cars had drum brakes it was discovered that those newfangled disc brakes needed high temp grease in their bearings.

Days ago I went to an Auto parts store and looking into the grease shelf, there were several brands and types of grease and one of those tubes has the note:
"For Disk brake bearings", with no more description, as you well mentioned many years ago the temperature WAS the difference, not now, but the manufacturers make a "special" product for a "special" application in order to increase the price of the products, thats why I said I do not know the difference!

R Garcia

An excellent method of loosening tight rotors (on modern cars anyways) is to loosen all the lug nuts and drive the car a very short distance. You can do this in the driveway. When you hit the brakes, lock them up, and it will usually break the rotors loose from the hubs.

Also, note that when packing the wheel bearings, you don't want to fill the entire bearing area with grease. You want to pack the bearings and add a little grease to the bearing housing. Too much grease will cause fun when it all heats up.
Mike Parker

Last night I performed half of the job!
I managed to take the rotors off, heavy set of blows with mallet and piece of wood, as suggested.

The trick for taking off the hub worked perfectly. I used a spark-plug socket with a piece of cloth wrapped around it to center it, and tightened the spinner.

Now I need to put the new rotor on.

However I wanted to rebuild the calipers now cos I have the impression that on one of them the pistons seem to be ceased (or at least will no push in by hand). I have ordered a rebuild kit for it.

The jobs are building up...

Thanks for 'Tricks'!
G Ramos


If the rotors are scoured and you're going to replace them anyway, (and you should indeed replace them because they have come way down in price over the years), then just use a heavy duty mallet and whack them off. Don't use any petroleum based solvent to clean your bearings and don't soak them in any cleaner and such. Use paper towels to clean and remove the old grease and then lube them with a good brand of axle grease. (The wheel bearings for drum brakes and disc brakes are the same and calling a grease "disc brake grease" is marketing talk. (It's basically all good quality soap and oil).

Yes indeed, rebuild those brake calipers but FIRST, suck out most of the brake fluid in the master brake cylinder reservoir first leaving a small amount at the bottom to avoid air bubbles in the line. When you replace the pads and begin to separate the two pads to fit onto the rotor, the fluid in the master cyl. will commence to overflow if you haven't removed some of it beforehand. Use caution here to avoid damaging the surfaces of the pads. Use an inverted 2 lb. coffee can to rest the caliper assembly on to avoid it hanging down by the rubber brake hose.

Use special care when rebuilding the brake calipers. Make note especially how the flanged rubber O-Ring is oriented. Use extreme caution when inserting the piston back into the housing. All parts have to be very clean and coated with brake fluid. Wear latex gloves and eye protection 'cause the fluid can really sting your eyes if anything squirts up at you unexpectedly.

Then, using a piece of wood about 1/2 inch thick x 6-8 inches long, insert the wood (pry) in between the pads to separate them. Gently wiggle the caliper with pads onto the rotor whilst removing the piece of wood. Double check the rubber brake hose to the caliper. If in doubt, replace it. Install the two hex bolts with the long tabbed combo washer to the brake plate and torque to about 35-40 lbs. Use a little machine oil on these studs.

Carefully re-install the hub w/ spacer, shims, tab washer and castellated hex nut. Shorten the length of a 1/8 inch cotter pin to fit inside the hub recess. Scratch a line w/ a screwdriver on the end of the axle face to remind yourself where the cotter pin hole is located. Then you can easily align the hex nut and cotter pin to the right location through the holes provided on the hub.

But wait, you're not finished. Snug up the axle hex castellated nut; wiggle the hub assembly and turn it a few times; then using a torque wrench, tighten the hex nut to 30-40 ft/lbs using a 1/2"x 1-1/8" socket with extension. BACK OFF the hex nut slightly to align the hex nut with the cotter pin hole. With your needle nosed pliers insert the cotter pin into one of the two holes provided in the splined hub. Make sure the longer leg of the cotter pin is facing you so that you can grab it with the pliers and pull and bend the leg of the cotter pin from inside the front of the hub recess.

Tighten the "lug nuts" from the axle plate to the hub with about 40-45 ft/lbs. (If they have been installed to tightly, previously, the studs will invariably fall out the next time the hub is removed. The studs are splined and they will get damaged if tightened beyond "reasonable" torque values.)

Special Note: Our good friend John Twist of University Motors stated on one of his videos to use 60 ft/lbs when tightening the axle nut.

With all due respect to John, I slightly disagree with that. I would recommend using what John refers to as the "American Method" of tightening the axle nut. That is to tighten to the 60 ft/lbs as he described but then backing off to align the cotter pin".

The reasoning behind using a castellated hex nut with cotter pin, (a technology that's been around since the wagon train days)was that the hex nut should be "tight" but not too tight. After 40 years of experience, I rely on the good old American Method (even tho I'm a Canuck).

Go ahead and tighten to 60 ft/lbs. to pre-load the bearings and then back off and re-tighten to 30-40 ft/lbs. and then back off ever so slightly to align and fit the cotter pin into the corresponding hole in the axle. Slightly backing off the hex nut will most likely yield a torque setting value of about 25-30 lbs which is very adequate for wheel bearings, even on Little British Cars.

Now you're almost finished. Add a small amount of grease to the inside of the hub cap. When the bearings get hot there will be some extra grease to lube 'em.

This is a lengthy discussion but the brakes are an extremely important and critical component of MG Cars in general. They have been a major "single point failure item" on many MGs resulting in many rear end colisions over the years. Back in the 1950's and '60s it was common to see an MG with front end damage due to faulty drivers with faulty brakes.

In the unlikely event that a DIY mechanic finds themselves involved in a collision, then having taken the most prudent care of refurbishing your car's brakes and corresponding braking system will go a long way to validate your repair and vindicate you of any irresponsibility should that question ever arise. (Save your receipts and be prepared to describe what you, as a responsible person, did to your car's braking system. Don't cut corners here.)

R Murray

For $120 for a new caliper, would you not be better going for new and having the peace of mind that it's going to be perfect? And they will last a long long time too.

When were the flex hoses last replaced - you should replace those (all three) as a matter of course : $45 for the set.

And with new rotors, you should fit new pads.
dominic clancy

Hey RM, thank you for the detailed explanation of the task.
I have managed to change the rotors this morning with no problem, and I think I got all the points you mentioned right.
The only thing in the back of my mind is that there was a very slight play on the right hub after tighten it all down. Not sure if there it could do with an additional shim, or the bearings are tired.

Now the calipers are a different story, I had some trouble getting both pistons out of the caliper, and of course I made a big mess with brake fluid everywhere.
Damn I hate that stuff!

In fact the pistons seem to be in good condition so I might just press them back in an install again...
I must say they are quite tight in the bore.

The front hoses are new and were changed recently...

I am also changing the pads.
G Ramos


Good job. Those pistons in the calipers are very difficult. Even though they may go in tightly and snug, they can leak if not installed correctly.

I've used Silicone Brake Fluid for the past 25 years in all my MG cars with much success. It's more "friendly" to British Rubber products and paint jobs. This argument, i.e., Dot 5 Brake Fluid vs. Silicone Brake Fluid is like the Republicans vs. the Democrats. Most race car drivers I've spoken to use Silicone brake fluid. Let's leave it there.

The end play of the hub/rotor has to be addressed. If you feel the end play is significant (a standard piece of paper is .003 inches thick for example), then try to install a .005 inch SHIM and then tighten down again and verify any end play. Check the runout as described by our good MG friend above, J.H. Cole in the UK.

In the COMPLETE OFFICIAL MGB 1962-'74, Workshop Manual, General Data, p.58, it indicates under TORQUE WRENCH SETTINGS-Rear Axle - "Axle Shaft Nut" - 150 lb/ft. and aligned to next (cotter) pin hole.

Op.Cit.: Page 55; It's not exactly determined what the author's intent was here: Pinion Bearing Nut = 135-140 lb.ft.; then just under this is the "Bearing Retaining Nut" = 180 lb.ft.

The author has chosen to change nomenclature several times in the manual but clearly, the Axle - Bearing Retaining Castellated Hex Nut is designed for 40-70 lb.ft. as referenced on page 55 (op.cit.). FRONT SUSPENSION: "Bearing Retaining Nut" - 40-70 lb/ft.

Ref. Haynes Automotive Repair Manual, MGB 1962 - 1980, Ch. 8 REAR AXLE, pp. 142: Axle Shaft Nut (Semi-Floating axle) = 150 lbf/ft (further tighten to align split pin hole).

MY OPINION: I would tighten to 40-70 ft/lbs as described in a few other MG & General Automotive Technical Articles. Tighten to about 60 ft/lbs, spin the wheel/rotor; wiggle from side to side, back off the nut and RE-TORQUE the axle nut to 60 ft/lbs as described by John Twists video on YouTube.


John indicated that you should tighten to the next cotter pin hole and I have been corrected on this by several people. But isn't what this MG Experience web site is all about?

Good hunting Gonzalo.
R Murray

Well, John Twist sure has it under control.

So what I understand (even if the MGA hub config is slightly different and has no tapper bearings), is that I would need to take out the outer bearing and remove a shim (or two). right?

I will try to measure the end float and also runout...

Any tricks to tighten to 60ft/lbs without a torque wrench? Do it involve putting an extension bar to the 1/2 inch rachet?

G Ramos


Unless your car is a twin cam or DeLuxe, you won't find any shims. I would try turning your castle nut 1/4 to 1/2 turn more (or even some more). With a 2-foot breaker bar, I generally give it a good pull. The play you describe sounds to me like the nut isn't tight enough.

Glen Perchie

I will try to tighten it a bit tomorrow to see if it goes away.
The float I am talking about is minimal. I can feel a little 'click' when I pull it.

This will be the easiest solution rather than having to take the bearings out, which I would like to avoid.
G Ramos


Is the play you mention found by rocking the wheel? If so it is probably in the front hub bearings and be down to bearing tolerances. This has been much discussed here in the past and is either a thing we live with or can be overcome by using tight tolerance bearings.

On the MGB, taper roller bearings were used allowing the play to be taken out by tightening the hub nut but with the ball races on the MGA we have to rely on the accuracy of the bearings themselves. Putting in shims or altering the bearing spacer tube will make things worse and should not be done.

You can try changing the bearings but I doubt it will make any difference. The people here who have researched bearing tolerances can probably tell you more.

Malcolm Asquith


Thanks for the additional info.
To me it seems like a very minimal play. It is in the direction of the axis of the hub, not side to side.

I am sure I can live with it as long as it doesn't lead to a dangerous/catastrophic situation in the future.

Cos I am sure the nut was pretty damn tight! But I can try backing it off and doing it again, see what happens.
G Ramos

This thread was discussed between 05/04/2010 and 12/04/2010

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