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MG MGA - caliper disassembly with a twist

Hi folks. I have a spare brake caliper that I want to rebuild. The calipers only has one piston remaining in it, the other having been already removed by a previous owner. The remaining piston is an original MGA piston, and is firmly seized in the bore. Not being able to generate hydraulic pressure using a grease gun, with a missing piston, I am wondering if this will work: Separate the caliper halves, put a metal /rubber gasket between the fluid channel passage, bolt the caliper back together, and then apply hydraulic pressure to the caliper. My reasoning is that with the fluid channel blocked, the hydraulic force will then act on only the one half of the caliper, that being the one with the seized caliper. Is there any flaw in my logic, or any concerns readers might have with my idea? Additionally does anyone have any other thoughts on how to remove my single seized caliper piston using a different method? As you may have guessed, I have been learning that MGA difficulties are often not as simple as they first appear! Thanks. Glenn
Glenn

Have you tried heat? I would use a piece of soft copper sheet between the caliper halves. If you don't have a torch, try and talk the wife into heating it in her self cleaning oven set on clean, (make sure you flush it out first)than apply the hydraulic pressure. If you can't develope enough hydraulic pressure, put in a few drops of oil and several drops of water, plug the hole for the line and heat the caliper, steam can be quite the persuader. Put a nice heavy rag in front of the piston.
John H

Glen; With due respect toJohn, don't heat it. The steel piston has a higher rate of linear expansion (.0000065 in/in / degree f than the iron housing(000006), which will make it tighter as it heats up. Please don't use steam, it expandes to 2000 times it orignal volume and can crate an explosive force. Could easily go thru the over door an sill have enough energy to penetrate several walls! I would soak it for several days in penetrating oil, then try to move the piston in. If it doesn't budge soak again then try your method. If that does't work i would use a batery charge an anode and a electolite to disolve the rust. Be safe.Good Luck, Bill
w.g cook

I've had success using a 3 or 4 jaw chuck on a lathe to grip the piston, then lever the caliper away from the chuck. People send us these things half disassembled quite often, and this has been the only reliable way. Peter
Peter Caldwell

Glenn. I have had good luck with a product called ZEP Restore, made by ZEP manufacturing. It is specifically designed for use on rusted parts and works better than most penetrating oils I have tried.

As to the open hole, I made steel cover that goes over the hole, used a piece of gasket material between the cover and the caliper, clamped it in place, then, used air pressure to force out the remaining piston. This system, combined with the ZEP Restore has worked well on even badly rusted calipers. Grease might work, as others have mentioned, but I have not tried it myself.

Peter's idea sounds like a good one if you have a sufficiently large lathe. A Unimat would have too small, and weak, a chuck. With my 17" LeBlond, however, it would be worth considering should I ever need to use that technique.

Les
Les Bengtson

Personally I wouldn't want to mess with seperating the caliper halves. I've used Les's method before (block off the open hole, penetrating fluid, air pressure)and it worked just fine. You may have to work it a few times, forcing the piston a fraction of an inch, pushing it back in, etc.

Oh yeah, and do mind your fingers.
Mark

Hi Mark. Stories about supposed "Problems" with separating the caliper halves were apparently a Girling marketing ploy, to entice owners to purchase brand new brake calipers, instead of rebuilding the old ones. Specialty brake shops routinely separate the caliper halves in order to facilitate proper cleaning, and ease of installing new pistons and seals. One simply needs to ensure that caliper halves are spotlessly clean, that a PROPER new fluid channel seal is used, and that the caliper halves are bolted back together using required torque values on the bolts. Cheers! Glenn
Glenn

Glenn- I think it would be wise to replace the bolts also. I think that Clarke Restorations used to have them and he also has the lock tabs. Gary
gary starr

Thanks for all the advice folks. I think I am back on track! Cheers! GLenn
Glenn

W.G.
John H

W.G. Thanks for being polite when saying I'm out of my mind. While you are correct about the expansion rates of the material, sometimes heating and than cooling can result in breaking the seized part loose just from the expansion and contraction. I have pulled many broken steel studs from cast iron using heat. I wasn't suggesting putting the whole part in the oven for steam, just heating the caliper slowly behind the piston. I fully agree that heating the whole caliper quickly could product some spectacular pressures.
John H

Yes, exactly John and I would go that way as well. When using heat we all have to think one step ahead and work out what we might destroy. In this case the piston seal would have no favours done but as this will be replaced anyway it doesnt matter. I agree that the expansion rates of the two materials work slightly against each other but as Jonn says this doesn't matter here. In addition depending on where the heat is applied first it could just be that the calliper bore is first to move. Not everyone has a lathe with chuck available. I for one would want the job done quickly and it that case heat would be the option.
Iain MacKintosh

MGA calipers, unlike almost every other caliper, have a pin in the bore which goes into a needlessly complicated bit in the piston. That is usually the cause of a seized piston and resists the usual techniques applied to other types of calipers. BTW, consider removing the pin and using the more common, and cheaper, MGB piston. The pin is not neccessary. Peter C
Peter Caldwell

Glenn,
Don't know how you decided to proceed, but, if you are interested I could send you a used piston to plug the opposite side. However, since it seems likely that you're going to install new pistons anyway, why not just install one of the new pistons first and save yourself the grief of dismantling the caliper? Also, I have to say I've never encounterd a piston that was truly seized. Hydraulic pressure ( I use a bench top master cylinder) always seems to pop them free, no matter how long they've been laying around.
Good luck,
GTF
G T Foster

John H.
I agree with you. I have used my heat gun to free up items like calipers and the M/C, with the addition of good penetrating oil like Les mentions. What W.G. says about the expansion rates will work in your favor if you allow the metals to expand and contract several times, allowing the penetrating oil to gain access to all the interfaces between the two metals.
Good Luck!
Cleve

Hi Gerry. Thanks for the kind offer. If I can't extract the piston any other way, I may take you up on use of your spare piston. However, I think that I should be able to get the offending piston out without resorting to a second piston being installed. I will keep you updated. Cheers! GLenn
Glenn

Hi Folks! I can happily report that I was finally successful in removing the stuck piston from my brake caliper! Thanks for everyone's input, it is appreciated! Cheers! GLenn
Glenn

Well done Glenn - so what was your final method in extracting the stuck piston? - just in case I need to do the same - Cheers Cam
Cam Cunningham

Hi Cam. The stuck piston was extracted by using hydraulic pressure generated by a grease gun. Before applying pressure, the caliper halves were split, and a thin sheet of copper with a paper gasket was installed over the internal fluid channel. The caliper was then bolted back together. With the internal fluid channel blocked, hydraulic pressure could not reach the caliper half with the missing piston, so hydraulic pressure forced the remaining piston out. The only downside to all this is that now the grease inside the caliper will have to be removed. I figure that a good soaking in strong solvent, followed by a compressed air blast should do the trick. An immersion in alcohol, followed by another extended air blast should ensure that the caliper is clean enough to rebuild. Thanks again for everyone's assistance! This forum rocks! Glenn
Glenn

Thanks for that Glenn - must remember that - by the way was the immersion in alcohol a refreshment needed for you after all the efforts! Cheers Cam
Cam Cunningham

Cool, you did just what you originaly wanted to do and it worked, it doesn't get any better than that.
John H

Hi Cam. hic, you got the alcoholic refreshment part 100 cer pent correct, hic................Glenn
Glenn

This thread was discussed between 15/02/2006 and 20/02/2006

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