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MG MGB Technical - Caliper piston removal

Any easy tricks on getting both pistons out of the caliper? One is east, but once one is out there seems to be no easy eay to blow the second out. Read somewhere about not separating the caliper halves. Can't see any other way frankly.


Lubricate the one that is out with brakefluid and push it back in.
Get an appropriate grease nipple and fix it instead of the flexi brake line. Apply a c-clamp to the brakepiston that you had out before. Attach your grease gun to the nipple and start pumping. Make sure you donīt pop out the stubborn piston fully. Switch the c-clamp to the other side and do the same trick over again. When both are out. Start cleaning.

Hope it helps


Tom, I've done the grease gun trick Erik describes and it works fine..... just be prepared to split the halves, clean out all the passages of grease, and replace the inner seal and assembly bolts with new parts.
Brian Denis

Put the free one back in and tie it in flush with the inside of the caliper, try a nylon tie strap, then blow compressed air in the flexi pipe but keep your fingers clear of the pistons it should come out with sufficient pressure behind it

C J Bryan

Brian - Where does one get new assembly bolts for the calipers and what torque is used when tightening them?
Cheers - Dave
David DuBois

Dave, the parts:
Bolts, Moss 320-135 $2.54 each, need 4
Inner seal, Moss 180-285 $1.00 each, need 2

I wouldn't reuse the pistons... new are about $18 each, and get a piston seal kit. I used a thinner solvent to get the last of the grease out of the passages after cleaning the gunk.

I don't recall the torque, but it is specified in Haynes & Chiltons.

Brian Denis

Bang the half caliper on a block of wood. piston should come out if not totally rusted in, a little heat always helps too.

If you don't like that approach...go to the hardware store's plumbing section and get an expandable rubber pipe plug that just fits into the piston's cavity. It really grabs hold and you can work the piston out. Splitting the halves won't gain you much--and I'd be leery of re-using the old bolts--they're supposed to be super high strength to sustain the forces of application, once they've been torqued, they may be stretched beyond their design limits. I'd not separate them unless there was absolutely no other choice. The bolts Brian suggests are caliper mounting bolts--Moss, nor any other supplier that I know of, offers replacement bolts for tying the sides together. Re-using the bolts is not a recommended proceedure. Brakes are one area where nothing should be left to chance. Shortcuts and
R. L Carleen

Check with Todd Clarke of Clarke Spares & Restorations. He has bolts & lock plates for MGA's maybe he has them for MGB's.
gary starr

Reusing the bolts is a more common procedure than you might think. I wouldn't be surprised if all the rebuilt units came back with the original bolts. I just split mine and used compressed air to pop the pistons out of both sides. The grease gun method probably works fine too, but what a mess!

I hope Jack never services my car!
Jeff Schlemmer

RL is right .... I listed the wrong bolts, the correct ones are: Proper MG 27H4353 Bolt-Caliper Bridge $2.75.

Jeff is also right, what a mess!
Brian Denis

You do what you want but the factory says do not split. As a mechanic you are takeing one heck of a chance spliting one. I would never split one. I don't want that on my conscience. Not to mention getting sued. They are not hard to do unsplit. Bob Thompson/International Auto
Bob Thompson

I guess there's one thing I don't understand about splitting the caliper halves. If torqueing the bolts once (original installation)ruins the bolts for reuse, how can it have the strength to work in the first place? That's like saying you MUST replace head, rod, and main bolts on every engine during every rebuild - or don't disassemble the engine. Or are the caliper bolts in question that low in quality? They certainly weren't torqued very tight. I removed mine while the caliper was off the car with little effort. Maybe the bolts on my calipers were bad before I even loosened them? Maybe there's an aircraft grade replacement? Why is everyone so scared of these things? I know the ramifications of a cliper splitting while on the road - total loss of brakes, but the odds of that happening have got to be about as low as the odds of being in a plane crash. Has anyone EVER heard of it happening? I'd bet not!
Jeff Schlemmer

Some bolts torque with something called 'stretch torque'. These bolts are torqued to a specific value and then turned an additional number of degrees. These bolts are NOT to be reused. When loosened they do not return to their original dimention. I doubt that caliper bolts fall into that catagory. I more think that Bob Thompson hit it closer by saying "Not to mention getting sued." Try to get brakes replaced and not having someone suggest replacing the rotors and calipers! Then when you don't, they note that on the repair sheet to cover their butts! I had someone check the 'Possible Dangerous Condition' box on a routine oil change/Lube when they did their 'inspection.' Two things happened here. 1) They covered their butts and 2), possibly made it uncomfortable for me if I had an accident after leaving there and have some lawyer attribute it to the 'Possible Dangerous Condition' that I ignored. Scary huh.


Can anyone cite just one case with the facts that splitting the calipers causes any failure? Or reusing the origional bolts? Do any of you actually think the rebuilders of calipers do it without splitting them?
gerry masterman

I don't understand why there is such strong concern about splitting the halves. There are three seperate brake lines, so even if a caliper leaked wouldn't there still be braking... at least temporarily? I recognize it'd brake uneven.
I'd think rebuilding the master cylinder would be of more concern. There you're impacting all braking.
I seperated mine only after a brake shop gave up trying to get the pistons out. The "grease gun" approach was my last idea, and that required spliting the calipers to clean them out.
I must be missing something....
Brian Denis

Hello, folks - midget owners certainly have the best position on this board - right between MGB Technical and MGA, so I occasionally drift over. What everyone is missing is that AP Lockheed once sold a P/N SSB-4 item called a Brake Survival Kit that explained how to split the MGB and Midget brake halves, and get them back together. I recently bought one of thes kits on eBay. Here are my findings:

A L Darnall


Clean the ouside with break fluid/ Gently push them back inside. Clamp one and test with air pressure. It worked for ma for 1964 callipres at rebuild stage.
Good luck.

GKN is the manufacturer and 'S' refers to the tensile strength of the bolt [50-55 Tons/inch sq.] I think you could reuse the bolt if you clean and dry it and use Locktite, because 37.5 lb ft is quite a low torque for a bolt of that size and quality of steel, it certainly wouldn't take it over the elastic limit as suggested earlier.
R. Algie

***If your brake calipers have been dismantled for any reason you MUST fit new bolts. The bolts are self-locking but only designed to be used once. Piston seals can be changed without dismantling the caliper. Thank you Alec for shedding light on this controversial subject!!!

I do believe this means that they are only designed to be self-locking once. In all other respects they are grade 5 bolts torqued to less than their maximum load range. So in other words, the myth is busted and an application of permanent Loctite will render the bolts re-useable, unless someone can prove otherwise. I plan on chasing the threads to insure proper torque - I imagine that is the only other concern.

Many bolts are designed to be re-used after torqueing with a clock gauge - by stretch. That is the proper way to torque any connecting rod bolts. While it is recommended that they be replaced, it is rarely done, and they rarely fail.
Jeff Schlemmer

After just cleaning the threads on the caliper half bolts, I can verify that the factory used a thread locking compound from the fourth thread to the 9th or 10th thread from the ends of the bolts. The amount of compound varied slightly from bolt to bolt, but the narrow band of application was consistent. The bolts do appear to be a high grade hardened steel, but I do not have the means for a Rockwell hardness test. There appears to be no form of mechanical locking such as thread distortion other than the compound applied to the threads. I do believe it is important to clean the threads in the calpier half to prevent an improper torque figure due to corrosion or paint in the area of unused threads. It is also important to remove the pre-existing locking compound. Use of a tap and die is not recommended, but rather a rethreading tap and die designed to clean and straighten the threads (these are readily available at Sears and probably many other outlets.) Using a regular tap and die can cause a weakness in the bolts by helping to cause stress fractures at the point of cutting. This cutting action would damage the rolled threads and reduce the maximum amount of torque the bolts could sustain.

Jeff Schlemmer

This thread was discussed between 03/12/2004 and 05/12/2004

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