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MG MGA - Caliper bolt torque/piston orientation

Hi people;
Any thoughts out there on a reasonably effective torque range on the 2 main bolts for the caliper halves?

Something else; on the Moss site, there's a .pdf file outlining instructions for using their caliper piston installation tool (386-280). It's used for both MGA and MGB, and within the instructions, they specify that the portion of the caliper piston that has been machined away from the face of each piston should be closest to the caliper mounting ears.
Whereas in both the MGA workshop manual and the Haynes MGA manual, on p. MM.5 and 126 respectively, the instructions call for the cutaway portion of the caliper piston facing downward towards the end opposite the bleed screw.
Silly me, I just finished rebuilding my first caliper (shy of bolting the 2 halves together) before noticing this discrepancy, and I followed the Moss instructions without verifying with the manuals before doing the rebuild.
So, my rebuilt caliper halves both have the cutaway portion of the pistons facing toward the caliper mounting ears.
Would this work ok, or should I re-orient the pistons? Thanks.
Rick deOlazarra

Bridge bolt 17H7873. I can't find a description of it. Give me the bolt diameter, and I can look look up the torque spec. Otherwise it is a high strength bolt, probably grade 5 like most other bolts on the car. Use a standard length end wrench, and give it a good pull. You won't break it.
Barney Gaylord

1/2 inch diameter. (20 threads per inch)
Rick deOlazarra

See here:
90 Dry, 65 Oiled for grade 5 bolts.

Also see here:
If there are grade markings on the bolt head it may allow different torque, directly proportional to tensile strength of the bolts. However, I wouldn't recommend exceeding Grade torque with the cast iron female thread.

Do use a high temperature thread lock adhesive. Original bolts had a thread locking plastic insert or adhesive built in, which is why it was advised not to disassemble the calipers. The original bolts were intended to be single use bolts, but they can be re-used if thread locker is applied.
Barney Gaylord

Rick - You need the pistons the right way round as per the manual. You might just get away with turning the pistons by the central pillar in the pistons with a pair of molegrips. Mine re-assembled exchange calipers arrived all wrong too.
P N Tipping

Barney; thanks for the info... :)

I emailed Moss about the discrepancy, but they haven't responded.
The machined portion of the brake piston was facing upwards on mine b4 I removed them for rebuilding, the manual calls for them to face downward, and Moss material specifies facing forward. It would help if one knew what the purpose of the machined portion is in the first place.
To hell with it; they'll get installed however they get installed.
I remember someone saying something about the design helps eliminate brake squeal. Maybe the machined portion facing forward encourages air to enter & help cooling. Who knows...
Rick deOlazarra

Rick - those pistons must be the right way round - the pads won't sit right and you won't like the squeals..! Have you tried turning them yet? They are very tight to turn...
P N Tipping

My best guess is the cut-out is an anti-vibration device to prevent brake squeal. Since lots of other cars don't have such a cut-out in the piston, I suppose it may not actually be necessary. But since it it there you should get it right. I hope people know how to read the Workshop Manual. The Book definitely dictates cut-out down. That makes me wonder if the disc brakes might work slightly different when backing up.

Later on the MGB Workshop Manual dictated position the cut-out forward, or toward the bearing hub. Perhaps the MG factory had learned something new by this time and had a better idea. So you might consider "upgrading" to later specifications. This position should make the brakes work the same going forward of backward.

This brings up the question, what if you install MGB calipers on the MGA? Does it start another religious war.
Barney Gaylord

Again, thanks for the feedback; it all helps.
FWIW, I have modified MGA calipers (pin removed) and am installing new MGB pistons. The left caliper already has the new pistons installed facing toward the mounting ears (forward), and the 2 halves are yet to be joined together. Pete; I can't think of any way to rotate them other than pushing them out w/air first...(there's nothing to grab onto), and repeating the tedium of gingerly pressing in the dust seal metal cover back into place. I'm encouraged by the fact that the correct orientation of the pistons on the MGB is the same as the caliper I've already done and, incidentally, the same orientation as given by Moss in their instructions for installing caliper pistons in both the MGA and the MGB. (facing forward). It'll be awhile b4 proceeding since I caught a nasty bug a couple days ago and spend most of my time in bed coughing up crap while going through nicotine withdrawal, insomnia, and watching Space: 1999...
Rick deOlazarra

This thread was discussed between 20/01/2013 and 23/01/2013

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