'69 S Brake Caliper Rebuild Question
 

I am rebuilding my Aluminum S brake calipers and have found that the pins that guide the pistons are different diameters and the one piston does not slide smoothly. Anyone else had that problem or have a set of the guide springs that go in the back of the piston. Thanks Bob Shafer

Bob, You have a PM. Mark S

Mark
You have a Pm Bob Shafer

There were early and late versions of the AL caliper, but I didn't know about pin differences. The thing to look for is another reason for the binding. That spring gizmo inside just slightly backs the piston off the rotor and racers many times don't use them. I'm not sure that the ones you have won't work with different pins. And, maybe someone substituted a pin. They don't "guide" anything. Most ATE calipers have no pins or the gizmo. There must have been a squealing issue at the time back in the late 60's that brought about the use of the pins.

Edit The thing is, people like Stomski Racing who make SS pistons for these don't have two different styles. And, the internals are very hard to find.

fwiw
if you haven't seen it already.
maybe you could ask eric, he is knowledgable.
http://www.early911sregistry.org/for...ad.php?t=17846

Rebuilding my 908 Magnesium Calipers...
 

Need the expert's opinion on this...

http://www.pbase.com/9146gt/gt_alloy_calipers_908_magnesium

Both of my 908 Magnesium calipers have the 'piston guide-pin' missing. I now wonder if the Porsche factory purposely did NOT include the guide-pins on these now very rare 908 Magnesium calipers (used mostly on Hill-climb race cars, so I was told) or are the guide-pins missing and needing to be reinstalled?

http://forums.pelicanparts.com/uploa...1216402770.jpg

http://forums.pelicanparts.com/uploa...1216402782.jpg

http://forums.pelicanparts.com/uploa...1216402800.jpg

http://forums.pelicanparts.com/uploa...1216402791.jpg

http://forums.pelicanparts.com/uploa...1216403402.jpg

Not sure at this point of what I should do, move forward and rebuild these calipers as they are, or try and replace the missing guide-pins.

The million dollar question...

I truly need to confirm or know "for-sure" how the "908 Magnesium Calipers" were configured from factory for these race cars. The Hill-climb cars may have very well not used the pins and it would be a shame for me to follow the wrong path at this point. Any thoughts?

Thanks

this caliper never had the pins, nor the parts inside the piston:
http://forums.pelicanparts.com/porsche-911-technical-forum/329280-identify-caliper.html
http://www.early911sregistry.org/forum/showthread.php?t=21244
It is obviously an early race caliper, and the forging predates the 'normal' S caliper.
Therefore I suspect that the magnesium 908 calipers did not have the pins either.

To answer Bob's question, I have to ask: do the calipers have an 'X' in the indented circles on the bridges? I have both X and non-X calipers, but have not quite figured out what the 'improvement' was that was designated by the X. Perhaps the different pin sizes is a clue. Eric has half of my calipers at his shop now. I still have my NOS early X and the pair of non-X at home.

It's not the 910 style of spring clip and U guide pin. It is Magnesium, not Aluminum and has the guides on the bottom.

Very interesting. Thanks for sharing.

These are not "guide" pins. They are knock-back pins. ATE literally did away with them in the mid-70's. Most calipers included this mechanism. Somehere around 1974 the M-Calipers began to come with the "cup" pistons we see in A-Calipers. Check rear M-Calipers from 74 on and you'll see what I mean. The cup pistons afforded better cooling by having more surface area.

Armando, the pins were never installed in the Mag calipers you have or the very early (if not the first) 908 caliper that Dave has. I've heard from an old pit boss that they used to practice simply breaking them off as they could bind in a racing situation costing the team the race.

Back to the original posters question dilemma; The pins have always been 5mm. I've measured all of them from 69 through 76. My guess is you have other things hanging up your piston. It is very common to have the piston off kilter only slightly and have it hold up, especially in an aluminum caliper. I would use some caliper grease liberally around the bore and lightly on the pin.

One other thing that may have happened is a bent pin. If you did not protect the opposite bore when removing your pistons, you may have bent the pin with the high pressure release of the opposite piston.

All S-Caliper (and M-Caliper) Owners. You have two options:

1. Leave the knock back mechanism in place when performing a rebuild.
2. Remove it. Rather than breaking off the pins (which we've done upon request in the past) I recommend leaving it in place and simply removing the mechanism from the piston. New SS pistons come without the mechanisms and you have to transfer them from your old pistons.

If you remove the mechanism and place it in a plastic bag in the garage, you have the option of using it again if you so desire. The mechanism is very easy to remove by simply inserting a small flat head screwdriver behind the head of the c-clip.

http://www.914world.com/bbs2/uploads...1194407423.jpg

Removed:

http://www.914world.com/bbs2/uploads...1194407463.jpg

Installing in a new SS piston:

http://www.914world.com/bbs2/uploads...1194407498.jpg

Finished caliper:

http://forums.pelicanparts.com/uploa...1216595219.jpg