Stan944

Another question from me on rebuilding brake calipers...

The piston is not symmetric, it has a section of the circumference machined down. This is to prevent brake squeeling, I think.
How do you orient it for assembly (I took notes before dissassembly, but want to make sure it was correct to begin with)?
FSM calls for "check 20 deg piston position and adjust if necessary". I'm not sure if this section of FSM describes my year of the car, but let's assume it does.
Question: 20 degrees measured from what? Could someone explain it clearly, post a picture, etc.?
Thanks again, Stan

89magic98

When I look at the following picture in the FSM, I see a metal/paper shim with a 20 degree angle crossing in front of the piston.

Does the piston surface itself have any kind of lines/marks on it for alignment purposes? If I couldn't find any, my thought was to make marks before I removed it.

Can you turn it with your hand? I can't find any source for a "piston turning pliers". I assume they just want to make sure you don't scratch/score the piston when you move it.

I'm a few days behind you in starting this job. Thanks for starting all these threads - they are very informative!

austin944

I recall that I made a mark on the caliper body before removing the piston that was aligned with one chamfer edge. I was able to rotate the piston by hand with some difficulty when the caliper was removed from the car. Once it's on the car you are going to need a tool to turn it if it's not aligned.

I think the 20 degrees refers to the angle of that flat plate tool that's shown in the picture that kjp posted. The plate fits against the chamfer on the piston. I did it by eye-ball with the mark that I made; I don't think it has to be exactly 20 degrees. If you didn't make a mark maybe you can make the plate out of sheet aluminum.

Stan944

I see it more clearly in the picture now. Yes, there are two "kinks" on the piston: a section about 60 degrees wide is cut out (chamfered?). Now I think it's one of these two kinks that needs to be aligned using the tool shown in the picture. Question: WHICH ONE? Keep in mind I know how it was done by PO - I just want to make sure it was done properly, and didn't move accidentally during brake action.

I can't tell for sure from this picture, but it looks like 20 degrees are measured from the horizontal line.

I have the piston out now, and can place it back any way I want, just want to make sure it is the right way. Yes, it may be hard to twist later on without a proper tool.

Kevin - my pleasure with starting these threads, and glad it's usefull for others too. I'm a kind of guy who pays attention to details, which are obvious to some people, and irrelevant to others, or useful to guys like you, :-)

I think this alignment of the piston is fairly important. In addition to making brakes squeel, wrong alignment could substantially modify the brake bias, if done randomly on four courners. On my mountail bike, for example, the feel of the brakes changes dramatically depending on the angle between the brake pads and the wheel rims.

cheers, Stan

austin944

Both. When the plate lays across the piston, I believe it's supposed to lie flush with the piston top. It fits in the chamfered 60 degree notch.

Edit: You can buy the alignment plate and the piston turning tool from Baum Tools. Go to Porsche->Brakes->Mechanical.

http://tools.baumtools.com

Stan944

Thanks for the hint, but I still can't visualize it properly without having this rather specialized tool. Could you draw a picture or say how the kinks are positioned, e.g. 20 deg and 80 deg clockwise from 12 o'clock looking towards the piston (this will depend whether it's the left or right side caliper).
Thanks.

Charlotte944

Just make sure the cut-out section of the caliper piston faces INTO the direction of the brake rotor's rotation.

If you look at the picture that 89magic98 posted you can see the cut-out section of the piston is facing "down" and the brake disk will be rotating "up".

The reason for the cut-out is to spread the clamping force towards the rear (trailing edge) of the brake pad. If the piston were flat (no cut-out), the caliper would apply more clamping force to the leading edge of the brake pad. This would cause the pads to wear unevently, with more wear on the pad's leading edge.

Stan944

Thanks.
Incidentally, a good drawing of positioning the template tool was included in the caliper rebuild kit - I should have checked earlier, silly me.
Stan

Stan944

and here is the drawing of the template: