Itinerant Air-Cooled

I am rebuilding my front disk braking system. Previous system was dirty and the pistons stuck. The calipers are Girling not ATE. I have cleaned both and the pistons and cups are good with no internal rust/corrosion. Good smooth action. All is well there. One of the caliper to steering knuckle locating bolts has a shoulder bolt and the other is all thread. These calipers have two bleed ports each.

My bolts are M12X1.5 not M14X1.5 as Bentley says they should be. The changeover from 12mm to 14mm apparently occurred in 73 necessitated by the increase in size of the heavier duty calipers. Other articles I have read all point to the 14mm bolts as standard from 73 on up for either Girling or ATE. To clean up the bolts and internal threads I bought a die/tap in 14mm size...nuts! Will fix that oversight shortly. Any thoughts on this or am I missing something obvious?

Psucamper wrote:I am rebuilding my front disk braking system. Previous system was dirty and the pistons stuck. The calipers are Girling not ATE. I have cleaned both and the pistons and cups are good with no internal rust/corrosion. Good smooth action. All is well there. One of the caliper to steering knuckle locating bolts has a shoulder bolt and the other is all thread. These calipers have two bleed ports each.

My bolts are M12X1.5 not M14X1.5 as Bentley says they should be. The changeover from 12mm to 14mm apparently occurred in 73 necessitated by the increase in size of the heavier duty calipers. Other articles I have read all point to the 14mm bolts as standard from 73 on up for either Girling or ATE. To clean up the bolts and internal threads I bought a die/tap in 14mm size...nuts! Will fix that oversight shortly. Any thoughts on this or am I missing something obvious?

Are the threaded portions of the bolts M12 and the locator shoulder portion 14?
Colin

Colin.Thanks for the quick reply. In essence, there is no mystery The bolt I measured was not from the calipers. Gross operator error! Both (correct bolts) are M14X1.5 and still need to be thread chased. But another question arises. Since these are Girling calipers, There are no cutouts on the Girling pistons where the thin plates are supposed to index to prevent piston rotation as seen on the ATE pistons. I have a new (anti-rotation?) set from Scott but there is no obvious way for the plates to grab the pistons and hold them in place to prevent rotation. The reason for the plates half-moon cutouts with their turned in lip is also not obvious. Don't see how these floppy plates do anything useful. Is it just a matter of anti-squeal or what? VW didn't make the calipers as they were outsourced from Girling so they must have had reason. There has been much chatter about when/why/how these plates operate on the other Forum. Stuff like keeping the pad wear pattern correct. Appears to be speculation or "my mechanic said". I'll install them but sure would like to understand the real story.

In the absence of any authoritative source (i.e. VW/ATE/Girling) of information on the exact function of those plates, there is nothing
left but speculation or amateur engineering analysis. The pretty clear fact that those Girling plates are unable to provide any anti-piston-rotation
effect must weigh against the argument that anti-rotation is the (or even one of the) main purposes of the plates. The bend in middle of the plates may
be there simply as a stiffening element. The Girling plates and the ATE plates both cause the piston's force to be directed mainly on the trailing
half of the brake pads, which I believe is their main intended purpose. Here is a good shot of a Girling plate, showing well where the piston contacts it:
http://images.thesamba.com/vw/gallery/pix/629778.jpg.

Thanks for the reply. It's as I suspected. As you say, sans real engineering data it's mainly speculation/guesses. I'll put them in anyway...can't hurt. Bob

Psucamper wrote:Thanks for the reply. It's as I suspected. As you say, sans real engineering data it's mainly speculation/guesses. I'll put them in anyway...can't hurt. Bob

The new system is better!

The old system asked the shims to pilot the cut-outs in the pistons to face downward.

Rather than forcing the pistons to stay in alignment with little shim plate cut-outs that always get mashed flat, VW decided to just put a gap in the shim along the perimeter of the piston. Now, the piston can rotate but the *engineering* is always in effect.

What is the engineering in effect?

To always keep the leading edge of the piston away from the pad so the pad "lays onto the disk" quietly!
Colin