Ok I thought all S4's had the same brake calipers. Guess thats not
the case. I have one thats sticking and happened apon on off a 87.
Thinking sweet quick fix I unbolted my 88 caliper and tried to
bolt on the 87. ummm no go. the 87 piece hits my rotor 1 inch before
the mounts hit the base. shure enough check the 2 and the 88 caliper
is 1 inch deeper.

Any one seen that before? Or so I have a mutant 88??

 

Methinks you have GTS rotors/calipers on the front.

 

Look at the pistons. The 'stock' ones havesame size pistons...the GT/GTS have 2 different size pistons.

 

nice I have gts brakes sweet!!!!! Guess I will have to rebuild this sticky one.

Thanks all..

 

I'm pretty certain my standard S4 calipers have different size pistons.

 

gKnight.............the piston sizes are all equal S4, GT, GTS..............the difference is in the caliper size itself to accomodate the larger GTS rotor. Easy test, pull the rotor and measure the diameter.............if its 322mm then you got GTS brakes.............304 is S4 brakes.

 

Not true the GTS has the Big black or Big Red painted black upfront. It uses the non cast whole rotor from the 94 turbo 3.6. The pistons are different sizes and so are the dampers. The pads are also larger/deeper.

 

When i painted my Brembos Yellow I accidentally scored one of the pistons on the left rear caliper. When looking for a replacement piston i was told by a gentelman at Stoptech the piston design changed for the S4 brakes sometime in mid-87. Basically the 86.5's and early 87's have a different piston outter seal design than the later S4 calipers, and the early style pistons are no longer available. The 86.5- early 87's have a wiper type seal on the ID of the cylinder bore, and the later brakes have a bellows type seal that is affixed to the ID of the cylinder and OD of the piston....According to Stoptech.

 

Anthony............yes, true.

For clarification I was comparing the front brake caliper piston sizes between the S4, GT and GTS. There are 4 pistons in each caliper 2 x 44 and 2 x 36 for all these models as stated in the WSM technical data 46-05 through 46-012. As I also stated the front brake rotor changes from 304mm to 322mm hence the caliper change to accomodate (reference same WSM pages). The larger rotor and calipers then provide a larger swept brake area.

If the rotor is a 322mm GTS Version the 'older' calipers will not reach the mounting holes before hitting the rotor outer edge. Hopefully that clears the tech data

 

I now see what you were saying. I thought you were saying the piston sizes were of equal size. ie all 4 were 44mm. I did not read it that way. Thanks for the clarification.

The size of the caliper also differs because of the brake pad size The Big black or GTS size caliper uses a much larger pad. I do not have the sizes available but the GTS is something like 142mm by 58mm vs the medium S4 which is something like 98mm x 48mm. These are not the correct sizes but they are close.

 

Anthony..........no worries, glad to share the info.

GKnight...........if indeed you have GTS brakes you should probably consider rebuilding...........not a difficult task with any caliper just everything very clean and use 'new' brake fluid at all times.

Can't recommend the power bleeder highly enough..........on sale from 928 specialists and the gang!

 

How can one tell from a photo?

 

At the risk of asking a dumb question......is there any difference between the GTS big red and big black calipers? (other than the color)

 

No difference other than color.

 

The holbert has 304mm rotors, and calipers with two different sized pistons on each caliper half. dont know the actual diameter difference, but they are different sized.

does this mean the Holbert car has GT brakes up front?

mk