Bluemach1

I got these as part of a "Big Black" set up for a 944 (obviously they have been painted red). I can not find any markings to identify them as big GTS or medium S4. Is there a way to ID these visually from the pictures I am posting? Or is there somewhere on the caliper where the markings are that are not visible when mounted?



Thanks!

Jim M.

Is the "Porsche" script a decal or is it cast raised letters? If they are raised then they are not GTS big blacks.

Bluemach1

Yes, they are raised cast lettering. So is that the definitive "tell" between GT/S4 and the GTS?

Rob Edwards

Maybe it's definitive, don't think there are any raised script GTS calipers. by '91 even the S4 front calipers had painted logos. The definitive 'tell' is the dimensions of the caliper. Bottom line, a Big Black GTS front caliper is 11-5/8" long, while an S4 caliper is 10-7/8" long. GTS front calipers are 3-1/4" thick measured at the mounting bolt boss, while S4 fronts are 2-5/8. And so on. Interestingly, S4 calipers are wider overall than GTS big blacks. Can't post pics tonight but I took a bunch, will post later.

Lizard928

I can tell that those are standard S4 calipers. Used from 1986.5 to 1988. In 89 they changed a few things....

Andre Hedrick

Those are straight S4 w/ 44/36mm pistons, I have a set on my car, and the rears would be 30mm only.

Bluemach1

Great. Thanks for all the good information.

So the major meat and potatoes difference S4 to GTS is piston size and the pad size??

Murray

Hey Colin, what changed in 89?

Rob Edwards

I think the major change is that the outer piston seals went from being scraper seals to dust boots. The old scraper seals are NLA, so '87-88 S4 front calipers cannot be rebuilt.

Here's a TSB that describes the changes:

https://webfiles.uci.edu/redwards/pu...%208-12-11.pdf

As to differentiating S4 vs. Big Black calipers- I took a bunch of measurements of different dimensions if anyone cares, but the pictures tell the story.

Top view:




Bottom view:




Side view:




Big Black pistons (same 36/44 mm as the S4 fronts, just more caliper 'meat' around them...):



S4 front pistons:




The caliper casting has very different shapes along the cleft for the rotor and the reinforcement of the 'wall' for the pad retainers:

GTS:





S4:

Bluemach1

Rob -
Good job with the pictures and discriptions!

martyp

FWIW - my late in year model '90 GT has painted logos. Nice photos!

Mako 928

If they are the earlier version (86.5) the seals are NLA and in order to replace them you have to upgrade/ replace the pistons.

Rob Edwards

My 90GT's build date was 1/10/90, it has the embossed/cast-in logos in front:



And painted logos in the rear:



Front 90GT/S4 caliper Part # 20.4564.00:

Vilhuer

They can also have 42/36 pistons if they are early version used in US '86.5 and early '87 S4. ROW cars had them in '86 only except Saudi-Arabia which had same setup as early '87 US model.

martyp

Interesting Rob. My build date is 6/28/90 (I have the much maligned #517 ).

Fronts are 89 20.4565.00 (I'm in the process of adding the logos post re-finish):



and rears are 90 20.4673.00: