[Crumpler]

Conversion from 86.5 (so S4 caliper and rotor set up) to GTS caliper and rotor set up on front. Rear will remain S4 set up for now. Good problem to have but with the SC, the current brakes don’t feel real “sufficient”.
18 inch wheels already on car, so clearance issue should be fine.

My initial research shows I need 322 diameter rotors, with 32mm thickness and height of 77 mm with hat height of 45mm.
Anybody think that’s wrong?

I’m using aftermarket stainless steel brake lines that i assume will convert in terms of length to new calipers ok.

My understanding is that OEM GTS was solid rotor and I’m looking for slotted/drilled. Anybody got a quick source? I was looking at the Mercedes 400 3654 20, which I think is a Zimmermann #000-421-30-12-07-M359 for the record. But I can’t find specs on height.

Any additional insight from my betters?

[maddog2020]

Roger can get you a GTS sized rotor with the correct offset from the hat in cast hole or slotted.

[Benno]

The stainless brake lines you have on your S4 calipers will work fine for the GTS calipers, they are the same length and mounting.

You can source OEM cast hole rotors that were used on the 965 for the GTS calipers. These are dimensionally identical to the GTS rotors. Here are the part numbers Left: 96535104500 and Right: 96535104600


Additionally, you can get the matched rears from a 951/968 part number 95135204101


Also, you may want to change your brake bias valve there a couple different variations.
18 bar - 951-355-305-01 (used in most 928)
35 bar - 928-355-305-02 (used in some 928, 911, 944 , 968)
45 bar - 964-355-305-00 (964-993)
55 bar - 964-355-305-10 (964 s/turbo)


If you are interested I have a spare 55 bar valve and rear pads,

[Crumpler]

Ahhhhhhh.
Perfect MB. Thank you for the run down.

I need to research the bias issue and figure out what I want.

[maddog2020]

I just found my receipt for those dust shields. I got them from DR years ago for $18 each.... damn parts have really gone up.

[worf]

I need to research the bias issue and figure out what I want.

Start with the 33 BAR.

A 928 track car typically has a more-forward weight bias than a stock 928 and as a result needs less rear brake pressure than an optimally set-up street 928. 18 BAR is not optimal for a street 928. 55 BAR is way too much for a street 928. 33 works well on a street 928.

[maddog2020]

Tire sizes are another thing to keep in mind when considering bias change. I'm running 335's on the back. When I upsized to the F50 fronts and the 993tt rear brakes the brake bias would be similar to stock, however adding so much rear tire, adds to the equation, meaning that I would need less braking in the rear because the larger tires can perform more braking before locking up, & I wanted a more balanced feel. Porsche tried to be conservative in the brake bias which is why so many people change it. We are also putting better tires with less sidewall, and wider tires and that can change the braking performance quite a bit.

[Crumpler]

Dug in on this today.
I was replacing the protective shields as well so that required pulling the hubs. I forgot how much fun it is to pull the dust caps. Curious if someone has a better hack, I use a muffler clamp and bang on it with a four pound hammer, repositioning clamp to work around the circumference.
Everything else was cleaning and plug and play.
I’m going to power bleed everything in the morning.

[Ed Scherer]

When I did this job 20+ years ago, I used Vice Grip 11R to get the dust caps off. Working around the perimeter while rapping on the tool's knuckles got it done.

I also seem to remember that chain-link fence post clamps (a variation of what you used) can be helpful, too. Not sure of size; something like this: https://www.homedepot.com/p/Everbilt-2- ... /312373224

[MFranke]

For the dust caps, I tap rather than pull. I've found it easiest/fastest to use a spare impact socket extension, which have somewhat rounded nose where they normal insert into the socket. I angle the extension nose into the groove lip on the cap, and just hammer tap the extension around the cap until it comes out. The angle is maybe 45deg, but I actually think this helps unbind the dust cap when tapping it out. Because of the rounded nose it does not mar the cap.

For inserting I use a rubber mallet.

[Crumpler]

Appreciate the inputs.

Bleeding system today with the motive power bleeder, didn’t take much to do it.
What volume do you think a caliper holds S4 or GTS sizing?

[worf]

What volume do you think a caliper holds S4 or GTS sizing?

With the pistons fully ‘pushed-in’ … less than a tablespoon.

[Crumpler]

Thanks, that’s consistent with what I saw. Interesting.

[Crumpler]

Brakes feel better. Trustworthy.
I will say I’m probably not done.
Front end now brakes solid, back end gets lighter at hard speed braking. I won’t say squirrelly, there’s 305’s on, but a little dancy if that’s a word.
Might need to change bias or even rear caliper size.

[worf]

Brakes feel better. Trustworthy.
I will say I’m probably not done.
Front end now brakes solid, back end gets lighter at hard speed braking. I won’t say squirrelly, there’s 305’s on, but a little dancy if that’s a word.
Might need to change bias or even rear caliper size.

Front tire size and front spring stiffness also need to be factored into this.

What bias valve are you running now? 33?

With 305s on the rear - and 86.5? rear calipers - I doubt you’re getting close to locking the rears up.

Anti-dive solutions and/or rear downforce…

Strap-in the biggest rear passengers you can find and see if the rear is still squirrely under braking. If it’s not, then dive and weight transfer’s the problem.

[Crumpler]

Gotta admit I’ve having some pesky brake squeal on one front caliper. Just enough to go back in and deal with it.
Traditionally, I clean any oil from rotor surface ( I’ve heard the manufacturers will coat them to prevent rust), Pads cleaned. Hardware points lubed with coper anti-seize. Back of pads treated with the red sticky brake quiet.
Any other tricks?
I’ve never used shims before, but I have a set coming from Roger. Shims more important for bigger calipers?

[worf]

I’ve never used shims before, but I have a set coming from Roger. Shims more important for bigger calipers?

Shims?

[Crumpler]

I call them shims;)
You could also call them dampers?

[Benno]

Nice work on the brake upgrade and good choice on the rotors. I am curious which bias valve you have in he car now. 18, 33 something else?

[worf]

I call them shims;)
You could also call them dampers?

I’ve heard them called many things, some things that Grandma would be shocked to hear, heretofore never shims.

There’s a trick to making the brakes not squeal (especially with the ‘shims.’) Find me at Frenzy and I’ll tell you. If you’re not going, remind me next week and I’ll post back here.

[Crumpler]

Hey guys, shims/dampers did the trick.

MB, the bias is currently stock, which I’m assuming is 33 percent.

Brake action is now appropriate for car, but I need to get more drives in to be sure about rear end.

The car’s weight is just under 3000 lbs.
It was professionally balanced last year to 51/49ish.
It has Bilstein dampers and a set of the 928 international Eibach springs that I got from BC.
She’s verrrry stiff on the road. Like knock your fillings out stiff if you hit a pothole;)
245’s front and 305’s rear.

[worf]

It has Bilstein dampers and a set of the 928 international Eibach springs that I got from BC.
She’s verrrry stiff on the road. Like knock your fillings out stiff if you hit a pothole;)

It shouldn't be that stiff. Sure doesn't need to be.

From hat source are the Bilstein's?

What ride heights?

[Crumpler]

Bilstein’s were rebuilt by manufacturer, is my memory. BC had not used them yet. A shame that he isn’t here with us, he was lured away by the easy HP of Corvettes:(

I will have to check on height in my log book, pretty close to stock though.

[worf]

I will have to check on height in my log book, pretty close to stock though.

If it is close to stock then the problem may be that you’ve winded out all the progressive part of the spring.

Eibachs are lowering springs.

If the Bilsteins were rebuilt to a non-original spec, then all bets are off.