SharkSkin

I picked up these calipers a long time ago as part of a set of 4. They were represented as a complete set of S4 calipers. They sat in a box in my garage for the past few years while I waited for my original rotors to wear down.

I'm pretty sure the front ones are in fact late S4 calipers, piston sizes are 44/36mm.

The rear calipers are a bit of a puzzle though. Every bit of info I've been able to dredge up indicates that rear calipers should have 30/28mm pistons. Using the Zeckhausen "how to measure pistons" page I came up with 40/36mm piston, whether I measured the hole in the pistons or the OD of the seal. I wanted to be sure, so I extracted the pistons on these calipers enough to measure them to be certain, and they are 40/36mm pistons. WTF, over?

Can anyone help me to ID them, and more importantly answer the following questions?

1. Will these bolt up and work using the normal "S4 brake retrofit" approach?
2. Will the larger pistons and resulting higher clamping force per unit of line pressure negatively impact my brake balance? Total area per caliper would be ~2600sq. mm for the smaller pistons and ~4500 sq.mm for the larger pistons, almost doubling the clamping force. OTOH, the fronts(compared to 42/36mm) will generate more force than the early calipers, but not by much. The increase in area is ~5075/4800. I have heard that the bias might be a bit light on the rear with the usual S4 retrofit but I'm afraid this might be too far in the other direction.

Mounting holes are 12mm dia. ~94mm spacing, I can take other measurements as needed to help ID the parts.

For reference, the OB brakes have F/R piston diameters of 54/36mm, area per caliper is ~2300/1000. It would seem that the 30/28 rears would get me much closer to the same bias as what I have currently.

Clicking any of the pics below will open a large version.

Leon Speed

Pre 89 front calipers are 42/36 mm, so that's not it. Rear calipers are 30/28 mm, also on GTS's. From the mounting points they are rear calipers. My guess, they are not 928 calipers.

Mrmerlin

They look like rear calipers due to the mounting points.
If you have the pads you can measure them then look up rear pads and see if they are the same measurement.

SharkSkin

Stan, pads are 98mm long -- where can I look up sizes? At best that would be a clue, but IMHO would not answer the other questions.

JPBNY

My Guess Could be 968 or 951 AKA 944 Turbo non S

stolarzj

According to the part numbers on those they are front calipers from a early 944 Turbo or 964.

SharkSkin

Lovely. Anyone have a set of S4 rears for swap or sale?

86'928S MeteorGrey

Those look a lot like the 85/86 928 calipers.... I'd need to see them side by side to be sure...

SharkSkin

Mike, AFAIK all 928 fronts(4-pot) have radial mounting bolts, all rears(4-pot) have axial mounting bolts(and smaller pistons than these).

ZEUS+

I've checked everywhere and cannot match those #'s to anything. The length of an rear S4 pad is 98mm, front 132. The raised letters and lines are pre 89 I believe. First pic is 928, second is 944/968. Just quick and hopefully accurate research.
928 LR caliper - 928.352.421.00
928RR caliper - 928.352.422.00
944/968 RR caliper - 951.352.422.02
944/968 LR caliper - 951.352.421.02
http://jenniskens.livedsl.nl/Technical/Brakes.htm

SharkSkin

Thanks for the input, Zeus. I expect that those are Brembo #'s.

Maybe they are front calipers for a different car. If the offsets and mounting work out, maybe someone will be interested as an upgrade to rear brakes, but I have heard no complaints about the performance of 928 rear calipers.

I have already posted these in the classifieds, since IMHO putting them on my non-ABS OB(assuming they fit) would cause the rears to lock up much too early.

Mrmerlin

Hi Dave i just pulled my 1988 set I have and the rear inner numbers are 20487300 2/87 and 20487200 2/87
the pistons are 30 and 28mm and the pads are 98mm long
From your numbers it sounds like you might have a newer set of calipers, as far as the rear pistons go you might try it and if the rears are locking then you would change the bias valve if you have one.
You might also have to rework the MC to the newer 2 outlet system to do this to fit a bias valve

SharkSkin

Thanks for looking into it Stan. Trial an error is not an option here. I know of OB owners who have had good results overall with the S4 rears. My plan is/was to pull out all of the pistons and seals, get the calipers powdercoated and the guide plates replated, put it all together and do the swap in an afternoon. Once I get some S4 rears I'll move forward with that plan.

Changing the bias valves is not a trivial effort like it is on the later cars. I do have twin bias valves(diagonal circuits) and a rat's nest of plumbing. Ideally I'd rather not f with that end of it.

Mrmerlin

a suggestion but it may be void if you go the powder coating route, is to leave the outer piston wipers in place and clean the pistons and clean the bores and replace the inner seals.
I just did the front on my 86.5 and it was going to be major PITA to remove the outer seals I did as I instructed you it seems to work fine.
In fact i didnt replace the seals as they were good just cleaned them and put some DCorning111 on the seals prior to installing them.
I did find the pistons were full of crud and that took a bit of scraping with a razor blade held perpendicular to the piston to clean off the baked on crud

Giovanni

http://www.hydroponicsonline.com/sto...310807407.html

Seems to fit a 1986 944 Turbo.. ;-(