fiftyfive

Anyone know the diameters of the pistons in the S4 front/rear calipers? Also, the diameter of the master cylinder? I recently bought James Walker's brake book (good one) and want to plug actual numbers into his formulas.

Thanks,
John

AO

From the Tech Spec book. You'll want the 86 onward specs.

fiftyfive

Thanks Andrew!

John

928autobahndreamer

Wasn't there a different piston size in 87 vs 88 on. I was thinking this is why there haven't been caliper rebuild sets for the 87. Please correct me if I'm wrong because I am getting ready to take apart my 87s brakes and was just going to clean and reassemble.

John Veninger

Yes, the piston size did change on the "S4" calipers. Andrew shows it above. 86.5 and most of 87 had 42 and 36 pistons. Sometime in 88 they went to 44 and 36. then in 1989 the pistons stayed the same, but the dust seals changed.

46-6c of the Workshop manual:

Note
From 1987 models on*, the front brake caliper
piston diameters were changed to 44/36
mm (previously 42/36 mm).
During the 1989 model year, a further modification
was introduced. The piston seal was
changed from the scraper ring type to the protective
cap type. (Technical Information Gr. 4
No. l/69) Notes on replacement: refer to
Pages 47-04 I 4705.

l*On some cars (those for the USA, Canada, Australia and Arab countries), fixed calipers with piston
diameters of 42/36 mm were still used in the 1987 model year. For notes on replacement, refer to
Page 47-04.

Vilhuer

Once again US cars got old stuff. Porsche people must have thought US drivers do not exceed 55 mph.

dr bob

Of course none of us did back when Porsche made this change. Now we get to rocket along at 65 and sometimes even 70 MPH. We are chewing through dinosaur squeezins faster than they can be replenished, thanks exclusively to the change in speed limits.

I know that my fuel comsumption is higher at 70 than at 55. Mostly because I get to slow down for folks cruising at 55 in the left lane, and accelerate back to 70 once I can do a totally unsafe pass-on-the-right, after flashing to alert them that they might be in the wrong lane.

-----

Traffic congestion here is a big subject of discussion these days. It's time to step up and remind our lawmakers that we can fit twice as many cars on the same roads for half the time if everyone would drive twice as fast as they are now. Congestion is caused by people who drive slow. In front of me.

Jim bailey - 928 International

Be sure to factor in the proportioning valve as it is what determines the front rear bias...

fraggle

Yes careful with those '87's... they are different and the seals not available anywhere. The only solution I was able to find (and abandoned) was all new pistons. ick.